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hsmusic-wiki - HSMusic - static wiki software cataloguing collaborative creation
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-rw-r--r--src/data/cacheable-object.js420
-rw-r--r--src/data/checks.js704
-rw-r--r--src/data/composite.js1297
-rw-r--r--src/data/composite/control-flow/exitWithoutDependency.js35
-rw-r--r--src/data/composite/control-flow/exitWithoutUpdateValue.js24
-rw-r--r--src/data/composite/control-flow/exposeConstant.js26
-rw-r--r--src/data/composite/control-flow/exposeDependency.js28
-rw-r--r--src/data/composite/control-flow/exposeDependencyOrContinue.js34
-rw-r--r--src/data/composite/control-flow/exposeUpdateValueOrContinue.js40
-rw-r--r--src/data/composite/control-flow/index.js14
-rw-r--r--src/data/composite/control-flow/inputAvailabilityCheckMode.js9
-rw-r--r--src/data/composite/control-flow/raiseOutputWithoutDependency.js39
-rw-r--r--src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js47
-rw-r--r--src/data/composite/control-flow/withResultOfAvailabilityCheck.js71
-rw-r--r--src/data/composite/data/excludeFromList.js55
-rw-r--r--src/data/composite/data/fillMissingListItems.js50
-rw-r--r--src/data/composite/data/index.js17
-rw-r--r--src/data/composite/data/withFilteredList.js50
-rw-r--r--src/data/composite/data/withFlattenedList.js47
-rw-r--r--src/data/composite/data/withMappedList.js39
-rw-r--r--src/data/composite/data/withPropertiesFromList.js92
-rw-r--r--src/data/composite/data/withPropertiesFromObject.js87
-rw-r--r--src/data/composite/data/withPropertyFromList.js82
-rw-r--r--src/data/composite/data/withPropertyFromObject.js69
-rw-r--r--src/data/composite/data/withSortedList.js121
-rw-r--r--src/data/composite/data/withUnflattenedList.js72
-rw-r--r--src/data/composite/data/withUniqueItemsOnly.js40
-rw-r--r--src/data/composite/things/album/index.js2
-rw-r--r--src/data/composite/things/album/withTrackSections.js127
-rw-r--r--src/data/composite/things/album/withTracks.js51
-rw-r--r--src/data/composite/things/flash-act/index.js1
-rw-r--r--src/data/composite/things/flash-act/withFlashSide.js22
-rw-r--r--src/data/composite/things/flash/index.js1
-rw-r--r--src/data/composite/things/flash/withFlashAct.js22
-rw-r--r--src/data/composite/things/track/exitWithoutUniqueCoverArt.js26
-rw-r--r--src/data/composite/things/track/index.js11
-rw-r--r--src/data/composite/things/track/inferredAdditionalNameList.js67
-rw-r--r--src/data/composite/things/track/inheritFromOriginalRelease.js50
-rw-r--r--src/data/composite/things/track/sharedAdditionalNameList.js38
-rw-r--r--src/data/composite/things/track/trackAdditionalNameList.js38
-rw-r--r--src/data/composite/things/track/trackReverseReferenceList.js38
-rw-r--r--src/data/composite/things/track/withAlbum.js22
-rw-r--r--src/data/composite/things/track/withAlwaysReferenceByDirectory.js78
-rw-r--r--src/data/composite/things/track/withContainingTrackSection.js42
-rw-r--r--src/data/composite/things/track/withHasUniqueCoverArt.js61
-rw-r--r--src/data/composite/things/track/withOriginalRelease.js78
-rw-r--r--src/data/composite/things/track/withOtherReleases.js41
-rw-r--r--src/data/composite/things/track/withPropertyFromAlbum.js40
-rw-r--r--src/data/composite/wiki-data/exitWithoutContribs.js47
-rw-r--r--src/data/composite/wiki-data/index.js16
-rw-r--r--src/data/composite/wiki-data/inputWikiData.js17
-rw-r--r--src/data/composite/wiki-data/withParsedCommentaryEntries.js179
-rw-r--r--src/data/composite/wiki-data/withResolvedContribs.js76
-rw-r--r--src/data/composite/wiki-data/withResolvedReference.js57
-rw-r--r--src/data/composite/wiki-data/withResolvedReferenceList.js101
-rw-r--r--src/data/composite/wiki-data/withReverseContributionList.js83
-rw-r--r--src/data/composite/wiki-data/withReverseReferenceList.js81
-rw-r--r--src/data/composite/wiki-data/withThingsSortedAlphabetically.js122
-rw-r--r--src/data/composite/wiki-data/withUniqueReferencingThing.js52
-rw-r--r--src/data/composite/wiki-properties/additionalFiles.js30
-rw-r--r--src/data/composite/wiki-properties/additionalNameList.js14
-rw-r--r--src/data/composite/wiki-properties/color.js12
-rw-r--r--src/data/composite/wiki-properties/commentary.js30
-rw-r--r--src/data/composite/wiki-properties/commentatorArtists.js49
-rw-r--r--src/data/composite/wiki-properties/contentString.js15
-rw-r--r--src/data/composite/wiki-properties/contribsPresent.js30
-rw-r--r--src/data/composite/wiki-properties/contributionList.js35
-rw-r--r--src/data/composite/wiki-properties/dimensions.js13
-rw-r--r--src/data/composite/wiki-properties/directory.js23
-rw-r--r--src/data/composite/wiki-properties/duration.js13
-rw-r--r--src/data/composite/wiki-properties/externalFunction.js11
-rw-r--r--src/data/composite/wiki-properties/fileExtension.js13
-rw-r--r--src/data/composite/wiki-properties/flag.js19
-rw-r--r--src/data/composite/wiki-properties/index.js28
-rw-r--r--src/data/composite/wiki-properties/name.js11
-rw-r--r--src/data/composite/wiki-properties/referenceList.js45
-rw-r--r--src/data/composite/wiki-properties/reverseContributionList.js24
-rw-r--r--src/data/composite/wiki-properties/reverseReferenceList.js30
-rw-r--r--src/data/composite/wiki-properties/simpleDate.js14
-rw-r--r--src/data/composite/wiki-properties/simpleString.js12
-rw-r--r--src/data/composite/wiki-properties/singleReference.js46
-rw-r--r--src/data/composite/wiki-properties/urls.js14
-rw-r--r--src/data/composite/wiki-properties/wikiData.js27
-rw-r--r--src/data/language.js331
-rw-r--r--src/data/patches.js598
-rw-r--r--src/data/serialize.js31
-rw-r--r--src/data/thing.js77
-rw-r--r--src/data/things.js1720
-rw-r--r--src/data/things/album.js434
-rw-r--r--src/data/things/art-tag.js107
-rw-r--r--src/data/things/artist.js392
-rw-r--r--src/data/things/flash.js370
-rw-r--r--src/data/things/group.js194
-rw-r--r--src/data/things/homepage-layout.js223
-rw-r--r--src/data/things/index.js188
-rw-r--r--src/data/things/language.js730
-rw-r--r--src/data/things/news-entry.js73
-rw-r--r--src/data/things/static-page.js80
-rw-r--r--src/data/things/track.js566
-rw-r--r--src/data/things/wiki-info.js110
-rw-r--r--src/data/validators.js1018
-rw-r--r--src/data/yaml.js1978
102 files changed, 11552 insertions, 3522 deletions
diff --git a/src/data/cacheable-object.js b/src/data/cacheable-object.js
index 4afb036..1e7c7aa 100644
--- a/src/data/cacheable-object.js
+++ b/src/data/cacheable-object.js
@@ -74,236 +74,296 @@
 //      function, which provides a mapping of exposed property names to whether
 //      or not their dependencies are yet met.
 
-import { color, ENABLE_COLOR } from '../util/cli.js';
+import {inspect as nodeInspect} from 'node:util';
 
-import { inspect as nodeInspect } from 'util';
+import {colors, ENABLE_COLOR} from '#cli';
 
 function inspect(value) {
-    return nodeInspect(value, {colors: ENABLE_COLOR});
+  return nodeInspect(value, {colors: ENABLE_COLOR});
 }
 
 export default class CacheableObject {
-    static instance = Symbol('CacheableObject `this` instance');
-
-    #propertyUpdateValues = Object.create(null);
-    #propertyUpdateCacheInvalidators = Object.create(null);
-
-    /*
-    // Note the constructor doesn't take an initial data source. Due to a quirk
-    // of JavaScript, private members can't be accessed before the superclass's
-    // constructor is finished processing - so if we call the overridden
-    // update() function from inside this constructor, it will error when
-    // writing to private members. Pretty bad!
-    //
-    // That means initial data must be provided by following up with update()
-    // after constructing the new instance of the Thing (sub)class.
-    */
-
-    constructor() {
-        this.#defineProperties();
-        this.#initializeUpdatingPropertyValues();
-
-        if (CacheableObject.DEBUG_SLOW_TRACK_INVALID_PROPERTIES) {
-            return new Proxy(this, {
-                get: (obj, key) => {
-                    if (!Object.hasOwn(obj, key)) {
-                        if (key !== 'constructor') {
-                            CacheableObject._invalidAccesses.add(`(${obj.constructor.name}).${key}`);
-                        }
-                    }
-                    return obj[key];
-                }
-            });
-        }
+  #propertyUpdateValues = Object.create(null);
+  #propertyUpdateCacheInvalidators = Object.create(null);
+
+  // Note the constructor doesn't take an initial data source. Due to a quirk
+  // of JavaScript, private members can't be accessed before the superclass's
+  // constructor is finished processing - so if we call the overridden
+  // update() function from inside this constructor, it will error when
+  // writing to private members. Pretty bad!
+  //
+  // That means initial data must be provided by following up with update()
+  // after constructing the new instance of the Thing (sub)class.
+
+  constructor() {
+    this.#defineProperties();
+    this.#initializeUpdatingPropertyValues();
+
+    if (CacheableObject.DEBUG_SLOW_TRACK_INVALID_PROPERTIES) {
+      return new Proxy(this, {
+        get: (obj, key) => {
+          if (!Object.hasOwn(obj, key)) {
+            if (key !== 'constructor') {
+              CacheableObject._invalidAccesses.add(`(${obj.constructor.name}).${key}`);
+            }
+          }
+          return obj[key];
+        },
+      });
     }
+  }
 
-    #initializeUpdatingPropertyValues() {
-        for (const [ property, descriptor ] of Object.entries(this.constructor.propertyDescriptors)) {
-            const { flags, update } = descriptor;
+  #initializeUpdatingPropertyValues() {
+    for (const [property, descriptor] of Object.entries(this.constructor.propertyDescriptors)) {
+      const {flags, update} = descriptor;
 
-            if (!flags.update) {
-                continue;
-            }
+      if (!flags.update) {
+        continue;
+      }
 
-            if (update?.default) {
-                this[property] = update?.default;
-            } else {
-                this[property] = null;
-            }
-        }
+      if (update?.default) {
+        this[property] = update?.default;
+      } else {
+        this[property] = null;
+      }
     }
+  }
 
-    #defineProperties() {
-        if (!this.constructor.propertyDescriptors) {
-            throw new Error(`Expected constructor ${this.constructor.name} to define propertyDescriptors`);
-        }
+  #defineProperties() {
+    if (!this.constructor.propertyDescriptors) {
+      throw new Error(`Expected constructor ${this.constructor.name} to define propertyDescriptors`);
+    }
 
-        for (const [ property, descriptor ] of Object.entries(this.constructor.propertyDescriptors)) {
-            const { flags } = descriptor;
+    for (const [property, descriptor] of Object.entries(this.constructor.propertyDescriptors)) {
+      const {flags} = descriptor;
 
-            const definition = {
-                configurable: false,
-                enumerable: true
-            };
+      const definition = {
+        configurable: false,
+        enumerable: flags.expose,
+      };
 
-            if (flags.update) {
-                definition.set = this.#getUpdateObjectDefinitionSetterFunction(property);
-            }
+      if (flags.update) {
+        definition.set = this.#getUpdateObjectDefinitionSetterFunction(property);
+      }
 
-            if (flags.expose) {
-                definition.get = this.#getExposeObjectDefinitionGetterFunction(property);
-            }
+      if (flags.expose) {
+        definition.get = this.#getExposeObjectDefinitionGetterFunction(property);
+      }
+
+      Object.defineProperty(this, property, definition);
+    }
 
-            Object.defineProperty(this, property, definition);
+    Object.seal(this);
+  }
+
+  #getUpdateObjectDefinitionSetterFunction(property) {
+    const {update} = this.#getPropertyDescriptor(property);
+    const validate = update?.validate;
+
+    return (newValue) => {
+      const oldValue = this.#propertyUpdateValues[property];
+
+      if (newValue === undefined) {
+        throw new TypeError(`Properties cannot be set to undefined`);
+      }
+
+      if (newValue === oldValue) {
+        return;
+      }
+
+      if (newValue !== null && validate) {
+        try {
+          const result = validate(newValue);
+          if (result === undefined) {
+            throw new TypeError(`Validate function returned undefined`);
+          } else if (result !== true) {
+            throw new TypeError(`Validation failed for value ${newValue}`);
+          }
+        } catch (caughtError) {
+          throw new CacheableObjectPropertyValueError(
+            property, oldValue, newValue, {cause: caughtError});
         }
+      }
 
-        Object.seal(this);
-    }
+      this.#propertyUpdateValues[property] = newValue;
+      this.#invalidateCachesDependentUpon(property);
+    };
+  }
 
-    #getUpdateObjectDefinitionSetterFunction(property) {
-        const { update } = this.#getPropertyDescriptor(property);
-        const validate = update?.validate;
-        const allowNull = update?.allowNull;
+  #getPropertyDescriptor(property) {
+    return this.constructor.propertyDescriptors[property];
+  }
 
-        return (newValue) => {
-            const oldValue = this.#propertyUpdateValues[property];
+  #invalidateCachesDependentUpon(property) {
+    const invalidators = this.#propertyUpdateCacheInvalidators[property];
+    if (!invalidators) {
+      return;
+    }
 
-            if (newValue === undefined) {
-                throw new TypeError(`Properties cannot be set to undefined`);
-            }
+    for (const invalidate of invalidators) {
+      invalidate();
+    }
+  }
+
+  #getExposeObjectDefinitionGetterFunction(property) {
+    const {flags} = this.#getPropertyDescriptor(property);
+    const compute = this.#getExposeComputeFunction(property);
+
+    if (compute) {
+      let cachedValue;
+      const checkCacheValid = this.#getExposeCheckCacheValidFunction(property);
+      return () => {
+        if (checkCacheValid()) {
+          return cachedValue;
+        } else {
+          return (cachedValue = compute());
+        }
+      };
+    } else if (!flags.update && !compute) {
+      throw new Error(`Exposed property ${property} does not update and is missing compute function`);
+    } else {
+      return () => this.#propertyUpdateValues[property];
+    }
+  }
 
-            if (newValue === oldValue) {
-                return;
-            }
+  #getExposeComputeFunction(property) {
+    const {flags, expose} = this.#getPropertyDescriptor(property);
 
-            if (newValue !== null && validate) {
-                try {
-                    const result = validate(newValue);
-                    if (result === undefined) {
-                        throw new TypeError(`Validate function returned undefined`);
-                    } else if (result !== true) {
-                        throw new TypeError(`Validation failed for value ${newValue}`);
-                    }
-                } catch (error) {
-                    error.message = `Property ${color.green(property)} (${inspect(this[property])} -> ${inspect(newValue)}): ${error.message}`;
-                    throw error;
-                }
-            }
+    const compute = expose?.compute;
+    const transform = expose?.transform;
 
-            this.#propertyUpdateValues[property] = newValue;
-            this.#invalidateCachesDependentUpon(property);
-        };
+    if (flags.update && !transform) {
+      return null;
+    } else if (flags.update && compute) {
+      throw new Error(`Updating property ${property} has compute function, should be formatted as transform`);
+    } else if (!flags.update && !compute) {
+      throw new Error(`Exposed property ${property} does not update and is missing compute function`);
     }
 
-    #getUpdatePropertyValidateFunction(property) {
-        const descriptor = this.#getPropertyDescriptor(property);
-    }
+    let getAllDependencies;
 
-    #getPropertyDescriptor(property) {
-        return this.constructor.propertyDescriptors[property];
-    }
+    if (expose.dependencies?.length > 0) {
+      const dependencyKeys = expose.dependencies.slice();
+      const shouldReflect = dependencyKeys.includes('this');
+
+      getAllDependencies = () => {
+        const dependencies = Object.create(null);
 
-    #invalidateCachesDependentUpon(property) {
-        for (const invalidate of this.#propertyUpdateCacheInvalidators[property] || []) {
-            invalidate();
+        for (const key of dependencyKeys) {
+          dependencies[key] = this.#propertyUpdateValues[key];
         }
-    }
 
-    #getExposeObjectDefinitionGetterFunction(property) {
-        const { flags } = this.#getPropertyDescriptor(property);
-        const compute = this.#getExposeComputeFunction(property);
-
-        if (compute) {
-            let cachedValue;
-            const checkCacheValid = this.#getExposeCheckCacheValidFunction(property);
-            return () => {
-                if (checkCacheValid()) {
-                    return cachedValue;
-                } else {
-                    return (cachedValue = compute());
-                }
-            };
-        } else if (!flags.update && !compute) {
-            throw new Error(`Exposed property ${property} does not update and is missing compute function`);
-        } else {
-            return () => this.#propertyUpdateValues[property];
+        if (shouldReflect) {
+          dependencies.this = this;
         }
+
+        return dependencies;
+      };
+    } else {
+      const dependencies = Object.create(null);
+      Object.freeze(dependencies);
+      getAllDependencies = () => dependencies;
     }
 
-    #getExposeComputeFunction(property) {
-        const { flags, expose } = this.#getPropertyDescriptor(property);
+    if (flags.update) {
+      return () => transform(this.#propertyUpdateValues[property], getAllDependencies());
+    } else {
+      return () => compute(getAllDependencies());
+    }
+  }
 
-        const compute = expose?.compute;
-        const transform = expose?.transform;
+  #getExposeCheckCacheValidFunction(property) {
+    const {flags, expose} = this.#getPropertyDescriptor(property);
 
-        if (flags.update && !transform) {
-            return null;
-        } else if (flags.update && compute) {
-            throw new Error(`Updating property ${property} has compute function, should be formatted as transform`);
-        } else if (!flags.update && !compute) {
-            throw new Error(`Exposed property ${property} does not update and is missing compute function`);
-        }
+    let valid = false;
 
-        const dependencyKeys = expose.dependencies || [];
-        const dependencyGetters = dependencyKeys.map(key => () => [key, this.#propertyUpdateValues[key]]);
-        const getAllDependencies = () => Object.fromEntries(dependencyGetters.map(f => f())
-            .concat([[this.constructor.instance, this]]));
+    const invalidate = () => {
+      valid = false;
+    };
 
-        if (flags.update) {
-            return () => transform(this.#propertyUpdateValues[property], getAllDependencies());
-        } else {
-            return () => compute(getAllDependencies());
-        }
+    const dependencyKeys = new Set(expose?.dependencies);
+
+    if (flags.update) {
+      dependencyKeys.add(property);
     }
 
-    #getExposeCheckCacheValidFunction(property) {
-        const { flags, expose } = this.#getPropertyDescriptor(property);
+    for (const key of dependencyKeys) {
+      if (this.#propertyUpdateCacheInvalidators[key]) {
+        this.#propertyUpdateCacheInvalidators[key].push(invalidate);
+      } else {
+        this.#propertyUpdateCacheInvalidators[key] = [invalidate];
+      }
+    }
 
-        let valid = false;
+    return () => {
+      if (!valid) {
+        valid = true;
+        return false;
+      } else {
+        return true;
+      }
+    };
+  }
+
+  static cacheAllExposedProperties(obj) {
+    if (!(obj instanceof CacheableObject)) {
+      console.warn('Not a CacheableObject:', obj);
+      return;
+    }
 
-        const invalidate = () => {
-            valid = false;
-        };
+    const {propertyDescriptors} = obj.constructor;
 
-        const dependencyKeys = new Set(expose?.dependencies);
+    if (!propertyDescriptors) {
+      console.warn('Missing property descriptors:', obj);
+      return;
+    }
 
-        if (flags.update) {
-            dependencyKeys.add(property);
-        }
+    for (const [property, descriptor] of Object.entries(propertyDescriptors)) {
+      const {flags} = descriptor;
 
-        for (const key of dependencyKeys) {
-            if (this.#propertyUpdateCacheInvalidators[key]) {
-                this.#propertyUpdateCacheInvalidators[key].push(invalidate);
-            } else {
-                this.#propertyUpdateCacheInvalidators[key] = [invalidate];
-            }
-        }
+      if (!flags.expose) {
+        continue;
+      }
 
-        return () => {
-            if (!valid) {
-                valid = true;
-                return false;
-            } else {
-                return true;
-            }
-        };
+      obj[property];
     }
+  }
 
-    static DEBUG_SLOW_TRACK_INVALID_PROPERTIES = false;
-    static _invalidAccesses = new Set();
+  static DEBUG_SLOW_TRACK_INVALID_PROPERTIES = false;
+  static _invalidAccesses = new Set();
 
-    static showInvalidAccesses() {
-        if (!this.DEBUG_SLOW_TRACK_INVALID_PROPERTIES) {
-            return;
-        }
+  static showInvalidAccesses() {
+    if (!this.DEBUG_SLOW_TRACK_INVALID_PROPERTIES) {
+      return;
+    }
 
-        if (!this._invalidAccesses.size) {
-            return;
-        }
+    if (!this._invalidAccesses.size) {
+      return;
+    }
 
-        console.log(`${this._invalidAccesses.size} unique invalid accesses:`);
-        for (const line of this._invalidAccesses) {
-            console.log(` - ${line}`);
-        }
+    console.log(`${this._invalidAccesses.size} unique invalid accesses:`);
+    for (const line of this._invalidAccesses) {
+      console.log(` - ${line}`);
     }
+  }
+
+  static getUpdateValue(object, key) {
+    if (!Object.hasOwn(object, key)) {
+      return undefined;
+    }
+
+    return object.#propertyUpdateValues[key] ?? null;
+  }
+}
+
+export class CacheableObjectPropertyValueError extends Error {
+  [Symbol.for('hsmusic.aggregate.translucent')] = true;
+
+  constructor(property, oldValue, newValue, options) {
+    super(
+      `Error setting ${colors.green(property)} (${inspect(oldValue)} -> ${inspect(newValue)})`,
+      options);
+
+    this.property = property;
+  }
 }
diff --git a/src/data/checks.js b/src/data/checks.js
new file mode 100644
index 0000000..44f3efd
--- /dev/null
+++ b/src/data/checks.js
@@ -0,0 +1,704 @@
+// checks.js - general validation and error/warning reporting for data objects
+
+import {inspect as nodeInspect} from 'node:util';
+import {colors, ENABLE_COLOR} from '#cli';
+
+import CacheableObject from '#cacheable-object';
+import {replacerSpec, parseInput} from '#replacer';
+import {compareArrays, cut, cutStart, empty, getNestedProp, iterateMultiline}
+  from '#sugar';
+import Thing from '#thing';
+import thingConstructors from '#things';
+import {commentaryRegexCaseSensitive} from '#wiki-data';
+
+import {
+  annotateErrorWithIndex,
+  conditionallySuppressError,
+  decorateErrorWithIndex,
+  filterAggregate,
+  openAggregate,
+  withAggregate,
+} from '#aggregate';
+
+function inspect(value, opts = {}) {
+  return nodeInspect(value, {colors: ENABLE_COLOR, ...opts});
+}
+
+// Warn about directories which are reused across more than one of the same type
+// of Thing. Directories are the unique identifier for most data objects across
+// the wiki, so we have to make sure they aren't duplicated!
+export function reportDuplicateDirectories(wikiData, {
+  getAllFindSpecs,
+}) {
+  const duplicateSets = [];
+
+  for (const findSpec of Object.values(getAllFindSpecs())) {
+    if (!findSpec.bindTo) continue;
+
+    const directoryPlaces = Object.create(null);
+    const duplicateDirectories = new Set();
+    const thingData = wikiData[findSpec.bindTo];
+
+    for (const thing of thingData) {
+      if (findSpec.include && !findSpec.include(thing)) {
+        continue;
+      }
+
+      const directories =
+        (findSpec.getMatchableDirectories
+          ? findSpec.getMatchableDirectories(thing)
+          : [thing.directory]);
+
+      for (const directory of directories) {
+        if (directory in directoryPlaces) {
+          directoryPlaces[directory].push(thing);
+          duplicateDirectories.add(directory);
+        } else {
+          directoryPlaces[directory] = [thing];
+        }
+      }
+    }
+
+    if (empty(duplicateDirectories)) continue;
+
+    const sortedDuplicateDirectories =
+      Array.from(duplicateDirectories)
+        .sort((a, b) => {
+          const aL = a.toLowerCase();
+          const bL = b.toLowerCase();
+          return aL < bL ? -1 : aL > bL ? 1 : 0;
+        });
+
+    for (const directory of sortedDuplicateDirectories) {
+      const places = directoryPlaces[directory];
+      duplicateSets.push({directory, places});
+    }
+  }
+
+  if (empty(duplicateSets)) return;
+
+  // Multiple find functions may effectively have duplicates across the same
+  // things. These only need to be reported once, because resolving one of them
+  // will resolve the rest, so cut out duplicate sets before reporting.
+
+  const seenDuplicateSets = new Map();
+  const deduplicateDuplicateSets = [];
+
+  for (const set of duplicateSets) {
+    if (seenDuplicateSets.has(set.directory)) {
+      const placeLists = seenDuplicateSets.get(set.directory);
+
+      for (const places of placeLists) {
+        // We're iterating globally over all duplicate directories, which may
+        // span multiple kinds of things, but that isn't going to cause an
+        // issue because we're comparing the contents by identity, anyway.
+        // Two artists named Foodog aren't going to match two tracks named
+        // Foodog.
+        if (compareArrays(places, set.places, {checkOrder: false})) {
+          continue;
+        }
+      }
+
+      placeLists.push(set.places);
+    } else {
+      seenDuplicateSets.set(set.directory, [set.places]);
+    }
+
+    deduplicateDuplicateSets.push(set);
+  }
+
+  withAggregate({message: `Duplicate directories found`}, ({push}) => {
+    for (const {directory, places} of deduplicateDuplicateSets) {
+      push(new Error(
+        `Duplicate directory ${colors.green(`"${directory}"`)}:\n` +
+        places.map(thing => ` - ` + inspect(thing)).join('\n')));
+    }
+  });
+}
+
+function bindFindArtistOrAlias(boundFind) {
+  return artistRef => {
+    const alias = boundFind.artistAlias(artistRef, {mode: 'quiet'});
+    if (alias) {
+      // No need to check if the original exists here. Aliases are automatically
+      // created from a field on the original, so the original certainly exists.
+      const original = alias.aliasedArtist;
+      throw new Error(`Reference ${colors.red(artistRef)} is to an alias, should be ${colors.green(original.name)}`);
+    }
+
+    return boundFind.artist(artistRef);
+  };
+}
+
+function getFieldPropertyMessage(yamlDocumentSpec, property) {
+  const {fields} = yamlDocumentSpec;
+
+  const field =
+    Object.entries(fields ?? {})
+      .find(([field, fieldSpec]) => fieldSpec.property === property)
+      ?.[0];
+
+  const fieldPropertyMessage =
+    (field
+      ? ` in field ${colors.green(field)}`
+      : ` in property ${colors.green(property)}`);
+
+  return fieldPropertyMessage;
+}
+
+// Warn about references across data which don't match anything.  This involves
+// using the find() functions on all references, setting it to 'error' mode, and
+// collecting everything in a structured logged (which gets logged if there are
+// any errors). At the same time, we remove errored references from the thing's
+// data array.
+export function filterReferenceErrors(wikiData, {
+  bindFind,
+}) {
+  const referenceSpec = [
+    ['albumData', {
+      artistContribs: '_contrib',
+      coverArtistContribs: '_contrib',
+      trackCoverArtistContribs: '_contrib',
+      wallpaperArtistContribs: '_contrib',
+      bannerArtistContribs: '_contrib',
+      groups: 'group',
+      artTags: '_artTag',
+      commentary: '_commentary',
+    }],
+
+    ['flashData', {
+      commentary: '_commentary',
+    }],
+
+    ['groupCategoryData', {
+      groups: 'group',
+    }],
+
+    ['homepageLayout.rows', {
+      sourceGroup: '_homepageSourceGroup',
+      sourceAlbums: 'album',
+    }],
+
+    ['flashData', {
+      contributorContribs: '_contrib',
+      featuredTracks: 'track',
+    }],
+
+    ['flashActData', {
+      flashes: 'flash',
+    }],
+
+    ['trackData', {
+      artistContribs: '_contrib',
+      contributorContribs: '_contrib',
+      coverArtistContribs: '_contrib',
+      referencedTracks: '_trackNotRerelease',
+      sampledTracks: '_trackNotRerelease',
+      artTags: '_artTag',
+      originalReleaseTrack: '_trackNotRerelease',
+      commentary: '_commentary',
+    }],
+
+    ['wikiInfo', {
+      divideTrackListsByGroups: 'group',
+    }],
+  ];
+
+  const boundFind = bindFind(wikiData, {mode: 'error'});
+  const findArtistOrAlias = bindFindArtistOrAlias(boundFind);
+
+  const aggregate = openAggregate({message: `Errors validating between-thing references in data`});
+  for (const [thingDataProp, propSpec] of referenceSpec) {
+    const thingData = getNestedProp(wikiData, thingDataProp);
+    const things = Array.isArray(thingData) ? thingData : [thingData];
+    aggregate.nest({message: `Reference errors in ${colors.green('wikiData.' + thingDataProp)}`}, ({nest}) => {
+      for (const thing of things) {
+        nest({message: `Reference errors in ${inspect(thing)}`}, ({nest, push, filter}) => {
+          for (const [property, findFnKey] of Object.entries(propSpec)) {
+            let value = CacheableObject.getUpdateValue(thing, property);
+            let writeProperty = true;
+
+            switch (findFnKey) {
+              case '_commentary':
+                if (value) {
+                  value =
+                    Array.from(value.matchAll(commentaryRegexCaseSensitive))
+                      .map(({groups}) => groups.artistReferences)
+                      .map(text => text.split(',').map(text => text.trim()));
+                }
+
+                writeProperty = false;
+                break;
+
+              case '_contrib':
+                // Don't write out contributions - these'll be filtered out
+                // for content and data purposes automatically, and they're
+                // handy to keep around when update values get checked for
+                // art tags below. (Possibly no reference-related properties
+                // need writing, humm...)
+                writeProperty = false;
+                break;
+            }
+
+            if (value === undefined) {
+              push(new TypeError(`Property ${colors.red(property)} isn't valid for ${colors.green(thing.constructor.name)}`));
+              continue;
+            }
+
+            if (value === null) {
+              continue;
+            }
+
+            let findFn;
+
+            switch (findFnKey) {
+              case '_artTag':
+                findFn = boundFind.artTag;
+                break;
+
+              case '_commentary':
+                findFn = findArtistOrAlias;
+                break;
+
+              case '_contrib':
+                findFn = contribRef => findArtistOrAlias(contribRef.who);
+                break;
+
+              case '_homepageSourceGroup':
+                findFn = groupRef => {
+                  if (groupRef === 'new-additions' || groupRef === 'new-releases') {
+                    return true;
+                  }
+
+                  return boundFind.group(groupRef);
+                };
+                break;
+
+              case '_trackNotRerelease':
+                findFn = trackRef => {
+                  const track = boundFind.track(trackRef);
+                  const originalRef = track && CacheableObject.getUpdateValue(track, 'originalReleaseTrack');
+
+                  if (originalRef) {
+                    // It's possible for the original to not actually exist, in this case.
+                    // It should still be reported since the 'Originally Released As' field
+                    // was present.
+                    const original = boundFind.track(originalRef, {mode: 'quiet'});
+
+                    // Prefer references by name, but only if it's unambiguous.
+                    const originalByName =
+                      (original
+                        ? boundFind.track(original.name, {mode: 'quiet'})
+                        : null);
+
+                    const shouldBeMessage =
+                      (originalByName
+                        ? colors.green(original.name)
+                     : original
+                        ? colors.green('track:' + original.directory)
+                        : colors.green(originalRef));
+
+                    throw new Error(`Reference ${colors.red(trackRef)} is to a rerelease, should be ${shouldBeMessage}`);
+                  }
+
+                  return track;
+                };
+                break;
+
+              default:
+                findFn = boundFind[findFnKey];
+                break;
+            }
+
+            const suppress = fn => conditionallySuppressError(error => {
+              if (property === 'sampledTracks') {
+                // Suppress "didn't match anything" errors in particular, just for samples.
+                // In hsmusic-data we have a lot of "stub" sample data which don't have
+                // corresponding tracks yet, so it won't be useful to report such reference
+                // errors until we take the time to address that. But other errors, like
+                // malformed reference strings or miscapitalized existing tracks, should
+                // still be reported, as samples of existing tracks *do* display on the
+                // website!
+                if (error.message.includes(`Didn't match anything`)) {
+                  return true;
+                }
+              }
+
+              return false;
+            }, fn);
+
+            const fieldPropertyMessage =
+              getFieldPropertyMessage(
+                thing.constructor[Thing.yamlDocumentSpec],
+                property);
+
+            const findFnMessage =
+              (findFnKey.startsWith('_')
+                ? ``
+                : ` (${colors.green('find.' + findFnKey)})`);
+
+            const errorMessage =
+              (Array.isArray(value)
+                ? `Reference errors` + fieldPropertyMessage + findFnMessage
+                : `Reference error` + fieldPropertyMessage + findFnMessage);
+
+            let newPropertyValue = value;
+
+            determineNewPropertyValue: {
+              // TODO: The special-casing for artTag is obviously a bit janky.
+              // It would be nice if this could be moved to processDocument ala
+              // fieldCombinationErrors, but art tags are only an error if the
+              // thing doesn't have an artwork - which can't be determined from
+              // the track document on its own, thanks to inheriting contribs
+              // from the album.
+              if (findFnKey === '_artTag') {
+                let hasCoverArtwork =
+                  !empty(CacheableObject.getUpdateValue(thing, 'coverArtistContribs'));
+
+                if (thing.constructor === thingConstructors.Track) {
+                  if (thing.album) {
+                    hasCoverArtwork ||=
+                      !empty(CacheableObject.getUpdateValue(thing.album, 'trackCoverArtistContribs'));
+                  }
+
+                  if (thing.disableUniqueCoverArt) {
+                    hasCoverArtwork = false;
+                  }
+                }
+
+                if (!hasCoverArtwork) {
+                  nest({message: errorMessage}, ({push}) => {
+                    push(new TypeError(`No cover artwork, so this shouldn't have art tags specified`));
+                  });
+
+                  newPropertyValue = [];
+                  break determineNewPropertyValue;
+                }
+              }
+
+              if (findFnKey === '_commentary') {
+                filter(
+                  value, {message: errorMessage},
+                  decorateErrorWithIndex(refs =>
+                    (refs.length === 1
+                      ? suppress(findFn)(refs[0])
+                      : filterAggregate(
+                          refs, {message: `Errors in entry's artist references`},
+                          decorateErrorWithIndex(suppress(findFn)))
+                            .aggregate
+                            .close())));
+
+                // Commentary doesn't write a property value, so no need to set
+                // anything on `newPropertyValue`.
+                break determineNewPropertyValue;
+              }
+
+              if (Array.isArray(value)) {
+                newPropertyValue = filter(
+                  value, {message: errorMessage},
+                  decorateErrorWithIndex(suppress(findFn)));
+                break determineNewPropertyValue;
+              }
+
+              nest({message: errorMessage},
+                suppress(({call}) => {
+                  try {
+                    call(findFn, value);
+                  } catch (error) {
+                    newPropertyValue = null;
+                    throw error;
+                  }
+                }));
+            }
+
+            if (writeProperty) {
+              thing[property] = newPropertyValue;
+            }
+          }
+        });
+      }
+    });
+  }
+
+  return aggregate;
+}
+
+export class ContentNodeError extends Error {
+  constructor({
+    length,
+    columnNumber,
+    containingLine,
+    where,
+    message,
+  }) {
+    const headingLine =
+      `(${where}) ${message}`;
+
+    const textUpToNode =
+      containingLine.slice(0, columnNumber);
+
+    const nodeText =
+      containingLine.slice(columnNumber, columnNumber + length);
+
+    const textPastNode =
+      containingLine.slice(columnNumber + length);
+
+    const containingLines =
+      containingLine.split('\n');
+
+    const formattedSourceLines =
+      containingLines.map((_, index, {length}) => {
+        let line = ' ⋮ ';
+
+        if (index === 0) {
+          line += colors.dim(cutStart(textUpToNode, 20));
+        }
+
+        line += nodeText;
+
+        if (index === length - 1) {
+          line += colors.dim(cut(textPastNode, 20));
+        }
+
+        return line;
+      });
+
+    super([
+      headingLine,
+      ...formattedSourceLines,
+    ].filter(Boolean).join('\n'));
+  }
+}
+
+export function reportContentTextErrors(wikiData, {
+  bindFind,
+}) {
+  const additionalFileShape = {
+    description: 'description',
+  };
+
+  const commentaryShape = {
+    body: 'commentary body',
+    artistDisplayText: 'commentary artist display text',
+    annotation: 'commentary annotation',
+  };
+
+  const contentTextSpec = [
+    ['albumData', {
+      additionalFiles: additionalFileShape,
+      commentary: commentaryShape,
+    }],
+
+    ['artistData', {
+      contextNotes: '_content',
+    }],
+
+    ['flashData', {
+      commentary: commentaryShape,
+    }],
+
+    ['flashActData', {
+      listTerminology: '_content',
+    }],
+
+    ['flashSideData', {
+      listTerminology: '_content',
+    }],
+
+    ['groupData', {
+      description: '_content',
+    }],
+
+    ['homepageLayout', {
+      sidebarContent: '_content',
+    }],
+
+    ['newsData', {
+      content: '_content',
+    }],
+
+    ['staticPageData', {
+      content: '_content',
+    }],
+
+    ['trackData', {
+      additionalFiles: additionalFileShape,
+      commentary: commentaryShape,
+      lyrics: '_content',
+      midiProjectFiles: additionalFileShape,
+      sheetMusicFiles: additionalFileShape,
+    }],
+
+    ['wikiInfo', {
+      description: '_content',
+      footerContent: '_content',
+    }],
+  ];
+
+  const boundFind = bindFind(wikiData, {mode: 'error'});
+  const findArtistOrAlias = bindFindArtistOrAlias(boundFind);
+
+  function* processContent(input) {
+    const nodes = parseInput(input);
+
+    for (const node of nodes) {
+      const index = node.i;
+      const length = node.iEnd - node.i;
+
+      if (node.type === 'tag') {
+        const replacerKeyImplied = !node.data.replacerKey;
+        const replacerKey = replacerKeyImplied ? 'track' : node.data.replacerKey.data;
+        const spec = replacerSpec[replacerKey];
+
+        if (!spec) {
+          yield {
+            index, length,
+            message:
+              `Unknown tag key ${colors.red(`"${replacerKey}"`)}`,
+          };
+
+          // No spec, no further errors to report.
+          continue;
+        }
+
+        const replacerValue = node.data.replacerValue[0].data;
+
+        if (spec.find) {
+          let findFn;
+
+          switch (spec.find) {
+            case 'artist':
+              findFn = findArtistOrAlias;
+              break;
+
+            default:
+              findFn = boundFind[spec.find];
+              break;
+          }
+
+          const findRef =
+            (replacerKeyImplied
+              ? replacerValue
+              : replacerKey + `:` + replacerValue);
+
+          try {
+            findFn(findRef);
+          } catch (error) {
+            yield {
+              index, length,
+              message: error.message,
+            };
+
+            // It's only possible to have one error per node at the moment.
+            continue;
+          }
+        }
+      } else if (node.type === 'external-link') {
+        try {
+          new URL(node.data.href);
+        } catch (error) {
+          yield {
+            index, length,
+            message:
+              `Invalid URL ${colors.red(`"${node.data.href}"`)}`,
+          };
+        }
+      }
+    }
+  }
+
+  function callProcessContent({
+    nest,
+    push,
+    value,
+    message,
+    annotateError = error => error,
+  }) {
+    const processContentIterator =
+      nest({message}, ({call}) =>
+        call(processContent, value));
+
+    if (!processContentIterator) return;
+
+    const multilineIterator =
+      iterateMultiline(value, processContentIterator, {
+        formatWhere: true,
+        getContainingLine: true,
+      });
+
+    const errors = [];
+
+    for (const result of multilineIterator) {
+      errors.push(new ContentNodeError(result));
+    }
+
+    if (empty(errors)) return;
+
+    push(
+      annotateError(
+        new AggregateError(errors, message)));
+  }
+
+  withAggregate({message: `Errors validating content text`}, ({nest}) => {
+    for (const [thingDataProp, propSpec] of contentTextSpec) {
+      const thingData = getNestedProp(wikiData, thingDataProp);
+      const things = Array.isArray(thingData) ? thingData : [thingData];
+      nest({message: `Content text errors in ${colors.green('wikiData.' + thingDataProp)}`}, ({nest}) => {
+        for (const thing of things) {
+          nest({message: `Content text errors in ${inspect(thing)}`}, ({nest, push}) => {
+
+            for (const [property, shape] of Object.entries(propSpec)) {
+              const value = thing[property];
+
+              if (value === undefined) {
+                push(new TypeError(`Property ${colors.red(property)} isn't valid for ${colors.green(thing.constructor.name)}`));
+                continue;
+              }
+
+              if (value === null) {
+                continue;
+              }
+
+              const fieldPropertyMessage =
+                getFieldPropertyMessage(
+                  thing.constructor[Thing.yamlDocumentSpec],
+                  property);
+
+              const topMessage =
+                `Content text errors` + fieldPropertyMessage;
+
+              if (shape === '_content') {
+                callProcessContent({
+                  nest,
+                  push,
+                  value,
+                  message: topMessage,
+                });
+              } else {
+                nest({message: topMessage}, ({push}) => {
+                  for (const [index, entry] of value.entries()) {
+                    for (const [key, annotation] of Object.entries(shape)) {
+                      const value = entry[key];
+
+                      // TODO: Should this check undefined/null similar to above?
+                      if (!value) continue;
+
+                      callProcessContent({
+                        nest,
+                        push,
+                        value,
+                        message: `Error in ${colors.green(annotation)}`,
+                        annotateError: error =>
+                          annotateErrorWithIndex(error, index),
+                      });
+                    }
+                  }
+                });
+              }
+            }
+          });
+        }
+      });
+    }
+  });
+}
diff --git a/src/data/composite.js b/src/data/composite.js
new file mode 100644
index 0000000..7a98c42
--- /dev/null
+++ b/src/data/composite.js
@@ -0,0 +1,1297 @@
+import {inspect} from 'node:util';
+
+import {decorateErrorWithIndex, openAggregate, withAggregate}
+  from '#aggregate';
+import {colors} from '#cli';
+import {empty, filterProperties, stitchArrays, typeAppearance, unique}
+  from '#sugar';
+import {a} from '#validators';
+import {TupleMap} from '#wiki-data';
+
+const globalCompositeCache = {};
+
+const _valueIntoToken = shape =>
+  (value = null) =>
+    (value === null
+      ? Symbol.for(`hsmusic.composite.${shape}`)
+   : typeof value === 'string'
+      ? Symbol.for(`hsmusic.composite.${shape}:${value}`)
+      : {
+          symbol: Symbol.for(`hsmusic.composite.input`),
+          shape,
+          value,
+        });
+
+export const input = _valueIntoToken('input');
+input.symbol = Symbol.for('hsmusic.composite.input');
+
+input.value = _valueIntoToken('input.value');
+input.dependency = _valueIntoToken('input.dependency');
+
+input.myself = () => Symbol.for(`hsmusic.composite.input.myself`);
+
+input.updateValue = _valueIntoToken('input.updateValue');
+
+input.staticDependency = _valueIntoToken('input.staticDependency');
+input.staticValue = _valueIntoToken('input.staticValue');
+
+function isInputToken(token) {
+  if (token === null) {
+    return false;
+  } else if (typeof token === 'object') {
+    return token.symbol === Symbol.for('hsmusic.composite.input');
+  } else if (typeof token === 'symbol') {
+    return token.description.startsWith('hsmusic.composite.input');
+  } else {
+    return false;
+  }
+}
+
+function getInputTokenShape(token) {
+  if (!isInputToken(token)) {
+    throw new TypeError(`Expected an input token, got ${typeAppearance(token)}`);
+  }
+
+  if (typeof token === 'object') {
+    return token.shape;
+  } else {
+    return token.description.match(/hsmusic\.composite\.(input.*?)(:|$)/)[1];
+  }
+}
+
+function getInputTokenValue(token) {
+  if (!isInputToken(token)) {
+    throw new TypeError(`Expected an input token, got ${typeAppearance(token)}`);
+  }
+
+  if (typeof token === 'object') {
+    return token.value;
+  } else {
+    return token.description.match(/hsmusic\.composite\.input.*?:(.*)/)?.[1] ?? null;
+  }
+}
+
+function getStaticInputMetadata(inputOptions) {
+  const metadata = {};
+
+  for (const [name, token] of Object.entries(inputOptions)) {
+    if (typeof token === 'string') {
+      metadata[input.staticDependency(name)] = token;
+      metadata[input.staticValue(name)] = null;
+    } else if (isInputToken(token)) {
+      const tokenShape = getInputTokenShape(token);
+      const tokenValue = getInputTokenValue(token);
+
+      metadata[input.staticDependency(name)] =
+        (tokenShape === 'input.dependency'
+          ? tokenValue
+          : null);
+
+      metadata[input.staticValue(name)] =
+        (tokenShape === 'input.value'
+          ? tokenValue
+          : null);
+    } else {
+      metadata[input.staticDependency(name)] = null;
+      metadata[input.staticValue(name)] = null;
+    }
+  }
+
+  return metadata;
+}
+
+function getCompositionName(description) {
+  return (
+    (description.annotation
+      ? description.annotation
+      : `unnamed composite`));
+}
+
+function validateInputValue(value, description) {
+  const tokenValue = getInputTokenValue(description);
+
+  const {acceptsNull, defaultValue, type, validate} = tokenValue || {};
+
+  if (value === null || value === undefined) {
+    if (acceptsNull || defaultValue === null) {
+      return true;
+    } else {
+      throw new TypeError(
+        (type
+          ? `Expected ${a(type)}, got ${typeAppearance(value)}`
+          : `Expected a value, got ${typeAppearance(value)}`));
+    }
+  }
+
+  if (type) {
+    // Note: null is already handled earlier in this function, so it won't
+    // cause any trouble here.
+    const typeofValue =
+      (typeof value === 'object'
+        ? Array.isArray(value) ? 'array' : 'object'
+        : typeof value);
+
+    if (typeofValue !== type) {
+      throw new TypeError(`Expected ${a(type)}, got ${typeAppearance(value)}`);
+    }
+  }
+
+  if (validate) {
+    validate(value);
+  }
+
+  return true;
+}
+
+export function templateCompositeFrom(description) {
+  const compositionName = getCompositionName(description);
+
+  withAggregate({message: `Errors in description for ${compositionName}`}, ({map, nest, push}) => {
+    if ('steps' in description) {
+      if (Array.isArray(description.steps)) {
+        push(new TypeError(`Wrap steps array in a function`));
+      } else if (typeof description.steps !== 'function') {
+        push(new TypeError(`Expected steps to be a function (returning an array)`));
+      }
+    }
+
+    validateInputs:
+    if ('inputs' in description) {
+      if (
+        Array.isArray(description.inputs) ||
+        typeof description.inputs !== 'object'
+      ) {
+        push(new Error(`Expected inputs to be object, got ${typeAppearance(description.inputs)}`));
+        break validateInputs;
+      }
+
+      nest({message: `Errors in static input descriptions for ${compositionName}`}, ({push}) => {
+        const missingCallsToInput = [];
+        const wrongCallsToInput = [];
+
+        for (const [name, value] of Object.entries(description.inputs)) {
+          if (!isInputToken(value)) {
+            missingCallsToInput.push(name);
+            continue;
+          }
+
+          if (!['input', 'input.staticDependency', 'input.staticValue'].includes(getInputTokenShape(value))) {
+            wrongCallsToInput.push(name);
+          }
+        }
+
+        for (const name of missingCallsToInput) {
+          push(new Error(`${name}: Missing call to input()`));
+        }
+
+        for (const name of wrongCallsToInput) {
+          const shape = getInputTokenShape(description.inputs[name]);
+          push(new Error(`${name}: Expected call to input, input.staticDependency, or input.staticValue, got ${shape}`));
+        }
+      });
+    }
+
+    validateOutputs:
+    if ('outputs' in description) {
+      if (
+        !Array.isArray(description.outputs) &&
+        typeof description.outputs !== 'function'
+      ) {
+        push(new Error(`Expected outputs to be array or function, got ${typeAppearance(description.outputs)}`));
+        break validateOutputs;
+      }
+
+      if (Array.isArray(description.outputs)) {
+        map(
+          description.outputs,
+          decorateErrorWithIndex(value => {
+            if (typeof value !== 'string') {
+              throw new Error(`${value}: Expected string, got ${typeAppearance(value)}`)
+            } else if (!value.startsWith('#')) {
+              throw new Error(`${value}: Expected "#" at start`);
+            }
+          }),
+          {message: `Errors in output descriptions for ${compositionName}`});
+      }
+    }
+  });
+
+  const expectedInputNames =
+    (description.inputs
+      ? Object.keys(description.inputs)
+      : []);
+
+  const instantiate = (inputOptions = {}) => {
+    withAggregate({message: `Errors in input options passed to ${compositionName}`}, ({push}) => {
+      const providedInputNames = Object.keys(inputOptions);
+
+      const misplacedInputNames =
+        providedInputNames
+          .filter(name => !expectedInputNames.includes(name));
+
+      const missingInputNames =
+        expectedInputNames
+          .filter(name => !providedInputNames.includes(name))
+          .filter(name => {
+            const inputDescription = getInputTokenValue(description.inputs[name]);
+            if (!inputDescription) return true;
+            if ('defaultValue' in inputDescription) return false;
+            if ('defaultDependency' in inputDescription) return false;
+            return true;
+          });
+
+      const wrongTypeInputNames = [];
+
+      const expectedStaticValueInputNames = [];
+      const expectedStaticDependencyInputNames = [];
+      const expectedValueProvidingTokenInputNames = [];
+
+      const validateFailedErrors = [];
+
+      for (const [name, value] of Object.entries(inputOptions)) {
+        if (misplacedInputNames.includes(name)) {
+          continue;
+        }
+
+        if (typeof value !== 'string' && !isInputToken(value)) {
+          wrongTypeInputNames.push(name);
+          continue;
+        }
+
+        const descriptionShape = getInputTokenShape(description.inputs[name]);
+
+        const tokenShape = (isInputToken(value) ? getInputTokenShape(value) : null);
+        const tokenValue = (isInputToken(value) ? getInputTokenValue(value) : null);
+
+        switch (descriptionShape) {
+          case'input.staticValue':
+            if (tokenShape !== 'input.value') {
+              expectedStaticValueInputNames.push(name);
+              continue;
+            }
+            break;
+
+          case 'input.staticDependency':
+            if (typeof value !== 'string' && tokenShape !== 'input.dependency') {
+              expectedStaticDependencyInputNames.push(name);
+              continue;
+            }
+            break;
+
+          case 'input':
+            if (typeof value !== 'string' && ![
+              'input',
+              'input.value',
+              'input.dependency',
+              'input.myself',
+              'input.updateValue',
+            ].includes(tokenShape)) {
+              expectedValueProvidingTokenInputNames.push(name);
+              continue;
+            }
+            break;
+        }
+
+        if (tokenShape === 'input.value') {
+          try {
+            validateInputValue(tokenValue, description.inputs[name]);
+          } catch (error) {
+            error.message = `${name}: ${error.message}`;
+            validateFailedErrors.push(error);
+          }
+        }
+      }
+
+      if (!empty(misplacedInputNames)) {
+        push(new Error(`Unexpected input names: ${misplacedInputNames.join(', ')}`));
+      }
+
+      if (!empty(missingInputNames)) {
+        push(new Error(`Required these inputs: ${missingInputNames.join(', ')}`));
+      }
+
+      const inputAppearance = name =>
+        (isInputToken(inputOptions[name])
+          ? `${getInputTokenShape(inputOptions[name])}() call`
+          : `dependency name`);
+
+      for (const name of expectedStaticDependencyInputNames) {
+        const appearance = inputAppearance(name);
+        push(new Error(`${name}: Expected dependency name, got ${appearance}`));
+      }
+
+      for (const name of expectedStaticValueInputNames) {
+        const appearance = inputAppearance(name)
+        push(new Error(`${name}: Expected input.value() call, got ${appearance}`));
+      }
+
+      for (const name of expectedValueProvidingTokenInputNames) {
+        const appearance = getInputTokenShape(inputOptions[name]);
+        push(new Error(`${name}: Expected dependency name or value-providing input() call, got ${appearance}`));
+      }
+
+      for (const name of wrongTypeInputNames) {
+        const type = typeAppearance(inputOptions[name]);
+        push(new Error(`${name}: Expected dependency name or input() call, got ${type}`));
+      }
+
+      for (const error of validateFailedErrors) {
+        push(error);
+      }
+    });
+
+    const inputMetadata = getStaticInputMetadata(inputOptions);
+
+    const expectedOutputNames =
+      (Array.isArray(description.outputs)
+        ? description.outputs
+     : typeof description.outputs === 'function'
+        ? description.outputs(inputMetadata)
+            .map(name =>
+              (name.startsWith('#')
+                ? name
+                : '#' + name))
+        : []);
+
+    const ownUpdateDescription =
+      (typeof description.update === 'object'
+        ? description.update
+     : typeof description.update === 'function'
+        ? description.update(inputMetadata)
+        : null);
+
+    const outputOptions = {};
+
+    const instantiatedTemplate = {
+      symbol: templateCompositeFrom.symbol,
+
+      outputs(providedOptions) {
+        withAggregate({message: `Errors in output options passed to ${compositionName}`}, ({push}) => {
+          const misplacedOutputNames = [];
+          const wrongTypeOutputNames = [];
+
+          for (const [name, value] of Object.entries(providedOptions)) {
+            if (!expectedOutputNames.includes(name)) {
+              misplacedOutputNames.push(name);
+              continue;
+            }
+
+            if (typeof value !== 'string') {
+              wrongTypeOutputNames.push(name);
+              continue;
+            }
+          }
+
+          if (!empty(misplacedOutputNames)) {
+            push(new Error(`Unexpected output names: ${misplacedOutputNames.join(', ')}`));
+          }
+
+          for (const name of wrongTypeOutputNames) {
+            const appearance = typeAppearance(providedOptions[name]);
+            push(new Error(`${name}: Expected string, got ${appearance}`));
+          }
+        });
+
+        Object.assign(outputOptions, providedOptions);
+        return instantiatedTemplate;
+      },
+
+      toDescription() {
+        const finalDescription = {};
+
+        if ('annotation' in description) {
+          finalDescription.annotation = description.annotation;
+        }
+
+        if ('compose' in description) {
+          finalDescription.compose = description.compose;
+        }
+
+        if (ownUpdateDescription) {
+          finalDescription.update = ownUpdateDescription;
+        }
+
+        if ('inputs' in description) {
+          const inputMapping = {};
+
+          for (const [name, token] of Object.entries(description.inputs)) {
+            const tokenValue = getInputTokenValue(token);
+            if (name in inputOptions) {
+              if (typeof inputOptions[name] === 'string') {
+                inputMapping[name] = input.dependency(inputOptions[name]);
+              } else {
+                inputMapping[name] = inputOptions[name];
+              }
+            } else if (tokenValue.defaultValue) {
+              inputMapping[name] = input.value(tokenValue.defaultValue);
+            } else if (tokenValue.defaultDependency) {
+              inputMapping[name] = input.dependency(tokenValue.defaultDependency);
+            } else {
+              inputMapping[name] = input.value(null);
+            }
+          }
+
+          finalDescription.inputMapping = inputMapping;
+          finalDescription.inputDescriptions = description.inputs;
+        }
+
+        if ('outputs' in description) {
+          const finalOutputs = {};
+
+          for (const name of expectedOutputNames) {
+            if (name in outputOptions) {
+              finalOutputs[name] = outputOptions[name];
+            } else {
+              finalOutputs[name] = name;
+            }
+          }
+
+          finalDescription.outputs = finalOutputs;
+        }
+
+        if ('steps' in description) {
+          finalDescription.steps = description.steps;
+        }
+
+        return finalDescription;
+      },
+
+      toResolvedComposition() {
+        const ownDescription = instantiatedTemplate.toDescription();
+
+        const finalDescription = {...ownDescription};
+
+        const aggregate = openAggregate({message: `Errors resolving ${compositionName}`});
+
+        const steps = ownDescription.steps();
+
+        const resolvedSteps =
+          aggregate.map(
+            steps,
+            decorateErrorWithIndex(step =>
+              (step.symbol === templateCompositeFrom.symbol
+                ? compositeFrom(step.toResolvedComposition())
+                : step)),
+            {message: `Errors resolving steps`});
+
+        aggregate.close();
+
+        finalDescription.steps = resolvedSteps;
+
+        return finalDescription;
+      },
+    };
+
+    return instantiatedTemplate;
+  };
+
+  instantiate.inputs = instantiate;
+
+  return instantiate;
+}
+
+templateCompositeFrom.symbol = Symbol();
+
+export const continuationSymbol = Symbol.for('compositeFrom: continuation symbol');
+export const noTransformSymbol = Symbol.for('compositeFrom: no-transform symbol');
+
+export function compositeFrom(description) {
+  const {annotation} = description;
+  const compositionName = getCompositionName(description);
+
+  const debug = fn => {
+    if (compositeFrom.debug === true) {
+      const label =
+        (annotation
+          ? colors.dim(`[composite: ${annotation}]`)
+          : colors.dim(`[composite]`));
+      const result = fn();
+      if (Array.isArray(result)) {
+        console.log(label, ...result.map(value =>
+          (typeof value === 'object'
+            ? inspect(value, {depth: 1, colors: true, compact: true, breakLength: Infinity})
+            : value)));
+      } else {
+        console.log(label, result);
+      }
+    }
+  };
+
+  if (!Array.isArray(description.steps)) {
+    throw new TypeError(
+      `Expected steps to be array, got ${typeAppearance(description.steps)}` +
+      (annotation ? ` (${annotation})` : ''));
+  }
+
+  const composition =
+    description.steps.map(step =>
+      ('toResolvedComposition' in step
+        ? compositeFrom(step.toResolvedComposition())
+        : step));
+
+  const inputMetadata = getStaticInputMetadata(description.inputMapping ?? {});
+
+  function _mapDependenciesToOutputs(providedDependencies) {
+    if (!description.outputs) {
+      return {};
+    }
+
+    if (!providedDependencies) {
+      return {};
+    }
+
+    return (
+      Object.fromEntries(
+        Object.entries(description.outputs)
+          .map(([continuationName, outputName]) => [
+            outputName,
+            (continuationName in providedDependencies
+              ? providedDependencies[continuationName]
+              : providedDependencies[continuationName.replace(/^#/, '')]),
+          ])));
+  }
+
+  // These dependencies were all provided by the composition which this one is
+  // nested inside, so input('name')-shaped tokens are going to be evaluated
+  // in the context of the containing composition.
+  const dependenciesFromInputs =
+    Object.values(description.inputMapping ?? {})
+      .map(token => {
+        const tokenShape = getInputTokenShape(token);
+        const tokenValue = getInputTokenValue(token);
+        switch (tokenShape) {
+          case 'input.dependency':
+            return tokenValue;
+          case 'input':
+          case 'input.updateValue':
+            return token;
+          case 'input.myself':
+            return 'this';
+          default:
+            return null;
+        }
+      })
+      .filter(Boolean);
+
+  const anyInputsUseUpdateValue =
+    dependenciesFromInputs
+      .filter(dependency => isInputToken(dependency))
+      .some(token => getInputTokenShape(token) === 'input.updateValue');
+
+  const inputNames =
+    Object.keys(description.inputMapping ?? {});
+
+  const inputSymbols =
+    inputNames.map(name => input(name));
+
+  const inputsMayBeDynamicValue =
+    stitchArrays({
+      mappingToken: Object.values(description.inputMapping ?? {}),
+      descriptionToken: Object.values(description.inputDescriptions ?? {}),
+    }).map(({mappingToken, descriptionToken}) => {
+        if (getInputTokenShape(descriptionToken) === 'input.staticValue') return false;
+        if (getInputTokenShape(mappingToken) === 'input.value') return false;
+        return true;
+      });
+
+  const inputDescriptions =
+    Object.values(description.inputDescriptions ?? {});
+
+  /*
+  const inputsAcceptNull =
+    Object.values(description.inputDescriptions ?? {})
+      .map(token => {
+        const tokenValue = getInputTokenValue(token);
+        if (!tokenValue) return false;
+        if ('acceptsNull' in tokenValue) return tokenValue.acceptsNull;
+        if ('defaultValue' in tokenValue) return tokenValue.defaultValue === null;
+        return false;
+      });
+  */
+
+  // Update descriptions passed as the value in an input.updateValue() token,
+  // as provided as inputs for this composition.
+  const inputUpdateDescriptions =
+    Object.values(description.inputMapping ?? {})
+      .map(token =>
+        (getInputTokenShape(token) === 'input.updateValue'
+          ? getInputTokenValue(token)
+          : null))
+      .filter(Boolean);
+
+  const base = composition.at(-1);
+  const steps = composition.slice();
+
+  const aggregate = openAggregate({
+    message:
+      `Errors preparing composition` +
+      (annotation ? ` (${annotation})` : ''),
+  });
+
+  const compositionNests = description.compose ?? true;
+
+  if (compositionNests && empty(steps)) {
+    aggregate.push(new TypeError(`Expected at least one step`));
+  }
+
+  // Steps default to exposing if using a shorthand syntax where flags aren't
+  // specified at all.
+  const stepsExpose =
+    steps
+      .map(step =>
+        (step.flags
+          ? step.flags.expose ?? false
+          : true));
+
+  // Steps default to composing if using a shorthand syntax where flags aren't
+  // specified at all - *and* aren't the base (final step), unless the whole
+  // composition is nestable.
+  const stepsCompose =
+    steps
+      .map((step, index, {length}) =>
+        (step.flags
+          ? step.flags.compose ?? false
+          : (index === length - 1
+              ? compositionNests
+              : true)));
+
+  // Steps update if the corresponding flag is explicitly set, if a transform
+  // function is provided, or if the dependencies include an input.updateValue
+  // token.
+  const stepsUpdate =
+    steps
+      .map(step =>
+        (step.flags
+          ? step.flags.update ?? false
+          : !!step.transform ||
+            !!step.dependencies?.some(dependency =>
+                isInputToken(dependency) &&
+                getInputTokenShape(dependency) === 'input.updateValue')));
+
+  // The expose description for a step is just the entire step object, when
+  // using the shorthand syntax where {flags: {expose: true}} is left implied.
+  const stepExposeDescriptions =
+    steps
+      .map((step, index) =>
+        (stepsExpose[index]
+          ? (step.flags
+              ? step.expose ?? null
+              : step)
+          : null));
+
+  // The update description for a step, if present at all, is always set
+  // explicitly. There may be multiple per step - namely that step's own
+  // {update} description, and any descriptions passed as the value in an
+  // input.updateValue({...}) token.
+  const stepUpdateDescriptions =
+    steps
+      .map((step, index) =>
+        (stepsUpdate[index]
+          ? [
+              step.update ?? null,
+              ...(stepExposeDescriptions[index]?.dependencies ?? [])
+                .filter(dependency => isInputToken(dependency))
+                .filter(token => getInputTokenShape(token) === 'input.updateValue')
+                .map(token => getInputTokenValue(token)),
+            ].filter(Boolean)
+          : []));
+
+  // Indicates presence of a {compute} function on the expose description.
+  const stepsCompute =
+    stepExposeDescriptions
+      .map(expose => !!expose?.compute);
+
+  // Indicates presence of a {transform} function on the expose description.
+  const stepsTransform =
+    stepExposeDescriptions
+      .map(expose => !!expose?.transform);
+
+  const dependenciesFromSteps =
+    unique(
+      stepExposeDescriptions
+        .flatMap(expose => expose?.dependencies ?? [])
+        .map(dependency => {
+          if (typeof dependency === 'string')
+            return (dependency.startsWith('#') ? null : dependency);
+
+          const tokenShape = getInputTokenShape(dependency);
+          const tokenValue = getInputTokenValue(dependency);
+          switch (tokenShape) {
+            case 'input.dependency':
+              return (tokenValue.startsWith('#') ? null : tokenValue);
+            case 'input.myself':
+              return 'this';
+            default:
+              return null;
+          }
+        })
+        .filter(Boolean));
+
+  const anyStepsUseUpdateValue =
+    stepExposeDescriptions
+      .some(expose =>
+        (expose?.dependencies
+          ? expose.dependencies.includes(input.updateValue())
+          : false));
+
+  const anyStepsExpose =
+    stepsExpose.includes(true);
+
+  const anyStepsUpdate =
+    stepsUpdate.includes(true);
+
+  const anyStepsCompute =
+    stepsCompute.includes(true);
+
+  const compositionExposes =
+    anyStepsExpose;
+
+  const compositionUpdates =
+    'update' in description ||
+    anyInputsUseUpdateValue ||
+    anyStepsUseUpdateValue ||
+    anyStepsUpdate;
+
+  const stepEntries = stitchArrays({
+    step: steps,
+    stepComposes: stepsCompose,
+    stepComputes: stepsCompute,
+    stepTransforms: stepsTransform,
+  });
+
+  for (let i = 0; i < stepEntries.length; i++) {
+    const {
+      step,
+      stepComposes,
+      stepComputes,
+      stepTransforms,
+    } = stepEntries[i];
+
+    const isBase = i === stepEntries.length - 1;
+    const message =
+      `Errors in step #${i + 1}` +
+      (isBase ? ` (base)` : ``) +
+      (step.annotation ? ` (${step.annotation})` : ``);
+
+    aggregate.nest({message}, ({push}) => {
+      if (isBase && stepComposes !== compositionNests) {
+        return push(new TypeError(
+          (compositionNests
+            ? `Base must compose, this composition is nestable`
+            : `Base must not compose, this composition isn't nestable`)));
+      } else if (!isBase && !stepComposes) {
+        return push(new TypeError(
+          (compositionNests
+            ? `All steps must compose`
+            : `All steps (except base) must compose`)));
+      }
+
+      if (
+        !compositionNests && !compositionUpdates &&
+        stepTransforms && !stepComputes
+      ) {
+        return push(new TypeError(
+          `Steps which only transform can't be used in a composition that doesn't update`));
+      }
+    });
+  }
+
+  if (!compositionNests && !compositionUpdates && !anyStepsCompute) {
+    aggregate.push(new TypeError(`Expected at least one step to compute`));
+  }
+
+  aggregate.close();
+
+  function _prepareContinuation(callingTransformForThisStep) {
+    const continuationStorage = {
+      returnedWith: null,
+      providedDependencies: undefined,
+      providedValue: undefined,
+    };
+
+    const continuation =
+      (callingTransformForThisStep
+        ? (providedValue, providedDependencies = null) => {
+            continuationStorage.returnedWith = 'continuation';
+            continuationStorage.providedDependencies = providedDependencies;
+            continuationStorage.providedValue = providedValue;
+            return continuationSymbol;
+          }
+        : (providedDependencies = null) => {
+            continuationStorage.returnedWith = 'continuation';
+            continuationStorage.providedDependencies = providedDependencies;
+            return continuationSymbol;
+          });
+
+    continuation.exit = (providedValue) => {
+      continuationStorage.returnedWith = 'exit';
+      continuationStorage.providedValue = providedValue;
+      return continuationSymbol;
+    };
+
+    if (compositionNests) {
+      const makeRaiseLike = returnWith =>
+        (callingTransformForThisStep
+          ? (providedValue, providedDependencies = null) => {
+              continuationStorage.returnedWith = returnWith;
+              continuationStorage.providedDependencies = providedDependencies;
+              continuationStorage.providedValue = providedValue;
+              return continuationSymbol;
+            }
+          : (providedDependencies = null) => {
+              continuationStorage.returnedWith = returnWith;
+              continuationStorage.providedDependencies = providedDependencies;
+              return continuationSymbol;
+            });
+
+      continuation.raiseOutput = makeRaiseLike('raiseOutput');
+      continuation.raiseOutputAbove = makeRaiseLike('raiseOutputAbove');
+    }
+
+    return {continuation, continuationStorage};
+  }
+
+  function _computeOrTransform(initialValue, continuationIfApplicable, initialDependencies) {
+    const expectingTransform = initialValue !== noTransformSymbol;
+
+    let valueSoFar =
+      (expectingTransform
+        ? initialValue
+        : undefined);
+
+    const availableDependencies = {...initialDependencies};
+
+    const inputValues =
+      Object.values(description.inputMapping ?? {})
+        .map(token => {
+          const tokenShape = getInputTokenShape(token);
+          const tokenValue = getInputTokenValue(token);
+          switch (tokenShape) {
+            case 'input.dependency':
+              return initialDependencies[tokenValue];
+            case 'input.value':
+              return tokenValue;
+            case 'input.updateValue':
+              if (!expectingTransform)
+                throw new Error(`Unexpected input.updateValue() accessed on non-transform call`);
+              return valueSoFar;
+            case 'input.myself':
+              return initialDependencies['this'];
+            case 'input':
+              return initialDependencies[token];
+            default:
+              throw new TypeError(`Unexpected input shape ${tokenShape}`);
+          }
+        });
+
+    withAggregate({message: `Errors in input values provided to ${compositionName}`}, ({push}) => {
+      for (const {dynamic, name, value, description} of stitchArrays({
+        dynamic: inputsMayBeDynamicValue,
+        name: inputNames,
+        value: inputValues,
+        description: inputDescriptions,
+      })) {
+        if (!dynamic) continue;
+        try {
+          validateInputValue(value, description);
+        } catch (error) {
+          error.message = `${name}: ${error.message}`;
+          push(error);
+        }
+      }
+    });
+
+    if (expectingTransform) {
+      debug(() => [colors.bright(`begin composition - transforming from:`), initialValue]);
+    } else {
+      debug(() => colors.bright(`begin composition - not transforming`));
+    }
+
+    for (let i = 0; i < steps.length; i++) {
+      const step = steps[i];
+      const isBase = i === steps.length - 1;
+
+      debug(() => [
+        `step #${i+1}` +
+        (isBase
+          ? ` (base):`
+          : ` of ${steps.length}:`),
+        step]);
+
+      const expose =
+        (step.flags
+          ? step.expose
+          : step);
+
+      if (!expose) {
+        if (!isBase) {
+          debug(() => `step #${i+1} - no expose description, nothing to do for this step`);
+          continue;
+        }
+
+        if (expectingTransform) {
+          debug(() => `step #${i+1} (base) - no expose description, returning so-far update value:`, valueSoFar);
+          if (continuationIfApplicable) {
+            debug(() => colors.bright(`end composition - raise (inferred - composing)`));
+            return continuationIfApplicable(valueSoFar);
+          } else {
+            debug(() => colors.bright(`end composition - exit (inferred - not composing)`));
+            return valueSoFar;
+          }
+        } else {
+          debug(() => `step #${i+1} (base) - no expose description, nothing to continue with`);
+          if (continuationIfApplicable) {
+            debug(() => colors.bright(`end composition - raise (inferred - composing)`));
+            return continuationIfApplicable();
+          } else {
+            debug(() => colors.bright(`end composition - exit (inferred - not composing)`));
+            return null;
+          }
+        }
+      }
+
+      const callingTransformForThisStep =
+        expectingTransform && expose.transform;
+
+      let continuationStorage;
+
+      const inputDictionary =
+        Object.fromEntries(
+          stitchArrays({symbol: inputSymbols, value: inputValues})
+            .map(({symbol, value}) => [symbol, value]));
+
+      const filterableDependencies = {
+        ...availableDependencies,
+        ...inputMetadata,
+        ...inputDictionary,
+        ...
+          (expectingTransform
+            ? {[input.updateValue()]: valueSoFar}
+            : {}),
+        [input.myself()]: initialDependencies?.['this'] ?? null,
+      };
+
+      const selectDependencies =
+        (expose.dependencies ?? []).map(dependency => {
+          if (!isInputToken(dependency)) return dependency;
+          const tokenShape = getInputTokenShape(dependency);
+          const tokenValue = getInputTokenValue(dependency);
+          switch (tokenShape) {
+            case 'input':
+            case 'input.staticDependency':
+            case 'input.staticValue':
+              return dependency;
+            case 'input.myself':
+              return input.myself();
+            case 'input.dependency':
+              return tokenValue;
+            case 'input.updateValue':
+              return input.updateValue();
+            default:
+              throw new Error(`Unexpected token ${tokenShape} as dependency`);
+          }
+        })
+
+      const filteredDependencies =
+        filterProperties(filterableDependencies, selectDependencies);
+
+      debug(() => [
+        `step #${i+1} - ${callingTransformForThisStep ? 'transform' : 'compute'}`,
+        `with dependencies:`, filteredDependencies,
+        `selecting:`, selectDependencies,
+        `from available:`, filterableDependencies,
+        ...callingTransformForThisStep ? [`from value:`, valueSoFar] : []]);
+
+      let result;
+
+      const getExpectedEvaluation = () =>
+        (callingTransformForThisStep
+          ? (filteredDependencies
+              ? ['transform', valueSoFar, continuationSymbol, filteredDependencies]
+              : ['transform', valueSoFar, continuationSymbol])
+          : (filteredDependencies
+              ? ['compute', continuationSymbol, filteredDependencies]
+              : ['compute', continuationSymbol]));
+
+      const naturalEvaluate = () => {
+        const [name, ...argsLayout] = getExpectedEvaluation();
+
+        let args;
+
+        if (isBase && !compositionNests) {
+          args =
+            argsLayout.filter(arg => arg !== continuationSymbol);
+        } else {
+          let continuation;
+
+          ({continuation, continuationStorage} =
+            _prepareContinuation(callingTransformForThisStep));
+
+          args =
+            argsLayout.map(arg =>
+              (arg === continuationSymbol
+                ? continuation
+                : arg));
+        }
+
+        return expose[name](...args);
+      }
+
+      switch (step.cache) {
+        // Warning! Highly WIP!
+        case 'aggressive': {
+          const hrnow = () => {
+            const hrTime = process.hrtime();
+            return hrTime[0] * 1000000000 + hrTime[1];
+          };
+
+          const [name, ...args] = getExpectedEvaluation();
+
+          let cache = globalCompositeCache[step.annotation];
+          if (!cache) {
+            cache = globalCompositeCache[step.annotation] = {
+              transform: new TupleMap(),
+              compute: new TupleMap(),
+              times: {
+                read: [],
+                evaluate: [],
+              },
+            };
+          }
+
+          const tuplefied = args
+            .flatMap(arg => [
+              Symbol.for('compositeFrom: tuplefied arg divider'),
+              ...(typeof arg !== 'object' || Array.isArray(arg)
+                ? [arg]
+                : Object.entries(arg).flat()),
+            ]);
+
+          const readTime = hrnow();
+          const cacheContents = cache[name].get(tuplefied);
+          cache.times.read.push(hrnow() - readTime);
+
+          if (cacheContents) {
+            ({result, continuationStorage} = cacheContents);
+          } else {
+            const evaluateTime = hrnow();
+            result = naturalEvaluate();
+            cache.times.evaluate.push(hrnow() - evaluateTime);
+            cache[name].set(tuplefied, {result, continuationStorage});
+          }
+
+          break;
+        }
+
+        default: {
+          result = naturalEvaluate();
+          break;
+        }
+      }
+
+      if (result !== continuationSymbol) {
+        debug(() => [`step #${i+1} - result: exit (inferred) ->`, result]);
+
+        if (compositionNests) {
+          throw new TypeError(`Inferred early-exit is disallowed in nested compositions`);
+        }
+
+        debug(() => colors.bright(`end composition - exit (inferred)`));
+
+        return result;
+      }
+
+      const {returnedWith} = continuationStorage;
+
+      if (returnedWith === 'exit') {
+        const {providedValue} = continuationStorage;
+
+        debug(() => [`step #${i+1} - result: exit (explicit) ->`, providedValue]);
+        debug(() => colors.bright(`end composition - exit (explicit)`));
+
+        if (compositionNests) {
+          return continuationIfApplicable.exit(providedValue);
+        } else {
+          return providedValue;
+        }
+      }
+
+      const {providedValue, providedDependencies} = continuationStorage;
+
+      const continuationArgs = [];
+      if (expectingTransform) {
+        continuationArgs.push(
+          (callingTransformForThisStep
+            ? providedValue ?? null
+            : valueSoFar ?? null));
+      }
+
+      debug(() => {
+        const base = `step #${i+1} - result: ` + returnedWith;
+        const parts = [];
+
+        if (callingTransformForThisStep) {
+          parts.push('value:', providedValue);
+        }
+
+        if (providedDependencies !== null) {
+          parts.push(`deps:`, providedDependencies);
+        } else {
+          parts.push(`(no deps)`);
+        }
+
+        if (empty(parts)) {
+          return base;
+        } else {
+          return [base + ' ->', ...parts];
+        }
+      });
+
+      switch (returnedWith) {
+        case 'raiseOutput':
+          debug(() =>
+            (isBase
+              ? colors.bright(`end composition - raiseOutput (base: explicit)`)
+              : colors.bright(`end composition - raiseOutput`)));
+          continuationArgs.push(_mapDependenciesToOutputs(providedDependencies));
+          return continuationIfApplicable(...continuationArgs);
+
+        case 'raiseOutputAbove':
+          debug(() => colors.bright(`end composition - raiseOutputAbove`));
+          continuationArgs.push(_mapDependenciesToOutputs(providedDependencies));
+          return continuationIfApplicable.raiseOutput(...continuationArgs);
+
+        case 'continuation':
+          if (isBase) {
+            debug(() => colors.bright(`end composition - raiseOutput (inferred)`));
+            continuationArgs.push(_mapDependenciesToOutputs(providedDependencies));
+            return continuationIfApplicable(...continuationArgs);
+          } else {
+            Object.assign(availableDependencies, providedDependencies);
+            if (callingTransformForThisStep && providedValue !== null) {
+              valueSoFar = providedValue;
+            }
+            break;
+          }
+      }
+    }
+  }
+
+  const constructedDescriptor = {};
+
+  if (annotation) {
+    constructedDescriptor.annotation = annotation;
+  }
+
+  constructedDescriptor.flags = {
+    update: compositionUpdates,
+    expose: compositionExposes,
+    compose: compositionNests,
+  };
+
+  if (compositionUpdates) {
+    // TODO: This is a dumb assign statement, and it could probably do more
+    // interesting things, like combining validation functions.
+    constructedDescriptor.update =
+      Object.assign(
+        {...description.update ?? {}},
+        ...inputUpdateDescriptions,
+        ...stepUpdateDescriptions.flat());
+  }
+
+  if (compositionExposes) {
+    const expose = constructedDescriptor.expose = {};
+
+    expose.dependencies =
+      unique([
+        ...dependenciesFromInputs,
+        ...dependenciesFromSteps,
+      ]);
+
+    const _wrapper = (...args) => {
+      try {
+        return _computeOrTransform(...args);
+      } catch (thrownError) {
+        const error = new Error(
+          `Error computing composition` +
+          (annotation ? ` ${annotation}` : ''));
+        error.cause = thrownError;
+        throw error;
+      }
+    };
+
+    if (compositionNests) {
+      if (compositionUpdates) {
+        expose.transform = (value, continuation, dependencies) =>
+          _wrapper(value, continuation, dependencies);
+      }
+
+      if (anyStepsCompute && !anyStepsUseUpdateValue && !anyInputsUseUpdateValue) {
+        expose.compute = (continuation, dependencies) =>
+          _wrapper(noTransformSymbol, continuation, dependencies);
+      }
+
+      if (base.cacheComposition) {
+        expose.cache = base.cacheComposition;
+      }
+    } else if (compositionUpdates) {
+      if (!empty(steps)) {
+        expose.transform = (value, dependencies) =>
+          _wrapper(value, null, dependencies);
+      }
+    } else {
+      expose.compute = (dependencies) =>
+        _wrapper(noTransformSymbol, null, dependencies);
+    }
+  }
+
+  return constructedDescriptor;
+}
+
+export function displayCompositeCacheAnalysis() {
+  const showTimes = (cache, key) => {
+    const times = cache.times[key].slice().sort();
+
+    const all = times;
+    const worst10pc = times.slice(-times.length / 10);
+    const best10pc = times.slice(0, times.length / 10);
+    const middle50pc = times.slice(times.length / 4, -times.length / 4);
+    const middle80pc = times.slice(times.length / 10, -times.length / 10);
+
+    const fmt = val => `${(val / 1000).toFixed(2)}ms`.padStart(9);
+    const avg = times => times.reduce((a, b) => a + b, 0) / times.length;
+
+    const left = ` - ${key}: `;
+    const indn = ' '.repeat(left.length);
+    console.log(left + `${fmt(avg(all))} (all ${all.length})`);
+    console.log(indn + `${fmt(avg(worst10pc))} (worst 10%)`);
+    console.log(indn + `${fmt(avg(best10pc))} (best 10%)`);
+    console.log(indn + `${fmt(avg(middle80pc))} (middle 80%)`);
+    console.log(indn + `${fmt(avg(middle50pc))} (middle 50%)`);
+  };
+
+  for (const [annotation, cache] of Object.entries(globalCompositeCache)) {
+    console.log(`Cached ${annotation}:`);
+    showTimes(cache, 'evaluate');
+    showTimes(cache, 'read');
+  }
+}
+
+// Evaluates a function with composite debugging enabled, turns debugging
+// off again, and returns the result of the function. This is mostly syntax
+// sugar, but also helps avoid unit tests avoid accidentally printing debug
+// info for a bunch of unrelated composites (due to property enumeration
+// when displaying an unexpected result). Use as so:
+//
+//   Without debugging:
+//     t.same(thing.someProp, value)
+//
+//   With debugging:
+//     t.same(debugComposite(() => thing.someProp), value)
+//
+export function debugComposite(fn) {
+  compositeFrom.debug = true;
+  const value = fn();
+  compositeFrom.debug = false;
+  return value;
+}
diff --git a/src/data/composite/control-flow/exitWithoutDependency.js b/src/data/composite/control-flow/exitWithoutDependency.js
new file mode 100644
index 0000000..c660a7e
--- /dev/null
+++ b/src/data/composite/control-flow/exitWithoutDependency.js
@@ -0,0 +1,35 @@
+// Early exits if a dependency isn't available.
+// See withResultOfAvailabilityCheck for {mode} options.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+import withResultOfAvailabilityCheck from './withResultOfAvailabilityCheck.js';
+
+export default templateCompositeFrom({
+  annotation: `exitWithoutDependency`,
+
+  inputs: {
+    dependency: input({acceptsNull: true}),
+    mode: inputAvailabilityCheckMode(),
+    value: input({defaultValue: null}),
+  },
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('dependency'),
+      mode: input('mode'),
+    }),
+
+    {
+      dependencies: ['#availability', input('value')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('value')]: value,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.exit(value)),
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/exitWithoutUpdateValue.js b/src/data/composite/control-flow/exitWithoutUpdateValue.js
new file mode 100644
index 0000000..244b323
--- /dev/null
+++ b/src/data/composite/control-flow/exitWithoutUpdateValue.js
@@ -0,0 +1,24 @@
+// Early exits if this property's update value isn't available.
+// See withResultOfAvailabilityCheck for {mode} options.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import exitWithoutDependency from './exitWithoutDependency.js';
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+
+export default templateCompositeFrom({
+  annotation: `exitWithoutUpdateValue`,
+
+  inputs: {
+    mode: inputAvailabilityCheckMode(),
+    value: input({defaultValue: null}),
+  },
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input.updateValue(),
+      mode: input('mode'),
+      value: input('value'),
+    }),
+  ],
+});
diff --git a/src/data/composite/control-flow/exposeConstant.js b/src/data/composite/control-flow/exposeConstant.js
new file mode 100644
index 0000000..e76699c
--- /dev/null
+++ b/src/data/composite/control-flow/exposeConstant.js
@@ -0,0 +1,26 @@
+// Exposes a constant value exactly as it is; like exposeDependency, this
+// is typically the base of a composition serving as a particular property
+// descriptor. It generally follows steps which will conditionally early
+// exit with some other value, with the exposeConstant base serving as the
+// fallback default value.
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `exposeConstant`,
+
+  compose: false,
+
+  inputs: {
+    value: input.staticValue({acceptsNull: true}),
+  },
+
+  steps: () => [
+    {
+      dependencies: [input('value')],
+      compute: ({
+        [input('value')]: value,
+      }) => value,
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/exposeDependency.js b/src/data/composite/control-flow/exposeDependency.js
new file mode 100644
index 0000000..3aa3d03
--- /dev/null
+++ b/src/data/composite/control-flow/exposeDependency.js
@@ -0,0 +1,28 @@
+// Exposes a dependency exactly as it is; this is typically the base of a
+// composition which was created to serve as one property's descriptor.
+//
+// Please note that this *doesn't* verify that the dependency exists, so
+// if you provide the wrong name or it hasn't been set by a previous
+// compositional step, the property will be exposed as undefined instead
+// of null.
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `exposeDependency`,
+
+  compose: false,
+
+  inputs: {
+    dependency: input.staticDependency({acceptsNull: true}),
+  },
+
+  steps: () => [
+    {
+      dependencies: [input('dependency')],
+      compute: ({
+        [input('dependency')]: dependency
+      }) => dependency,
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/exposeDependencyOrContinue.js b/src/data/composite/control-flow/exposeDependencyOrContinue.js
new file mode 100644
index 0000000..0f7f223
--- /dev/null
+++ b/src/data/composite/control-flow/exposeDependencyOrContinue.js
@@ -0,0 +1,34 @@
+// Exposes a dependency as it is, or continues if it's unavailable.
+// See withResultOfAvailabilityCheck for {mode} options.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+import withResultOfAvailabilityCheck from './withResultOfAvailabilityCheck.js';
+
+export default templateCompositeFrom({
+  annotation: `exposeDependencyOrContinue`,
+
+  inputs: {
+    dependency: input({acceptsNull: true}),
+    mode: inputAvailabilityCheckMode(),
+  },
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('dependency'),
+      mode: input('mode'),
+    }),
+
+    {
+      dependencies: ['#availability', input('dependency')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('dependency')]: dependency,
+      }) =>
+        (availability
+          ? continuation.exit(dependency)
+          : continuation()),
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/exposeUpdateValueOrContinue.js b/src/data/composite/control-flow/exposeUpdateValueOrContinue.js
new file mode 100644
index 0000000..1f94b33
--- /dev/null
+++ b/src/data/composite/control-flow/exposeUpdateValueOrContinue.js
@@ -0,0 +1,40 @@
+// Exposes the update value of an {update: true} property as it is,
+// or continues if it's unavailable.
+//
+// See withResultOfAvailabilityCheck for {mode} options.
+//
+// Provide {validate} here to conveniently set a custom validation check
+// for this property's update value.
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+import exposeDependencyOrContinue from './exposeDependencyOrContinue.js';
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+
+export default templateCompositeFrom({
+  annotation: `exposeUpdateValueOrContinue`,
+
+  inputs: {
+    mode: inputAvailabilityCheckMode(),
+
+    validate: input({
+      type: 'function',
+      defaultValue: null,
+    }),
+  },
+
+  update: ({
+    [input.staticValue('validate')]: validate,
+  }) =>
+    (validate
+      ? {validate}
+      : {}),
+
+  steps: () => [
+    exposeDependencyOrContinue({
+      dependency: input.updateValue(),
+      mode: input('mode'),
+    }),
+  ],
+});
diff --git a/src/data/composite/control-flow/index.js b/src/data/composite/control-flow/index.js
new file mode 100644
index 0000000..7fad88b
--- /dev/null
+++ b/src/data/composite/control-flow/index.js
@@ -0,0 +1,14 @@
+// #composite/control-flow
+//
+// No entries depend on any other entries, except siblings in this directory.
+//
+
+export {default as exitWithoutDependency} from './exitWithoutDependency.js';
+export {default as exitWithoutUpdateValue} from './exitWithoutUpdateValue.js';
+export {default as exposeConstant} from './exposeConstant.js';
+export {default as exposeDependency} from './exposeDependency.js';
+export {default as exposeDependencyOrContinue} from './exposeDependencyOrContinue.js';
+export {default as exposeUpdateValueOrContinue} from './exposeUpdateValueOrContinue.js';
+export {default as raiseOutputWithoutDependency} from './raiseOutputWithoutDependency.js';
+export {default as raiseOutputWithoutUpdateValue} from './raiseOutputWithoutUpdateValue.js';
+export {default as withResultOfAvailabilityCheck} from './withResultOfAvailabilityCheck.js';
diff --git a/src/data/composite/control-flow/inputAvailabilityCheckMode.js b/src/data/composite/control-flow/inputAvailabilityCheckMode.js
new file mode 100644
index 0000000..8008fde
--- /dev/null
+++ b/src/data/composite/control-flow/inputAvailabilityCheckMode.js
@@ -0,0 +1,9 @@
+import {input} from '#composite';
+import {is} from '#validators';
+
+export default function inputAvailabilityCheckMode() {
+  return input({
+    validate: is('null', 'empty', 'falsy', 'index'),
+    defaultValue: 'null',
+  });
+}
diff --git a/src/data/composite/control-flow/raiseOutputWithoutDependency.js b/src/data/composite/control-flow/raiseOutputWithoutDependency.js
new file mode 100644
index 0000000..3d04f8a
--- /dev/null
+++ b/src/data/composite/control-flow/raiseOutputWithoutDependency.js
@@ -0,0 +1,39 @@
+// Raises if a dependency isn't available.
+// See withResultOfAvailabilityCheck for {mode} options.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+import withResultOfAvailabilityCheck from './withResultOfAvailabilityCheck.js';
+
+export default templateCompositeFrom({
+  annotation: `raiseOutputWithoutDependency`,
+
+  inputs: {
+    dependency: input({acceptsNull: true}),
+    mode: inputAvailabilityCheckMode(),
+    output: input.staticValue({defaultValue: {}}),
+  },
+
+  outputs: ({
+    [input.staticValue('output')]: output,
+  }) => Object.keys(output ?? {}),
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('dependency'),
+      mode: input('mode'),
+    }),
+
+    {
+      dependencies: ['#availability', input('output')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('output')]: output,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.raiseOutputAbove(output)),
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js b/src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js
new file mode 100644
index 0000000..ffa83a9
--- /dev/null
+++ b/src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js
@@ -0,0 +1,47 @@
+// Raises if this property's update value isn't available.
+// See withResultOfAvailabilityCheck for {mode} options!
+
+import {input, templateCompositeFrom} from '#composite';
+
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+import withResultOfAvailabilityCheck from './withResultOfAvailabilityCheck.js';
+
+export default templateCompositeFrom({
+  annotation: `raiseOutputWithoutUpdateValue`,
+
+  inputs: {
+    mode: inputAvailabilityCheckMode(),
+    output: input.staticValue({defaultValue: {}}),
+  },
+
+  outputs: ({
+    [input.staticValue('output')]: output,
+  }) => Object.keys(output ?? {}),
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input.updateValue(),
+      mode: input('mode'),
+    }),
+
+    // TODO: A bit of a kludge, below. Other "do something with the update
+    // value" type functions can get by pretty much just passing that value
+    // as an input (input.updateValue()) into the corresponding "do something
+    // with a dependency/arbitrary value" function. But we can't do that here,
+    // because the special behavior, raiseOutputAbove(), only works to raise
+    // output above the composition it's *directly* nested in. Other languages
+    // have a throw/catch system that might serve as inspiration for something
+    // better here.
+
+    {
+      dependencies: ['#availability', input('output')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('output')]: output,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.raiseOutputAbove(output)),
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/withResultOfAvailabilityCheck.js b/src/data/composite/control-flow/withResultOfAvailabilityCheck.js
new file mode 100644
index 0000000..a694201
--- /dev/null
+++ b/src/data/composite/control-flow/withResultOfAvailabilityCheck.js
@@ -0,0 +1,71 @@
+// Checks the availability of a dependency and provides the result to later
+// steps under '#availability' (by default). This is mainly intended for use
+// by the more specific utilities, which you should consider using instead.
+//
+// Customize {mode} to select one of these modes, or default to 'null':
+//
+// * 'null':  Check that the value isn't null (and not undefined either).
+// * 'empty': Check that the value is neither null, undefined, nor an empty
+//            array.
+// * 'falsy': Check that the value isn't false when treated as a boolean
+//            (nor an empty array). Keep in mind this will also be false
+//            for values like zero and the empty string!
+// * 'index': Check that the value is a number, and is at least zero.
+//
+// See also:
+//  - exitWithoutDependency
+//  - exitWithoutUpdateValue
+//  - exposeDependencyOrContinue
+//  - exposeUpdateValueOrContinue
+//  - raiseOutputWithoutDependency
+//  - raiseOutputWithoutUpdateValue
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {empty} from '#sugar';
+
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+
+export default templateCompositeFrom({
+  annotation: `withResultOfAvailabilityCheck`,
+
+  inputs: {
+    from: input({acceptsNull: true}),
+    mode: inputAvailabilityCheckMode(),
+  },
+
+  outputs: ['#availability'],
+
+  steps: () => [
+    {
+      dependencies: [input('from'), input('mode')],
+
+      compute: (continuation, {
+        [input('from')]: value,
+        [input('mode')]: mode,
+      }) => {
+        let availability;
+
+        switch (mode) {
+          case 'null':
+            availability = value !== undefined && value !== null;
+            break;
+
+          case 'empty':
+            availability = value !== undefined && !empty(value);
+            break;
+
+          case 'falsy':
+            availability = !!value && (!Array.isArray(value) || !empty(value));
+            break;
+
+          case 'index':
+            availability = typeof value === 'number' && value >= 0;
+            break;
+        }
+
+        return continuation({'#availability': availability});
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/data/excludeFromList.js b/src/data/composite/data/excludeFromList.js
new file mode 100644
index 0000000..d798dcd
--- /dev/null
+++ b/src/data/composite/data/excludeFromList.js
@@ -0,0 +1,55 @@
+// Filters particular values out of a list. Note that this will always
+// completely skip over null, but can be used to filter out any other
+// primitive or object value.
+//
+// See also:
+//  - fillMissingListItems
+//
+// More list utilities:
+//  - withFilteredList, withMappedList, withSortedList
+//  - withFlattenedList, withUnflattenedList
+//  - withPropertyFromList, withPropertiesFromList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {empty} from '#sugar';
+
+export default templateCompositeFrom({
+  annotation: `excludeFromList`,
+
+  inputs: {
+    list: input(),
+
+    item: input({defaultValue: null}),
+    items: input({type: 'array', defaultValue: null}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+  }) => [list ?? '#list'],
+
+  steps: () => [
+    {
+      dependencies: [
+        input.staticDependency('list'),
+        input('list'),
+        input('item'),
+        input('items'),
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('list')]: listName,
+        [input('list')]: listContents,
+        [input('item')]: excludeItem,
+        [input('items')]: excludeItems,
+      }) => continuation({
+        [listName ?? '#list']:
+          listContents.filter(item => {
+            if (excludeItem !== null && item === excludeItem) return false;
+            if (!empty(excludeItems) && excludeItems.includes(item)) return false;
+            return true;
+          }),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/data/fillMissingListItems.js b/src/data/composite/data/fillMissingListItems.js
new file mode 100644
index 0000000..4f818a7
--- /dev/null
+++ b/src/data/composite/data/fillMissingListItems.js
@@ -0,0 +1,50 @@
+// Replaces items of a list, which are null or undefined, with some fallback
+// value. By default, this replaces the passed dependency.
+//
+// See also:
+//  - excludeFromList
+//
+// More list utilities:
+//  - withFilteredList, withMappedList, withSortedList
+//  - withFlattenedList, withUnflattenedList
+//  - withPropertyFromList, withPropertiesFromList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `fillMissingListItems`,
+
+  inputs: {
+    list: input({type: 'array'}),
+    fill: input({acceptsNull: true}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+  }) => [list ?? '#list'],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('fill')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('fill')]: fill,
+      }) => continuation({
+        ['#filled']:
+          list.map(item => item ?? fill),
+      }),
+    },
+
+    {
+      dependencies: [input.staticDependency('list'), '#filled'],
+      compute: (continuation, {
+        [input.staticDependency('list')]: list,
+        ['#filled']: filled,
+      }) => continuation({
+        [list ?? '#list']:
+          filled,
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/data/index.js b/src/data/composite/data/index.js
new file mode 100644
index 0000000..256c049
--- /dev/null
+++ b/src/data/composite/data/index.js
@@ -0,0 +1,17 @@
+// #composite/data
+//
+// Entries here may depend on entries in #composite/control-flow.
+//
+
+export {default as excludeFromList} from './excludeFromList.js';
+export {default as fillMissingListItems} from './fillMissingListItems.js';
+export {default as withFilteredList} from './withFilteredList.js';
+export {default as withFlattenedList} from './withFlattenedList.js';
+export {default as withMappedList} from './withMappedList.js';
+export {default as withPropertiesFromList} from './withPropertiesFromList.js';
+export {default as withPropertiesFromObject} from './withPropertiesFromObject.js';
+export {default as withPropertyFromList} from './withPropertyFromList.js';
+export {default as withPropertyFromObject} from './withPropertyFromObject.js';
+export {default as withSortedList} from './withSortedList.js';
+export {default as withUnflattenedList} from './withUnflattenedList.js';
+export {default as withUniqueItemsOnly} from './withUniqueItemsOnly.js';
diff --git a/src/data/composite/data/withFilteredList.js b/src/data/composite/data/withFilteredList.js
new file mode 100644
index 0000000..82e5690
--- /dev/null
+++ b/src/data/composite/data/withFilteredList.js
@@ -0,0 +1,50 @@
+// Applies a filter - an array of truthy and falsy values - to the index-
+// corresponding items in a list. Items which correspond to a truthy value
+// are kept, and the rest are excluded from the output list.
+//
+// TODO: It would be neat to apply an availability check here, e.g. to allow
+// not providing a filter at all and performing the check on the contents of
+// the list (though on the filter, if present, is fine too). But that's best
+// done by some shmancy-fancy mapping support in composite.js, so a bit out
+// of reach for now (apart from proving uses built on top of a more boring
+// implementation).
+//
+// TODO: There should be two outputs - one for the items included according to
+// the filter, and one for the items excluded.
+//
+// See also:
+//  - withMappedList
+//  - withSortedList
+//
+// More list utilities:
+//  - excludeFromList
+//  - fillMissingListItems
+//  - withFlattenedList, withUnflattenedList
+//  - withPropertyFromList, withPropertiesFromList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `withFilteredList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+    filter: input({type: 'array'}),
+  },
+
+  outputs: ['#filteredList'],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('filter')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('filter')]: filter,
+      }) => continuation({
+        '#filteredList':
+          list.filter((item, index) => filter[index]),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withFlattenedList.js b/src/data/composite/data/withFlattenedList.js
new file mode 100644
index 0000000..edfa340
--- /dev/null
+++ b/src/data/composite/data/withFlattenedList.js
@@ -0,0 +1,47 @@
+// Flattens an array with one level of nested arrays, providing as dependencies
+// both the flattened array as well as the original starting indices of each
+// successive source array.
+//
+// See also:
+//  - withUnflattenedList
+//
+// More list utilities:
+//  - excludeFromList
+//  - fillMissingListItems
+//  - withFilteredList, withMappedList, withSortedList
+//  - withPropertyFromList, withPropertiesFromList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `withFlattenedList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+  },
+
+  outputs: ['#flattenedList', '#flattenedIndices'],
+
+  steps: () => [
+    {
+      dependencies: [input('list')],
+      compute(continuation, {
+        [input('list')]: sourceList,
+      }) {
+        const flattenedList = sourceList.flat();
+        const indices = [];
+        let lastEndIndex = 0;
+        for (const {length} of sourceList) {
+          indices.push(lastEndIndex);
+          lastEndIndex += length;
+        }
+
+        return continuation({
+          ['#flattenedList']: flattenedList,
+          ['#flattenedIndices']: indices,
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/data/withMappedList.js b/src/data/composite/data/withMappedList.js
new file mode 100644
index 0000000..e0a700b
--- /dev/null
+++ b/src/data/composite/data/withMappedList.js
@@ -0,0 +1,39 @@
+// Applies a map function to each item in a list, just like a normal JavaScript
+// map.
+//
+// See also:
+//  - withFilteredList
+//  - withSortedList
+//
+// More list utilities:
+//  - excludeFromList
+//  - fillMissingListItems
+//  - withFlattenedList, withUnflattenedList
+//  - withPropertyFromList, withPropertiesFromList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `withMappedList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+    map: input({type: 'function'}),
+  },
+
+  outputs: ['#mappedList'],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('map')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('map')]: mapFn,
+      }) => continuation({
+        ['#mappedList']:
+          list.map(mapFn),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withPropertiesFromList.js b/src/data/composite/data/withPropertiesFromList.js
new file mode 100644
index 0000000..08907ba
--- /dev/null
+++ b/src/data/composite/data/withPropertiesFromList.js
@@ -0,0 +1,92 @@
+// Gets the listed properties from each of a list of objects, providing lists
+// of property values each into a dependency prefixed with the same name as the
+// list (by default).
+//
+// Like withPropertyFromList, this doesn't alter indices.
+//
+// See also:
+//  - withPropertiesFromObject
+//  - withPropertyFromList
+//
+// More list utilities:
+//  - excludeFromList
+//  - fillMissingListItems
+//  - withFilteredList, withMappedList, withSortedList
+//  - withFlattenedList, withUnflattenedList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {isString, validateArrayItems} from '#validators';
+
+export default templateCompositeFrom({
+  annotation: `withPropertiesFromList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+
+    properties: input({
+      validate: validateArrayItems(isString),
+    }),
+
+    prefix: input.staticValue({type: 'string', defaultValue: null}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+    [input.staticValue('properties')]: properties,
+    [input.staticValue('prefix')]: prefix,
+  }) =>
+    (properties
+      ? properties.map(property =>
+          (prefix
+            ? `${prefix}.${property}`
+         : list
+            ? `${list}.${property}`
+            : `#list.${property}`))
+      : ['#lists']),
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('properties')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('properties')]: properties,
+      }) => continuation({
+        ['#lists']:
+          Object.fromEntries(
+            properties.map(property => [
+              property,
+              list.map(item => item[property] ?? null),
+            ])),
+      }),
+    },
+
+    {
+      dependencies: [
+        input.staticDependency('list'),
+        input.staticValue('properties'),
+        input.staticValue('prefix'),
+        '#lists',
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('list')]: list,
+        [input.staticValue('properties')]: properties,
+        [input.staticValue('prefix')]: prefix,
+        ['#lists']: lists,
+      }) =>
+        (properties
+          ? continuation(
+              Object.fromEntries(
+                properties.map(property => [
+                  (prefix
+                    ? `${prefix}.${property}`
+                 : list
+                    ? `${list}.${property}`
+                    : `#list.${property}`),
+                  lists[property],
+                ])))
+          : continuation({'#lists': lists})),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withPropertiesFromObject.js b/src/data/composite/data/withPropertiesFromObject.js
new file mode 100644
index 0000000..21726b5
--- /dev/null
+++ b/src/data/composite/data/withPropertiesFromObject.js
@@ -0,0 +1,87 @@
+// Gets the listed properties from some object, providing each property's value
+// as a dependency prefixed with the same name as the object (by default).
+// If the object itself is null, all provided dependencies will be null;
+// if it's missing only select properties, those will be provided as null.
+//
+// See also:
+//  - withPropertiesFromList
+//  - withPropertyFromObject
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {isString, validateArrayItems} from '#validators';
+
+export default templateCompositeFrom({
+  annotation: `withPropertiesFromObject`,
+
+  inputs: {
+    object: input({type: 'object', acceptsNull: true}),
+
+    properties: input({
+      type: 'array',
+      validate: validateArrayItems(isString),
+    }),
+
+    prefix: input.staticValue({type: 'string', defaultValue: null}),
+  },
+
+  outputs: ({
+    [input.staticDependency('object')]: object,
+    [input.staticValue('properties')]: properties,
+    [input.staticValue('prefix')]: prefix,
+  }) =>
+    (properties
+      ? properties.map(property =>
+          (prefix
+            ? `${prefix}.${property}`
+         : object
+            ? `${object}.${property}`
+            : `#object.${property}`))
+      : ['#object']),
+
+  steps: () => [
+    {
+      dependencies: [input('object'), input('properties')],
+      compute: (continuation, {
+        [input('object')]: object,
+        [input('properties')]: properties,
+      }) => continuation({
+        ['#entries']:
+          (object === null
+            ? properties.map(property => [property, null])
+            : properties.map(property => [property, object[property]])),
+      }),
+    },
+
+    {
+      dependencies: [
+        input.staticDependency('object'),
+        input.staticValue('properties'),
+        input.staticValue('prefix'),
+        '#entries',
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('object')]: object,
+        [input.staticValue('properties')]: properties,
+        [input.staticValue('prefix')]: prefix,
+        ['#entries']: entries,
+      }) =>
+        (properties
+          ? continuation(
+              Object.fromEntries(
+                entries.map(([property, value]) => [
+                  (prefix
+                    ? `${prefix}.${property}`
+                 : object
+                    ? `${object}.${property}`
+                    : `#object.${property}`),
+                  value ?? null,
+                ])))
+          : continuation({
+              ['#object']:
+                Object.fromEntries(entries),
+            })),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withPropertyFromList.js b/src/data/composite/data/withPropertyFromList.js
new file mode 100644
index 0000000..a2c66d7
--- /dev/null
+++ b/src/data/composite/data/withPropertyFromList.js
@@ -0,0 +1,82 @@
+// Gets a property from each of a list of objects (in a dependency) and
+// provides the results.
+//
+// This doesn't alter any list indices, so positions which were null in the
+// original list are kept null here. Objects which don't have the specified
+// property are retained in-place as null.
+//
+// See also:
+//  - withPropertiesFromList
+//  - withPropertyFromObject
+//
+// More list utilities:
+//  - excludeFromList
+//  - fillMissingListItems
+//  - withFilteredList, withMappedList, withSortedList
+//  - withFlattenedList, withUnflattenedList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+function getOutputName({list, property, prefix}) {
+  if (!property) return `#values`;
+  if (prefix) return `${prefix}.${property}`;
+  if (list) return `${list}.${property}`;
+  return `#list.${property}`;
+}
+
+export default templateCompositeFrom({
+  annotation: `withPropertyFromList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+    property: input({type: 'string'}),
+    prefix: input.staticValue({type: 'string', defaultValue: null}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+    [input.staticValue('property')]: property,
+    [input.staticValue('prefix')]: prefix,
+  }) =>
+    [getOutputName({list, property, prefix})],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('property')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('property')]: property,
+      }) => continuation({
+        ['#values']:
+          list.map(item => item[property] ?? null),
+      }),
+    },
+
+    {
+      dependencies: [
+        input.staticDependency('list'),
+        input.staticValue('property'),
+        input.staticValue('prefix'),
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('list')]: list,
+        [input.staticValue('property')]: property,
+        [input.staticValue('prefix')]: prefix,
+      }) => continuation({
+        ['#outputName']:
+          getOutputName({list, property, prefix}),
+      }),
+    },
+
+    {
+      dependencies: ['#values', '#outputName'],
+      compute: (continuation, {
+        ['#values']: values,
+        ['#outputName']: outputName,
+      }) =>
+        continuation.raiseOutput({[outputName]: values}),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withPropertyFromObject.js b/src/data/composite/data/withPropertyFromObject.js
new file mode 100644
index 0000000..b31bab1
--- /dev/null
+++ b/src/data/composite/data/withPropertyFromObject.js
@@ -0,0 +1,69 @@
+// Gets a property of some object (in a dependency) and provides that value.
+// If the object itself is null, or the object doesn't have the listed property,
+// the provided dependency will also be null.
+//
+// See also:
+//  - withPropertiesFromObject
+//  - withPropertyFromList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `withPropertyFromObject`,
+
+  inputs: {
+    object: input({type: 'object', acceptsNull: true}),
+    property: input({type: 'string'}),
+  },
+
+  outputs: ({
+    [input.staticDependency('object')]: object,
+    [input.staticValue('property')]: property,
+  }) =>
+    (object && property
+      ? (object.startsWith('#')
+          ? [`${object}.${property}`]
+          : [`#${object}.${property}`])
+      : ['#value']),
+
+  steps: () => [
+    {
+      dependencies: [
+        input.staticDependency('object'),
+        input.staticValue('property'),
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('object')]: object,
+        [input.staticValue('property')]: property,
+      }) => continuation({
+        '#output':
+          (object && property
+            ? (object.startsWith('#')
+                ? `${object}.${property}`
+                : `#${object}.${property}`)
+            : '#value'),
+      }),
+    },
+
+    {
+      dependencies: [
+        '#output',
+        input('object'),
+        input('property'),
+      ],
+
+      compute: (continuation, {
+        ['#output']: output,
+        [input('object')]: object,
+        [input('property')]: property,
+      }) => continuation({
+        [output]:
+          (object === null
+            ? null
+            : object[property] ?? null),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withSortedList.js b/src/data/composite/data/withSortedList.js
new file mode 100644
index 0000000..dd81078
--- /dev/null
+++ b/src/data/composite/data/withSortedList.js
@@ -0,0 +1,121 @@
+// Applies a sort function across pairs of items in a list, just like a normal
+// JavaScript sort. Alongside the sorted results, so are outputted the indices
+// which each item in the unsorted list corresponds to in the sorted one,
+// allowing for the results of this sort to be composed in some more involved
+// operation. For example, using an alphabetical sort, the list ['banana',
+// 'apple', 'pterodactyl'] will output the expected alphabetical items, as well
+// as the indices list [1, 0, 2].
+//
+// If two items are equal (in the eyes of the sort operation), their placement
+// in the sorted list is arbitrary, though every input index will be present in
+// '#sortIndices' exactly once (and equal items will be bunched together).
+//
+// The '#sortIndices' output refers to the "true" index which each source item
+// occupies in the sorted list. This sacrifices information about equal items,
+// which can be obtained through '#unstableSortIndices' instead: each mapped
+// index may appear more than once, and rather than represent exact positions
+// in the sorted list, they represent relational values: if items A and B are
+// mapped to indices 3 and 5, then A certainly is positioned before B (and vice
+// versa); but there may be more than one item in-between. If items C and D are
+// both mapped to index 4, then their position relative to each other is
+// arbitrary - they are equal - but they both certainly appear after item A and
+// before item B.
+//
+// This implementation is based on the one used for sortMultipleArrays.
+//
+// See also:
+//  - withFilteredList
+//  - withMappedList
+//
+// More list utilities:
+//  - excludeFromList
+//  - fillMissingListItems
+//  - withFlattenedList, withUnflattenedList
+//  - withPropertyFromList, withPropertiesFromList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `withSortedList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+    sort: input({type: 'function'}),
+  },
+
+  outputs: ['#sortedList', '#sortIndices', '#unstableSortIndices'],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('sort')],
+      compute(continuation, {
+        [input('list')]: list,
+        [input('sort')]: sortFn,
+      }) {
+        const symbols = [];
+        const symbolToIndex = new Map();
+
+        for (const index of list.keys()) {
+          const symbol = Symbol();
+          symbols.push(symbol);
+          symbolToIndex.set(symbol, index);
+        }
+
+        const equalSymbols = new Map();
+
+        const assertEqual = (symbol1, symbol2) => {
+          if (equalSymbols.has(symbol1)) {
+            equalSymbols.get(symbol1).add(symbol2);
+          } else {
+            equalSymbols.set(symbol1, new Set([symbol2]));
+          }
+        };
+
+        const isEqual = (symbol1, symbol2) =>
+          !!equalSymbols.get(symbol1)?.has(symbol2);
+
+        symbols.sort((symbol1, symbol2) => {
+          const comparison =
+            sortFn(
+              list[symbolToIndex.get(symbol1)],
+              list[symbolToIndex.get(symbol2)]);
+
+          if (comparison === 0) {
+            assertEqual(symbol1, symbol2);
+            assertEqual(symbol2, symbol1);
+          }
+
+          return comparison;
+        });
+
+        const stableSortIndices = [];
+        const unstableSortIndices = [];
+        const sortedList = [];
+
+        let unstableIndex = 0;
+
+        for (const [stableIndex, symbol] of symbols.entries()) {
+          const sourceIndex = symbolToIndex.get(symbol);
+          sortedList.push(list[sourceIndex]);
+
+          if (stableIndex > 0) {
+            const previous = symbols[stableIndex - 1];
+            if (!isEqual(symbol, previous)) {
+              unstableIndex++;
+            }
+          }
+
+          stableSortIndices[sourceIndex] = stableIndex;
+          unstableSortIndices[sourceIndex] = unstableIndex;
+        }
+
+        return continuation({
+          ['#sortedList']: sortedList,
+          ['#sortIndices']: stableSortIndices,
+          ['#unstableSortIndices']: unstableSortIndices,
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/data/withUnflattenedList.js b/src/data/composite/data/withUnflattenedList.js
new file mode 100644
index 0000000..39a666d
--- /dev/null
+++ b/src/data/composite/data/withUnflattenedList.js
@@ -0,0 +1,72 @@
+// After mapping the contents of a flattened array in-place (being careful to
+// retain the original indices by replacing unmatched results with null instead
+// of filtering them out), this function allows for recombining them. It will
+// filter out null and undefined items by default (pass {filter: false} to
+// disable this).
+//
+// See also:
+//  - withFlattenedList
+//
+// More list utilities:
+//  - excludeFromList
+//  - fillMissingListItems
+//  - withFilteredList, withMappedList, withSortedList
+//  - withPropertyFromList, withPropertiesFromList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {isWholeNumber, validateArrayItems} from '#validators';
+
+export default templateCompositeFrom({
+  annotation: `withUnflattenedList`,
+
+  inputs: {
+    list: input({
+      type: 'array',
+      defaultDependency: '#flattenedList',
+    }),
+
+    indices: input({
+      validate: validateArrayItems(isWholeNumber),
+      defaultDependency: '#flattenedIndices',
+    }),
+
+    filter: input({
+      type: 'boolean',
+      defaultValue: true,
+    }),
+  },
+
+  outputs: ['#unflattenedList'],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('indices'), input('filter')],
+      compute(continuation, {
+        [input('list')]: list,
+        [input('indices')]: indices,
+        [input('filter')]: filter,
+      }) {
+        const unflattenedList = [];
+
+        for (let i = 0; i < indices.length; i++) {
+          const startIndex = indices[i];
+          const endIndex =
+            (i === indices.length - 1
+              ? list.length
+              : indices[i + 1]);
+
+          const values = list.slice(startIndex, endIndex);
+          unflattenedList.push(
+            (filter
+              ? values.filter(value => value !== null && value !== undefined)
+              : values));
+        }
+
+        return continuation({
+          ['#unflattenedList']: unflattenedList,
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/data/withUniqueItemsOnly.js b/src/data/composite/data/withUniqueItemsOnly.js
new file mode 100644
index 0000000..7ee08b0
--- /dev/null
+++ b/src/data/composite/data/withUniqueItemsOnly.js
@@ -0,0 +1,40 @@
+// Excludes duplicate items from a list and provides the results, overwriting
+// the list in-place, if possible.
+
+import {input, templateCompositeFrom} from '#composite';
+import {unique} from '#sugar';
+
+export default templateCompositeFrom({
+  annotation: `withUniqueItemsOnly`,
+
+  inputs: {
+    list: input({type: 'array'}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+  }) => [list ?? '#uniqueItems'],
+
+  steps: () => [
+    {
+      dependencies: [input('list')],
+      compute: (continuation, {
+        [input('list')]: list,
+      }) => continuation({
+        ['#values']:
+          unique(list),
+      }),
+    },
+
+    {
+      dependencies: ['#values', input.staticDependency('list')],
+      compute: (continuation, {
+        '#values': values,
+        [input.staticDependency('list')]: list,
+      }) => continuation({
+        [list ?? '#uniqueItems']:
+          values,
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/album/index.js b/src/data/composite/things/album/index.js
new file mode 100644
index 0000000..8139f10
--- /dev/null
+++ b/src/data/composite/things/album/index.js
@@ -0,0 +1,2 @@
+export {default as withTracks} from './withTracks.js';
+export {default as withTrackSections} from './withTrackSections.js';
diff --git a/src/data/composite/things/album/withTrackSections.js b/src/data/composite/things/album/withTrackSections.js
new file mode 100644
index 0000000..0a1ebeb
--- /dev/null
+++ b/src/data/composite/things/album/withTrackSections.js
@@ -0,0 +1,127 @@
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+import {empty, filterMultipleArrays, stitchArrays} from '#sugar';
+import {isTrackSectionList} from '#validators';
+
+import {exitWithoutDependency, exitWithoutUpdateValue}
+  from '#composite/control-flow';
+import {withResolvedReferenceList} from '#composite/wiki-data';
+
+import {
+  fillMissingListItems,
+  withFlattenedList,
+  withPropertiesFromList,
+  withUnflattenedList,
+} from '#composite/data';
+
+export default templateCompositeFrom({
+  annotation: `withTrackSections`,
+
+  outputs: ['#trackSections'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: 'ownTrackData',
+      value: input.value([]),
+    }),
+
+    exitWithoutUpdateValue({
+      mode: input.value('empty'),
+      value: input.value([]),
+    }),
+
+    // TODO: input.updateValue description down here is a kludge.
+    withPropertiesFromList({
+      list: input.updateValue({
+        validate: isTrackSectionList,
+      }),
+      prefix: input.value('#sections'),
+      properties: input.value([
+        'tracks',
+        'dateOriginallyReleased',
+        'isDefaultTrackSection',
+        'name',
+        'color',
+      ]),
+    }),
+
+    fillMissingListItems({
+      list: '#sections.tracks',
+      fill: input.value([]),
+    }),
+
+    fillMissingListItems({
+      list: '#sections.isDefaultTrackSection',
+      fill: input.value(false),
+    }),
+
+    fillMissingListItems({
+      list: '#sections.name',
+      fill: input.value('Unnamed Track Section'),
+    }),
+
+    fillMissingListItems({
+      list: '#sections.color',
+      fill: input.dependency('color'),
+    }),
+
+    withFlattenedList({
+      list: '#sections.tracks',
+    }).outputs({
+      ['#flattenedList']: '#trackRefs',
+      ['#flattenedIndices']: '#sections.startIndex',
+    }),
+
+    withResolvedReferenceList({
+      list: '#trackRefs',
+      data: 'ownTrackData',
+      notFoundMode: input.value('null'),
+      find: input.value(find.track),
+    }).outputs({
+      ['#resolvedReferenceList']: '#tracks',
+    }),
+
+    withUnflattenedList({
+      list: '#tracks',
+      indices: '#sections.startIndex',
+    }).outputs({
+      ['#unflattenedList']: '#sections.tracks',
+    }),
+
+    {
+      dependencies: [
+        '#sections.tracks',
+        '#sections.name',
+        '#sections.color',
+        '#sections.dateOriginallyReleased',
+        '#sections.isDefaultTrackSection',
+        '#sections.startIndex',
+      ],
+
+      compute: (continuation, {
+        '#sections.tracks': tracks,
+        '#sections.name': name,
+        '#sections.color': color,
+        '#sections.dateOriginallyReleased': dateOriginallyReleased,
+        '#sections.isDefaultTrackSection': isDefaultTrackSection,
+        '#sections.startIndex': startIndex,
+      }) => {
+        filterMultipleArrays(
+          tracks, name, color, dateOriginallyReleased, isDefaultTrackSection, startIndex,
+          tracks => !empty(tracks));
+
+        return continuation({
+          ['#trackSections']:
+            stitchArrays({
+              tracks,
+              name,
+              color,
+              dateOriginallyReleased,
+              isDefaultTrackSection,
+              startIndex,
+            }),
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/things/album/withTracks.js b/src/data/composite/things/album/withTracks.js
new file mode 100644
index 0000000..fff3d5a
--- /dev/null
+++ b/src/data/composite/things/album/withTracks.js
@@ -0,0 +1,51 @@
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+
+import {exitWithoutDependency, raiseOutputWithoutDependency}
+  from '#composite/control-flow';
+import {withResolvedReferenceList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `withTracks`,
+
+  outputs: ['#tracks'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: 'ownTrackData',
+      value: input.value([]),
+    }),
+
+    raiseOutputWithoutDependency({
+      dependency: 'trackSections',
+      mode: input.value('empty'),
+      output: input.value({
+        ['#tracks']: [],
+      }),
+    }),
+
+    {
+      dependencies: ['trackSections'],
+      compute: (continuation, {trackSections}) =>
+        continuation({
+          '#trackRefs': trackSections
+            .flatMap(section => section.tracks ?? []),
+        }),
+    },
+
+    withResolvedReferenceList({
+      list: '#trackRefs',
+      data: 'ownTrackData',
+      find: input.value(find.track),
+    }),
+
+    {
+      dependencies: ['#resolvedReferenceList'],
+      compute: (continuation, {
+        ['#resolvedReferenceList']: resolvedReferenceList,
+      }) => continuation({
+        ['#tracks']: resolvedReferenceList,
+      })
+    },
+  ],
+});
diff --git a/src/data/composite/things/flash-act/index.js b/src/data/composite/things/flash-act/index.js
new file mode 100644
index 0000000..40fecd2
--- /dev/null
+++ b/src/data/composite/things/flash-act/index.js
@@ -0,0 +1 @@
+export {default as withFlashSide} from './withFlashSide.js';
diff --git a/src/data/composite/things/flash-act/withFlashSide.js b/src/data/composite/things/flash-act/withFlashSide.js
new file mode 100644
index 0000000..64daa1f
--- /dev/null
+++ b/src/data/composite/things/flash-act/withFlashSide.js
@@ -0,0 +1,22 @@
+// Gets the flash act's side. This will early exit if flashSideData is missing.
+// If there's no side whose list of flash acts includes this act, the output
+// dependency will be null.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {withUniqueReferencingThing} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `withFlashSide`,
+
+  outputs: ['#flashSide'],
+
+  steps: () => [
+    withUniqueReferencingThing({
+      data: 'flashSideData',
+      list: input.value('acts'),
+    }).outputs({
+      ['#uniqueReferencingThing']: '#flashSide',
+    }),
+  ],
+});
diff --git a/src/data/composite/things/flash/index.js b/src/data/composite/things/flash/index.js
new file mode 100644
index 0000000..63ac13d
--- /dev/null
+++ b/src/data/composite/things/flash/index.js
@@ -0,0 +1 @@
+export {default as withFlashAct} from './withFlashAct.js';
diff --git a/src/data/composite/things/flash/withFlashAct.js b/src/data/composite/things/flash/withFlashAct.js
new file mode 100644
index 0000000..652b8bf
--- /dev/null
+++ b/src/data/composite/things/flash/withFlashAct.js
@@ -0,0 +1,22 @@
+// Gets the flash's act. This will early exit if flashActData is missing.
+// If there's no flash whose list of flashes includes this flash, the output
+// dependency will be null.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {withUniqueReferencingThing} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `withFlashAct`,
+
+  outputs: ['#flashAct'],
+
+  steps: () => [
+    withUniqueReferencingThing({
+      data: 'flashActData',
+      list: input.value('flashes'),
+    }).outputs({
+      ['#uniqueReferencingThing']: '#flashAct',
+    }),
+  ],
+});
diff --git a/src/data/composite/things/track/exitWithoutUniqueCoverArt.js b/src/data/composite/things/track/exitWithoutUniqueCoverArt.js
new file mode 100644
index 0000000..f47086d
--- /dev/null
+++ b/src/data/composite/things/track/exitWithoutUniqueCoverArt.js
@@ -0,0 +1,26 @@
+// Shorthand for checking if the track has unique cover art and exposing a
+// fallback value if it isn't.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency} from '#composite/control-flow';
+
+import withHasUniqueCoverArt from './withHasUniqueCoverArt.js';
+
+export default templateCompositeFrom({
+  annotation: `exitWithoutUniqueCoverArt`,
+
+  inputs: {
+    value: input({defaultValue: null}),
+  },
+
+  steps: () => [
+    withHasUniqueCoverArt(),
+
+    exitWithoutDependency({
+      dependency: '#hasUniqueCoverArt',
+      mode: input.value('falsy'),
+      value: input('value'),
+    }),
+  ],
+});
diff --git a/src/data/composite/things/track/index.js b/src/data/composite/things/track/index.js
new file mode 100644
index 0000000..cc723a2
--- /dev/null
+++ b/src/data/composite/things/track/index.js
@@ -0,0 +1,11 @@
+export {default as exitWithoutUniqueCoverArt} from './exitWithoutUniqueCoverArt.js';
+export {default as inferredAdditionalNameList} from './inferredAdditionalNameList.js';
+export {default as inheritFromOriginalRelease} from './inheritFromOriginalRelease.js';
+export {default as sharedAdditionalNameList} from './sharedAdditionalNameList.js';
+export {default as trackReverseReferenceList} from './trackReverseReferenceList.js';
+export {default as withAlbum} from './withAlbum.js';
+export {default as withAlwaysReferenceByDirectory} from './withAlwaysReferenceByDirectory.js';
+export {default as withContainingTrackSection} from './withContainingTrackSection.js';
+export {default as withHasUniqueCoverArt} from './withHasUniqueCoverArt.js';
+export {default as withOtherReleases} from './withOtherReleases.js';
+export {default as withPropertyFromAlbum} from './withPropertyFromAlbum.js';
diff --git a/src/data/composite/things/track/inferredAdditionalNameList.js b/src/data/composite/things/track/inferredAdditionalNameList.js
new file mode 100644
index 0000000..58e8d2a
--- /dev/null
+++ b/src/data/composite/things/track/inferredAdditionalNameList.js
@@ -0,0 +1,67 @@
+// Infers additional name entries from other releases that were titled
+// differently; the corresponding releases are stored in eacn entry's "from"
+// array, which will include multiple items, if more than one other release
+// shares the same name differing from this one's.
+
+import {input, templateCompositeFrom} from '#composite';
+import {chunkByProperties} from '#sugar';
+
+import {exitWithoutDependency} from '#composite/control-flow';
+import {withFilteredList, withPropertyFromList} from '#composite/data';
+import {withThingsSortedAlphabetically} from '#composite/wiki-data';
+
+import withOtherReleases from './withOtherReleases.js';
+
+export default templateCompositeFrom({
+  annotation: `inferredAdditionalNameList`,
+
+  compose: false,
+
+  steps: () => [
+    withOtherReleases(),
+
+    exitWithoutDependency({
+      dependency: '#otherReleases',
+      mode: input.value('empty'),
+      value: input.value([]),
+    }),
+
+    withPropertyFromList({
+      list: '#otherReleases',
+      property: input.value('name'),
+    }),
+
+    {
+      dependencies: ['#otherReleases.name', 'name'],
+      compute: (continuation, {
+        ['#otherReleases.name']: releaseNames,
+        ['name']: ownName,
+      }) => continuation({
+        ['#nameFilter']:
+          releaseNames.map(name => name !== ownName),
+      }),
+    },
+
+    withFilteredList({
+      list: '#otherReleases',
+      filter: '#nameFilter',
+    }).outputs({
+      '#filteredList': '#differentlyNamedReleases',
+    }),
+
+    withThingsSortedAlphabetically({
+      things: '#differentlyNamedReleases',
+    }).outputs({
+      '#sortedThings': '#differentlyNamedReleases',
+    }),
+
+    {
+      dependencies: ['#differentlyNamedReleases'],
+      compute: ({
+        ['#differentlyNamedReleases']: releases,
+      }) =>
+        chunkByProperties(releases, ['name'])
+          .map(({name, chunk}) => ({name, from: chunk})),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/inheritFromOriginalRelease.js b/src/data/composite/things/track/inheritFromOriginalRelease.js
new file mode 100644
index 0000000..27ed138
--- /dev/null
+++ b/src/data/composite/things/track/inheritFromOriginalRelease.js
@@ -0,0 +1,50 @@
+// Early exits with a value inherited from the original release, if
+// this track is a rerelease, and otherwise continues with no further
+// dependencies provided. If allowOverride is true, then the continuation
+// will also be called if the original release exposed the requested
+// property as null.
+//
+// Like withOriginalRelease, this will early exit (with notFoundValue) if the
+// original release is specified by reference and that reference doesn't
+// resolve to anything.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import withOriginalRelease from './withOriginalRelease.js';
+
+export default templateCompositeFrom({
+  annotation: `inheritFromOriginalRelease`,
+
+  inputs: {
+    property: input({type: 'string'}),
+    allowOverride: input({type: 'boolean', defaultValue: false}),
+    notFoundValue: input({defaultValue: null}),
+  },
+
+  steps: () => [
+    withOriginalRelease({
+      notFoundValue: input('notFoundValue'),
+    }),
+
+    {
+      dependencies: [
+        '#originalRelease',
+        input('property'),
+        input('allowOverride'),
+      ],
+
+      compute: (continuation, {
+        ['#originalRelease']: originalRelease,
+        [input('property')]: originalProperty,
+        [input('allowOverride')]: allowOverride,
+      }) => {
+        if (!originalRelease) return continuation();
+
+        const value = originalRelease[originalProperty];
+        if (allowOverride && value === null) return continuation();
+
+        return continuation.exit(value);
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/sharedAdditionalNameList.js b/src/data/composite/things/track/sharedAdditionalNameList.js
new file mode 100644
index 0000000..1806ec8
--- /dev/null
+++ b/src/data/composite/things/track/sharedAdditionalNameList.js
@@ -0,0 +1,38 @@
+// Compiles additional names directly provided by other releases.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency, exposeDependency}
+  from '#composite/control-flow';
+import {withFlattenedList, withPropertyFromList} from '#composite/data';
+
+import withOtherReleases from './withOtherReleases.js';
+
+export default templateCompositeFrom({
+  annotation: `sharedAdditionalNameList`,
+
+  compose: false,
+
+  steps: () => [
+    withOtherReleases(),
+
+    exitWithoutDependency({
+      dependency: '#otherReleases',
+      mode: input.value('empty'),
+      value: input.value([]),
+    }),
+
+    withPropertyFromList({
+      list: '#otherReleases',
+      property: input.value('additionalNames'),
+    }),
+
+    withFlattenedList({
+      list: '#otherReleases.additionalNames',
+    }),
+
+    exposeDependency({
+      dependency: '#flattenedList',
+    }),
+  ],
+});
diff --git a/src/data/composite/things/track/trackAdditionalNameList.js b/src/data/composite/things/track/trackAdditionalNameList.js
new file mode 100644
index 0000000..65a2263
--- /dev/null
+++ b/src/data/composite/things/track/trackAdditionalNameList.js
@@ -0,0 +1,38 @@
+// Compiles additional names from various sources.
+
+import {input, templateCompositeFrom} from '#composite';
+import {isAdditionalNameList} from '#validators';
+
+import withInferredAdditionalNames from './withInferredAdditionalNames.js';
+import withSharedAdditionalNames from './withSharedAdditionalNames.js';
+
+export default templateCompositeFrom({
+  annotation: `trackAdditionalNameList`,
+
+  compose: false,
+
+  update: {validate: isAdditionalNameList},
+
+  steps: () => [
+    withInferredAdditionalNames(),
+    withSharedAdditionalNames(),
+
+    {
+      dependencies: [
+        '#inferredAdditionalNames',
+        '#sharedAdditionalNames',
+        input.updateValue(),
+      ],
+
+      compute: ({
+        ['#inferredAdditionalNames']: inferredAdditionalNames,
+        ['#sharedAdditionalNames']: sharedAdditionalNames,
+        [input.updateValue()]: providedAdditionalNames,
+      }) => [
+        ...providedAdditionalNames ?? [],
+        ...sharedAdditionalNames,
+        ...inferredAdditionalNames,
+      ],
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/trackReverseReferenceList.js b/src/data/composite/things/track/trackReverseReferenceList.js
new file mode 100644
index 0000000..44940ae
--- /dev/null
+++ b/src/data/composite/things/track/trackReverseReferenceList.js
@@ -0,0 +1,38 @@
+// Like a normal reverse reference list ("objects which reference this object
+// under a specified property"), only excluding rereleases from the possible
+// outputs. While it's useful to travel from a rerelease to the tracks it
+// references, rereleases aren't generally relevant from the perspective of
+// the tracks *being* referenced. Apart from hiding rereleases from lists on
+// the site, it also excludes keeps them from relational data processing, such
+// as on the "Tracks - by Times Referenced" listing page.
+
+import {input, templateCompositeFrom} from '#composite';
+import {withReverseReferenceList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `trackReverseReferenceList`,
+
+  compose: false,
+
+  inputs: {
+    list: input({type: 'string'}),
+  },
+
+  steps: () => [
+    withReverseReferenceList({
+      data: 'trackData',
+      list: input('list'),
+    }),
+
+    {
+      flags: {expose: true},
+      expose: {
+        dependencies: ['#reverseReferenceList'],
+        compute: ({
+          ['#reverseReferenceList']: reverseReferenceList,
+        }) =>
+          reverseReferenceList.filter(track => !track.originalReleaseTrack),
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withAlbum.js b/src/data/composite/things/track/withAlbum.js
new file mode 100644
index 0000000..03b840d
--- /dev/null
+++ b/src/data/composite/things/track/withAlbum.js
@@ -0,0 +1,22 @@
+// Gets the track's album. This will early exit if albumData is missing.
+// If there's no album whose list of tracks includes this track, the output
+// dependency will be null.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {withUniqueReferencingThing} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `withAlbum`,
+
+  outputs: ['#album'],
+
+  steps: () => [
+    withUniqueReferencingThing({
+      data: 'albumData',
+      list: input.value('tracks'),
+    }).outputs({
+      ['#uniqueReferencingThing']: '#album',
+    }),
+  ],
+});
diff --git a/src/data/composite/things/track/withAlwaysReferenceByDirectory.js b/src/data/composite/things/track/withAlwaysReferenceByDirectory.js
new file mode 100644
index 0000000..fac8e21
--- /dev/null
+++ b/src/data/composite/things/track/withAlwaysReferenceByDirectory.js
@@ -0,0 +1,78 @@
+// Controls how find.track works - it'll never be matched by a reference
+// just to the track's name, which means you don't have to always reference
+// some *other* (much more commonly referenced) track by directory instead
+// of more naturally by name.
+
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+import {isBoolean} from '#validators';
+
+import {exitWithoutDependency, exposeUpdateValueOrContinue}
+  from '#composite/control-flow';
+import {withPropertyFromObject} from '#composite/data';
+import {withResolvedReference} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `withAlwaysReferenceByDirectory`,
+
+  outputs: ['#alwaysReferenceByDirectory'],
+
+  steps: () => [
+    exposeUpdateValueOrContinue({
+      validate: input.value(isBoolean),
+    }),
+
+    // Remaining code is for defaulting to true if this track is a rerelease of
+    // another with the same name, so everything further depends on access to
+    // trackData as well as originalReleaseTrack.
+
+    exitWithoutDependency({
+      dependency: 'trackData',
+      mode: input.value('empty'),
+      value: input.value(false),
+    }),
+
+    exitWithoutDependency({
+      dependency: 'originalReleaseTrack',
+      value: input.value(false),
+    }),
+
+    // It's necessary to use the custom trackOriginalReleasesOnly find function
+    // here, so as to avoid recursion issues - the find.track() function depends
+    // on accessing each track's alwaysReferenceByDirectory, which means it'll
+    // hit *this track* - and thus this step - and end up recursing infinitely.
+    // By definition, find.trackOriginalReleasesOnly excludes tracks which have
+    // an originalReleaseTrack update value set, which means even though it does
+    // still access each of tracks' `alwaysReferenceByDirectory` property, it
+    // won't access that of *this* track - it will never proceed past the
+    // `exitWithoutDependency` step directly above, so there's no opportunity
+    // for recursion.
+    withResolvedReference({
+      ref: 'originalReleaseTrack',
+      data: 'trackData',
+      find: input.value(find.trackOriginalReleasesOnly),
+    }).outputs({
+      '#resolvedReference': '#originalRelease',
+    }),
+
+    exitWithoutDependency({
+      dependency: '#originalRelease',
+      value: input.value(false),
+    }),
+
+    withPropertyFromObject({
+      object: '#originalRelease',
+      property: input.value('name'),
+    }),
+
+    {
+      dependencies: ['name', '#originalRelease.name'],
+      compute: (continuation, {
+        name,
+        ['#originalRelease.name']: originalName,
+      }) => continuation({
+        ['#alwaysReferenceByDirectory']: name === originalName,
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withContainingTrackSection.js b/src/data/composite/things/track/withContainingTrackSection.js
new file mode 100644
index 0000000..eaac14d
--- /dev/null
+++ b/src/data/composite/things/track/withContainingTrackSection.js
@@ -0,0 +1,42 @@
+// Gets the track section containing this track from its album's track list.
+
+import {input, templateCompositeFrom} from '#composite';
+import {is} from '#validators';
+
+import {raiseOutputWithoutDependency} from '#composite/control-flow';
+
+import withPropertyFromAlbum from './withPropertyFromAlbum.js';
+
+export default templateCompositeFrom({
+  annotation: `withContainingTrackSection`,
+
+  outputs: ['#trackSection'],
+
+  steps: () => [
+    withPropertyFromAlbum({
+      property: input.value('trackSections'),
+    }),
+
+    raiseOutputWithoutDependency({
+      dependency: '#album.trackSections',
+      output: input.value({'#trackSection': null}),
+    }),
+
+    {
+      dependencies: [
+        input.myself(),
+        '#album.trackSections',
+      ],
+
+      compute: (continuation, {
+        [input.myself()]: track,
+        [input('notFoundMode')]: notFoundMode,
+        ['#album.trackSections']: trackSections,
+      }) => continuation({
+        ['#trackSection']:
+          trackSections.find(({tracks}) => tracks.includes(track))
+            ?? null,
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withHasUniqueCoverArt.js b/src/data/composite/things/track/withHasUniqueCoverArt.js
new file mode 100644
index 0000000..96078d5
--- /dev/null
+++ b/src/data/composite/things/track/withHasUniqueCoverArt.js
@@ -0,0 +1,61 @@
+// Whether or not the track has "unique" cover artwork - a cover which is
+// specifically associated with this track in particular, rather than with
+// the track's album as a whole. This is typically used to select between
+// displaying the track artwork and a fallback, such as the album artwork
+// or a placeholder. (This property is named hasUniqueCoverArt instead of
+// the usual hasCoverArt to emphasize that it does not inherit from the
+// album.)
+
+import {input, templateCompositeFrom} from '#composite';
+import {empty} from '#sugar';
+
+import {withResolvedContribs} from '#composite/wiki-data';
+
+import withPropertyFromAlbum from './withPropertyFromAlbum.js';
+
+export default templateCompositeFrom({
+  annotation: 'withHasUniqueCoverArt',
+
+  outputs: ['#hasUniqueCoverArt'],
+
+  steps: () => [
+    {
+      dependencies: ['disableUniqueCoverArt'],
+      compute: (continuation, {disableUniqueCoverArt}) =>
+        (disableUniqueCoverArt
+          ? continuation.raiseOutput({
+              ['#hasUniqueCoverArt']: false,
+            })
+          : continuation()),
+    },
+
+    withResolvedContribs({from: 'coverArtistContribs'}),
+
+    {
+      dependencies: ['#resolvedContribs'],
+      compute: (continuation, {
+        ['#resolvedContribs']: contribsFromTrack,
+      }) =>
+        (empty(contribsFromTrack)
+          ? continuation()
+          : continuation.raiseOutput({
+              ['#hasUniqueCoverArt']: true,
+            })),
+    },
+
+    withPropertyFromAlbum({
+      property: input.value('trackCoverArtistContribs'),
+    }),
+
+    {
+      dependencies: ['#album.trackCoverArtistContribs'],
+      compute: (continuation, {
+        ['#album.trackCoverArtistContribs']: contribsFromAlbum,
+      }) =>
+        continuation.raiseOutput({
+          ['#hasUniqueCoverArt']:
+            !empty(contribsFromAlbum),
+        }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withOriginalRelease.js b/src/data/composite/things/track/withOriginalRelease.js
new file mode 100644
index 0000000..c7f4965
--- /dev/null
+++ b/src/data/composite/things/track/withOriginalRelease.js
@@ -0,0 +1,78 @@
+// Just includes the original release of this track as a dependency.
+// If this track isn't a rerelease, then it'll provide null, unless the
+// {selfIfOriginal} option is set, in which case it'll provide this track
+// itself. This will early exit (with notFoundValue) if the original release
+// is specified by reference and that reference doesn't resolve to anything.
+
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+import {validateWikiData} from '#validators';
+
+import {exitWithoutDependency, withResultOfAvailabilityCheck}
+  from '#composite/control-flow';
+import {withResolvedReference} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `withOriginalRelease`,
+
+  inputs: {
+    selfIfOriginal: input({type: 'boolean', defaultValue: false}),
+
+    data: input({
+      validate: validateWikiData({referenceType: 'track'}),
+      defaultDependency: 'trackData',
+    }),
+
+    notFoundValue: input({defaultValue: null}),
+  },
+
+  outputs: ['#originalRelease'],
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: 'originalReleaseTrack',
+    }),
+
+    {
+      dependencies: [
+        input.myself(),
+        input('selfIfOriginal'),
+        '#availability',
+      ],
+
+      compute: (continuation, {
+        [input.myself()]: track,
+        [input('selfIfOriginal')]: selfIfOriginal,
+        '#availability': availability,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.raiseOutput({
+              ['#originalRelease']:
+                (selfIfOriginal ? track : null),
+            })),
+    },
+
+    withResolvedReference({
+      ref: 'originalReleaseTrack',
+      data: input('data'),
+      find: input.value(find.track),
+    }),
+
+    exitWithoutDependency({
+      dependency: '#resolvedReference',
+      value: input('notFoundValue'),
+    }),
+
+    {
+      dependencies: ['#resolvedReference'],
+
+      compute: (continuation, {
+        ['#resolvedReference']: resolvedReference,
+      }) =>
+        continuation({
+          ['#originalRelease']: resolvedReference,
+        }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withOtherReleases.js b/src/data/composite/things/track/withOtherReleases.js
new file mode 100644
index 0000000..f8c1c3f
--- /dev/null
+++ b/src/data/composite/things/track/withOtherReleases.js
@@ -0,0 +1,41 @@
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency} from '#composite/control-flow';
+
+import withOriginalRelease from './withOriginalRelease.js';
+
+export default templateCompositeFrom({
+  annotation: `withOtherReleases`,
+
+  outputs: ['#otherReleases'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: 'trackData',
+      mode: input.value('empty'),
+    }),
+
+    withOriginalRelease({
+      selfIfOriginal: input.value(true),
+      notFoundValue: input.value([]),
+    }),
+
+    {
+      dependencies: [input.myself(), '#originalRelease', 'trackData'],
+      compute: (continuation, {
+        [input.myself()]: thisTrack,
+        ['#originalRelease']: originalRelease,
+        trackData,
+      }) => continuation({
+        ['#otherReleases']:
+          (originalRelease === thisTrack
+            ? []
+            : [originalRelease])
+            .concat(trackData.filter(track =>
+              track !== originalRelease &&
+              track !== thisTrack &&
+              track.originalReleaseTrack === originalRelease)),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withPropertyFromAlbum.js b/src/data/composite/things/track/withPropertyFromAlbum.js
new file mode 100644
index 0000000..d41390f
--- /dev/null
+++ b/src/data/composite/things/track/withPropertyFromAlbum.js
@@ -0,0 +1,40 @@
+// Gets a single property from this track's album, providing it as the same
+// property name prefixed with '#album.' (by default).
+
+import {input, templateCompositeFrom} from '#composite';
+import {is} from '#validators';
+
+import {withPropertyFromObject} from '#composite/data';
+
+import withAlbum from './withAlbum.js';
+
+export default templateCompositeFrom({
+  annotation: `withPropertyFromAlbum`,
+
+  inputs: {
+    property: input.staticValue({type: 'string'}),
+  },
+
+  outputs: ({
+    [input.staticValue('property')]: property,
+  }) => ['#album.' + property],
+
+  steps: () => [
+    withAlbum(),
+
+    withPropertyFromObject({
+      object: '#album',
+      property: input('property'),
+    }),
+
+    {
+      dependencies: ['#value', input.staticValue('property')],
+      compute: (continuation, {
+        ['#value']: value,
+        [input.staticValue('property')]: property,
+      }) => continuation({
+        ['#album.' + property]: value,
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/exitWithoutContribs.js b/src/data/composite/wiki-data/exitWithoutContribs.js
new file mode 100644
index 0000000..2c8219f
--- /dev/null
+++ b/src/data/composite/wiki-data/exitWithoutContribs.js
@@ -0,0 +1,47 @@
+// Shorthand for exiting if the contribution list (usually a property's update
+// value) resolves to empty - ensuring that the later computed results are only
+// returned if these contributions are present.
+
+import {input, templateCompositeFrom} from '#composite';
+import {isContributionList} from '#validators';
+
+import {withResultOfAvailabilityCheck} from '#composite/control-flow';
+
+import withResolvedContribs from './withResolvedContribs.js';
+
+export default templateCompositeFrom({
+  annotation: `exitWithoutContribs`,
+
+  inputs: {
+    contribs: input({
+      validate: isContributionList,
+      acceptsNull: true,
+    }),
+
+    value: input({defaultValue: null}),
+  },
+
+  steps: () => [
+    withResolvedContribs({
+      from: input('contribs'),
+    }),
+
+    // TODO: Fairly certain exitWithoutDependency would be sufficient here.
+
+    withResultOfAvailabilityCheck({
+      from: '#resolvedContribs',
+      mode: input.value('empty'),
+    }),
+
+    {
+      dependencies: ['#availability', input('value')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('value')]: value,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.exit(value)),
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/index.js b/src/data/composite/wiki-data/index.js
new file mode 100644
index 0000000..b4cf6d1
--- /dev/null
+++ b/src/data/composite/wiki-data/index.js
@@ -0,0 +1,16 @@
+// #composite/wiki-data
+//
+// Entries here may depend on entries in #composite/control-flow and in
+// #composite/data.
+//
+
+export {default as exitWithoutContribs} from './exitWithoutContribs.js';
+export {default as inputWikiData} from './inputWikiData.js';
+export {default as withParsedCommentaryEntries} from './withParsedCommentaryEntries.js';
+export {default as withResolvedContribs} from './withResolvedContribs.js';
+export {default as withResolvedReference} from './withResolvedReference.js';
+export {default as withResolvedReferenceList} from './withResolvedReferenceList.js';
+export {default as withReverseContributionList} from './withReverseContributionList.js';
+export {default as withReverseReferenceList} from './withReverseReferenceList.js';
+export {default as withThingsSortedAlphabetically} from './withThingsSortedAlphabetically.js';
+export {default as withUniqueReferencingThing} from './withUniqueReferencingThing.js';
diff --git a/src/data/composite/wiki-data/inputWikiData.js b/src/data/composite/wiki-data/inputWikiData.js
new file mode 100644
index 0000000..cf7a7c2
--- /dev/null
+++ b/src/data/composite/wiki-data/inputWikiData.js
@@ -0,0 +1,17 @@
+import {input} from '#composite';
+import {validateWikiData} from '#validators';
+
+// TODO: This doesn't access a class's own ThingSubclass[Thing.referenceType]
+// value because classes aren't initialized by when templateCompositeFrom gets
+// called (see: circular imports). So the reference types have to be hard-coded,
+// which somewhat defeats the point of storing them on the class in the first
+// place...
+export default function inputWikiData({
+  referenceType = '',
+  allowMixedTypes = false,
+} = {}) {
+  return input({
+    validate: validateWikiData({referenceType, allowMixedTypes}),
+    acceptsNull: true,
+  });
+}
diff --git a/src/data/composite/wiki-data/withParsedCommentaryEntries.js b/src/data/composite/wiki-data/withParsedCommentaryEntries.js
new file mode 100644
index 0000000..f0404a5
--- /dev/null
+++ b/src/data/composite/wiki-data/withParsedCommentaryEntries.js
@@ -0,0 +1,179 @@
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+import {stitchArrays} from '#sugar';
+import {isCommentary} from '#validators';
+import {commentaryRegexCaseSensitive} from '#wiki-data';
+
+import {
+  fillMissingListItems,
+  withFlattenedList,
+  withPropertiesFromList,
+  withUnflattenedList,
+} from '#composite/data';
+
+import withResolvedReferenceList from './withResolvedReferenceList.js';
+
+export default templateCompositeFrom({
+  annotation: `withParsedCommentaryEntries`,
+
+  inputs: {
+    from: input({validate: isCommentary}),
+  },
+
+  outputs: ['#parsedCommentaryEntries'],
+
+  steps: () => [
+    {
+      dependencies: [input('from')],
+
+      compute: (continuation, {
+        [input('from')]: commentaryText,
+      }) => continuation({
+        ['#rawMatches']:
+          Array.from(commentaryText.matchAll(commentaryRegexCaseSensitive)),
+      }),
+    },
+
+    withPropertiesFromList({
+      list: '#rawMatches',
+      properties: input.value([
+        '0', // The entire match as a string.
+        'groups',
+        'index',
+      ]),
+    }).outputs({
+      '#rawMatches.0': '#rawMatches.text',
+      '#rawMatches.groups': '#rawMatches.groups',
+      '#rawMatches.index': '#rawMatches.startIndex',
+    }),
+
+    {
+      dependencies: [
+        '#rawMatches.text',
+        '#rawMatches.startIndex',
+      ],
+
+      compute: (continuation, {
+        ['#rawMatches.text']: text,
+        ['#rawMatches.startIndex']: startIndex,
+      }) => continuation({
+        ['#rawMatches.endIndex']:
+          stitchArrays({text, startIndex})
+            .map(({text, startIndex}) => startIndex + text.length),
+      }),
+    },
+
+    {
+      dependencies: [
+        input('from'),
+        '#rawMatches.startIndex',
+        '#rawMatches.endIndex',
+      ],
+
+      compute: (continuation, {
+        [input('from')]: commentaryText,
+        ['#rawMatches.startIndex']: startIndex,
+        ['#rawMatches.endIndex']: endIndex,
+      }) => continuation({
+        ['#entries.body']:
+          stitchArrays({startIndex, endIndex})
+            .map(({endIndex}, index, stitched) =>
+              (index === stitched.length - 1
+                ? commentaryText.slice(endIndex)
+                : commentaryText.slice(
+                    endIndex,
+                    stitched[index + 1].startIndex)))
+            .map(body => body.trim()),
+      }),
+    },
+
+    withPropertiesFromList({
+      list: '#rawMatches.groups',
+      prefix: input.value('#entries'),
+      properties: input.value([
+        'artistReferences',
+        'artistDisplayText',
+        'annotation',
+        'date',
+      ]),
+    }),
+
+    // The artistReferences group will always have a value, since it's required
+    // for the line to match in the first place.
+
+    {
+      dependencies: ['#entries.artistReferences'],
+      compute: (continuation, {
+        ['#entries.artistReferences']: artistReferenceTexts,
+      }) => continuation({
+        ['#entries.artistReferences']:
+          artistReferenceTexts
+            .map(text => text.split(',').map(ref => ref.trim())),
+      }),
+    },
+
+    withFlattenedList({
+      list: '#entries.artistReferences',
+    }),
+
+    withResolvedReferenceList({
+      list: '#flattenedList',
+      data: 'artistData',
+      find: input.value(find.artist),
+      notFoundMode: input.value('null'),
+    }),
+
+    withUnflattenedList({
+      list: '#resolvedReferenceList',
+    }).outputs({
+      '#unflattenedList': '#entries.artists',
+    }),
+
+    fillMissingListItems({
+      list: '#entries.artistDisplayText',
+      fill: input.value(null),
+    }),
+
+    fillMissingListItems({
+      list: '#entries.annotation',
+      fill: input.value(null),
+    }),
+
+    {
+      dependencies: ['#entries.date'],
+      compute: (continuation, {
+        ['#entries.date']: date,
+      }) => continuation({
+        ['#entries.date']:
+          date.map(date => date ? new Date(date) : null),
+      }),
+    },
+
+    {
+      dependencies: [
+        '#entries.artists',
+        '#entries.artistDisplayText',
+        '#entries.annotation',
+        '#entries.date',
+        '#entries.body',
+      ],
+
+      compute: (continuation, {
+        ['#entries.artists']: artists,
+        ['#entries.artistDisplayText']: artistDisplayText,
+        ['#entries.annotation']: annotation,
+        ['#entries.date']: date,
+        ['#entries.body']: body,
+      }) => continuation({
+        ['#parsedCommentaryEntries']:
+          stitchArrays({
+            artists,
+            artistDisplayText,
+            annotation,
+            date,
+            body,
+          }),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/withResolvedContribs.js b/src/data/composite/wiki-data/withResolvedContribs.js
new file mode 100644
index 0000000..77b0f96
--- /dev/null
+++ b/src/data/composite/wiki-data/withResolvedContribs.js
@@ -0,0 +1,76 @@
+// Resolves the contribsByRef contained in the provided dependency,
+// providing (named by the second argument) the result. "Resolving"
+// means mapping the "who" reference of each contribution to an artist
+// object, and filtering out those whose "who" doesn't match any artist.
+
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+import {filterMultipleArrays, stitchArrays} from '#sugar';
+import {is, isContributionList} from '#validators';
+
+import {
+  raiseOutputWithoutDependency,
+} from '#composite/control-flow';
+
+import {
+  withPropertiesFromList,
+} from '#composite/data';
+
+import withResolvedReferenceList from './withResolvedReferenceList.js';
+
+export default templateCompositeFrom({
+  annotation: `withResolvedContribs`,
+
+  inputs: {
+    from: input({
+      validate: isContributionList,
+      acceptsNull: true,
+    }),
+
+    notFoundMode: input({
+      validate: is('exit', 'filter', 'null'),
+      defaultValue: 'null',
+    }),
+  },
+
+  outputs: ['#resolvedContribs'],
+
+  steps: () => [
+    raiseOutputWithoutDependency({
+      dependency: input('from'),
+      mode: input.value('empty'),
+      output: input.value({
+        ['#resolvedContribs']: [],
+      }),
+    }),
+
+    withPropertiesFromList({
+      list: input('from'),
+      properties: input.value(['who', 'what']),
+      prefix: input.value('#contribs'),
+    }),
+
+    withResolvedReferenceList({
+      list: '#contribs.who',
+      data: 'artistData',
+      find: input.value(find.artist),
+      notFoundMode: input('notFoundMode'),
+    }).outputs({
+      ['#resolvedReferenceList']: '#contribs.who',
+    }),
+
+    {
+      dependencies: ['#contribs.who', '#contribs.what'],
+
+      compute(continuation, {
+        ['#contribs.who']: who,
+        ['#contribs.what']: what,
+      }) {
+        filterMultipleArrays(who, what, (who, _what) => who);
+        return continuation({
+          ['#resolvedContribs']: stitchArrays({who, what}),
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/withResolvedReference.js b/src/data/composite/wiki-data/withResolvedReference.js
new file mode 100644
index 0000000..ea71707
--- /dev/null
+++ b/src/data/composite/wiki-data/withResolvedReference.js
@@ -0,0 +1,57 @@
+// Resolves a reference by using the provided find function to match it
+// within the provided thingData dependency. This will early exit if the
+// data dependency is null. Otherwise, the data object is provided on the
+// output dependency, or null, if the reference doesn't match anything or
+// itself was null to begin with.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {
+  exitWithoutDependency,
+  raiseOutputWithoutDependency,
+} from '#composite/control-flow';
+
+import inputWikiData from './inputWikiData.js';
+
+export default templateCompositeFrom({
+  annotation: `withResolvedReference`,
+
+  inputs: {
+    ref: input({type: 'string', acceptsNull: true}),
+
+    data: inputWikiData({allowMixedTypes: false}),
+    find: input({type: 'function'}),
+  },
+
+  outputs: ['#resolvedReference'],
+
+  steps: () => [
+    raiseOutputWithoutDependency({
+      dependency: input('ref'),
+      output: input.value({
+        ['#resolvedReference']: null,
+      }),
+    }),
+
+    exitWithoutDependency({
+      dependency: input('data'),
+    }),
+
+    {
+      dependencies: [
+        input('ref'),
+        input('data'),
+        input('find'),
+      ],
+
+      compute: (continuation, {
+        [input('ref')]: ref,
+        [input('data')]: data,
+        [input('find')]: findFunction,
+      }) => continuation({
+        ['#resolvedReference']:
+          findFunction(ref, data, {mode: 'quiet'}) ?? null,
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/withResolvedReferenceList.js b/src/data/composite/wiki-data/withResolvedReferenceList.js
new file mode 100644
index 0000000..1d39e5b
--- /dev/null
+++ b/src/data/composite/wiki-data/withResolvedReferenceList.js
@@ -0,0 +1,101 @@
+// Resolves a list of references, with each reference matched with provided
+// data in the same way as withResolvedReference. This will early exit if the
+// data dependency is null (even if the reference list is empty). By default
+// it will filter out references which don't match, but this can be changed
+// to early exit ({notFoundMode: 'exit'}) or leave null in place ('null').
+
+import {input, templateCompositeFrom} from '#composite';
+import {is, isString, validateArrayItems} from '#validators';
+
+import {
+  exitWithoutDependency,
+  raiseOutputWithoutDependency,
+} from '#composite/control-flow';
+
+import inputWikiData from './inputWikiData.js';
+
+export default templateCompositeFrom({
+  annotation: `withResolvedReferenceList`,
+
+  inputs: {
+    list: input({
+      validate: validateArrayItems(isString),
+      acceptsNull: true,
+    }),
+
+    data: inputWikiData({allowMixedTypes: false}),
+    find: input({type: 'function'}),
+
+    notFoundMode: input({
+      validate: is('exit', 'filter', 'null'),
+      defaultValue: 'filter',
+    }),
+  },
+
+  outputs: ['#resolvedReferenceList'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input('data'),
+      value: input.value([]),
+    }),
+
+    raiseOutputWithoutDependency({
+      dependency: input('list'),
+      mode: input.value('empty'),
+      output: input.value({
+        ['#resolvedReferenceList']: [],
+      }),
+    }),
+
+    {
+      dependencies: [input('list'), input('data'), input('find')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('data')]: data,
+        [input('find')]: findFunction,
+      }) =>
+        continuation({
+          '#matches': list.map(ref => findFunction(ref, data, {mode: 'quiet'})),
+        }),
+    },
+
+    {
+      dependencies: ['#matches'],
+      compute: (continuation, {'#matches': matches}) =>
+        (matches.every(match => match)
+          ? continuation.raiseOutput({
+              ['#resolvedReferenceList']: matches,
+            })
+          : continuation()),
+    },
+
+    {
+      dependencies: ['#matches', input('notFoundMode')],
+      compute(continuation, {
+        ['#matches']: matches,
+        [input('notFoundMode')]: notFoundMode,
+      }) {
+        switch (notFoundMode) {
+          case 'exit':
+            return continuation.exit([]);
+
+          case 'filter':
+            return continuation.raiseOutput({
+              ['#resolvedReferenceList']:
+                matches.filter(match => match),
+            });
+
+          case 'null':
+            return continuation.raiseOutput({
+              ['#resolvedReferenceList']:
+                matches.map(match => match ?? null),
+            });
+
+          default:
+            throw new TypeError(`Expected notFoundMode to be exit, filter, or null`);
+        }
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/withReverseContributionList.js b/src/data/composite/wiki-data/withReverseContributionList.js
new file mode 100644
index 0000000..eccb58b
--- /dev/null
+++ b/src/data/composite/wiki-data/withReverseContributionList.js
@@ -0,0 +1,83 @@
+// Analogous implementation for withReverseReferenceList, for contributions.
+// This is all duplicate code and both should be ported to the same underlying
+// data form later on.
+//
+// This implementation uses a global cache (via WeakMap) to attempt to speed
+// up subsequent similar accesses.
+//
+// This has absolutely not been rigorously tested with altering properties of
+// data objects in a wiki data array which is reused. If a new wiki data array
+// is used, a fresh cache will always be created.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency} from '#composite/control-flow';
+
+import inputWikiData from './inputWikiData.js';
+
+// Mapping of reference list property to WeakMap.
+// Each WeakMap maps a wiki data array to another weak map,
+// which in turn maps each referenced thing to an array of
+// things referencing it.
+const caches = new Map();
+
+export default templateCompositeFrom({
+  annotation: `withReverseContributionList`,
+
+  inputs: {
+    data: inputWikiData({allowMixedTypes: false}),
+    list: input({type: 'string'}),
+  },
+
+  outputs: ['#reverseContributionList'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input('data'),
+      value: input.value([]),
+      mode: input.value('empty'),
+    }),
+
+    {
+      dependencies: [input.myself(), input('data'), input('list')],
+
+      compute: (continuation, {
+        [input.myself()]: myself,
+        [input('data')]: data,
+        [input('list')]: list,
+      }) => {
+        if (!caches.has(list)) {
+          caches.set(list, new WeakMap());
+        }
+
+        const cache = caches.get(list);
+
+        if (!cache.has(data)) {
+          const cacheRecord = new WeakMap();
+
+          for (const referencingThing of data) {
+            const referenceList = referencingThing[list];
+
+            // Destructuring {who} is the only unique part of the
+            // withReverseContributionList implementation, compared to
+            // withReverseReferneceList.
+            for (const {who: referencedThing} of referenceList) {
+              if (cacheRecord.has(referencedThing)) {
+                cacheRecord.get(referencedThing).push(referencingThing);
+              } else {
+                cacheRecord.set(referencedThing, [referencingThing]);
+              }
+            }
+          }
+
+          cache.set(data, cacheRecord);
+        }
+
+        return continuation({
+          ['#reverseContributionList']:
+            cache.get(data).get(myself) ?? [],
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/withReverseReferenceList.js b/src/data/composite/wiki-data/withReverseReferenceList.js
new file mode 100644
index 0000000..2d7a421
--- /dev/null
+++ b/src/data/composite/wiki-data/withReverseReferenceList.js
@@ -0,0 +1,81 @@
+// Check out the info on reverseReferenceList!
+// This is its composable form.
+//
+// This implementation uses a global cache (via WeakMap) to attempt to speed
+// up subsequent similar accesses.
+//
+// This has absolutely not been rigorously tested with altering properties of
+// data objects in a wiki data array which is reused. If a new wiki data array
+// is used, a fresh cache will always be created.
+//
+// Note that this implementation is mirrored in withReverseContributionList,
+// so any changes should be reflected there (until these are combined).
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency} from '#composite/control-flow';
+
+import inputWikiData from './inputWikiData.js';
+
+// Mapping of reference list property to WeakMap.
+// Each WeakMap maps a wiki data array to another weak map,
+// which in turn maps each referenced thing to an array of
+// things referencing it.
+const caches = new Map();
+
+export default templateCompositeFrom({
+  annotation: `withReverseReferenceList`,
+
+  inputs: {
+    data: inputWikiData({allowMixedTypes: false}),
+    list: input({type: 'string'}),
+  },
+
+  outputs: ['#reverseReferenceList'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input('data'),
+      value: input.value([]),
+      mode: input.value('empty'),
+    }),
+
+    {
+      dependencies: [input.myself(), input('data'), input('list')],
+
+      compute: (continuation, {
+        [input.myself()]: myself,
+        [input('data')]: data,
+        [input('list')]: list,
+      }) => {
+        if (!caches.has(list)) {
+          caches.set(list, new WeakMap());
+        }
+
+        const cache = caches.get(list);
+
+        if (!cache.has(data)) {
+          const cacheRecord = new WeakMap();
+
+          for (const referencingThing of data) {
+            const referenceList = referencingThing[list];
+            for (const referencedThing of referenceList) {
+              if (cacheRecord.has(referencedThing)) {
+                cacheRecord.get(referencedThing).push(referencingThing);
+              } else {
+                cacheRecord.set(referencedThing, [referencingThing]);
+              }
+            }
+          }
+
+          cache.set(data, cacheRecord);
+        }
+
+        return continuation({
+          ['#reverseReferenceList']:
+            cache.get(data).get(myself) ?? [],
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/withThingsSortedAlphabetically.js b/src/data/composite/wiki-data/withThingsSortedAlphabetically.js
new file mode 100644
index 0000000..5e85fa6
--- /dev/null
+++ b/src/data/composite/wiki-data/withThingsSortedAlphabetically.js
@@ -0,0 +1,122 @@
+// Sorts a list of live, generic wiki data objects alphabetically.
+// Note that this uses localeCompare but isn't specialized to a particular
+// language; where localization is concerned (in content), a follow-up, locale-
+// specific sort should be performed. But this function does serve to organize
+// a list so same-name entries are beside each other.
+
+import {input, templateCompositeFrom} from '#composite';
+import {compareCaseLessSensitive, normalizeName} from '#sort';
+import {validateWikiData} from '#validators';
+
+import {raiseOutputWithoutDependency} from '#composite/control-flow';
+import {withMappedList, withSortedList, withPropertiesFromList}
+  from '#composite/data';
+
+export default templateCompositeFrom({
+  annotation: `withThingsSortedAlphabetically`,
+
+  inputs: {
+    things: input({validate: validateWikiData}),
+  },
+
+  outputs: ['#sortedThings'],
+
+  steps: () => [
+    raiseOutputWithoutDependency({
+      dependency: input('things'),
+      mode: input.value('empty'),
+      output: input.value({'#sortedThings': []}),
+    }),
+
+    withPropertiesFromList({
+      list: input('things'),
+      properties: input.value(['name', 'directory']),
+    }).outputs({
+      '#list.name': '#names',
+      '#list.directory': '#directories',
+    }),
+
+    withMappedList({
+      list: '#names',
+      map: input.value(normalizeName),
+    }).outputs({
+      '#mappedList': '#normalizedNames',
+    }),
+
+    withSortedList({
+      list: '#normalizedNames',
+      sort: input.value(compareCaseLessSensitive),
+    }).outputs({
+      '#unstableSortIndices': '#normalizedNameSortIndices',
+    }),
+
+    withSortedList({
+      list: '#names',
+      sort: input.value(compareCaseLessSensitive),
+    }).outputs({
+      '#unstableSortIndices': '#nonNormalizedNameSortIndices',
+    }),
+
+    withSortedList({
+      list: '#directories',
+      sort: input.value(compareCaseLessSensitive),
+    }).outputs({
+      '#unstableSortIndices': '#directorySortIndices',
+    }),
+
+    // TODO: No primitive for the next two-three steps, yet...
+
+    {
+      dependencies: [input('things')],
+      compute: (continuation, {
+        [input('things')]: things,
+      }) => continuation({
+        ['#combinedSortIndices']:
+          Array.from(
+            {length: things.length},
+            (_item, index) => index),
+      }),
+    },
+
+    {
+      dependencies: [
+        '#combinedSortIndices',
+        '#normalizedNameSortIndices',
+        '#nonNormalizedNameSortIndices',
+        '#directorySortIndices',
+      ],
+
+      compute: (continuation, {
+        ['#combinedSortIndices']: combined,
+        ['#normalizedNameSortIndices']: normalized,
+        ['#nonNormalizedNameSortIndices']: nonNormalized,
+        ['#directorySortIndices']: directory,
+      }) => continuation({
+        ['#combinedSortIndices']:
+          combined.sort((index1, index2) => {
+            if (normalized[index1] !== normalized[index2])
+              return normalized[index1] - normalized[index2];
+
+            if (nonNormalized[index1] !== nonNormalized[index2])
+              return nonNormalized[index1] - nonNormalized[index2];
+
+            if (directory[index1] !== directory[index2])
+              return directory[index1] - directory[index2];
+
+            return 0;
+          }),
+      }),
+    },
+
+    {
+      dependencies: [input('things'), '#combinedSortIndices'],
+      compute: (continuation, {
+        [input('things')]: things,
+        ['#combinedSortIndices']: combined,
+      }) => continuation({
+        ['#sortedThings']:
+          combined.map(index => things[index]),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/withUniqueReferencingThing.js b/src/data/composite/wiki-data/withUniqueReferencingThing.js
new file mode 100644
index 0000000..ce04f83
--- /dev/null
+++ b/src/data/composite/wiki-data/withUniqueReferencingThing.js
@@ -0,0 +1,52 @@
+// Like withReverseReferenceList, but this is specifically for special "unique"
+// references, meaning this thing is referenced by exactly one or zero things
+// in the data list.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency, raiseOutputWithoutDependency}
+  from '#composite/control-flow';
+
+import inputWikiData from './inputWikiData.js';
+import withReverseReferenceList from './withReverseReferenceList.js';
+
+export default templateCompositeFrom({
+  annotation: `withUniqueReferencingThing`,
+
+  inputs: {
+    data: inputWikiData({allowMixedTypes: false}),
+    list: input({type: 'string'}),
+  },
+
+  outputs: ['#uniqueReferencingThing'],
+
+  steps: () => [
+    // withReverseRefernceList does this check too, but it early exits with
+    // an empty array. That's no good here!
+    exitWithoutDependency({
+      dependency: input('data'),
+      mode: input.value('empty'),
+    }),
+
+    withReverseReferenceList({
+      data: input('data'),
+      list: input('list'),
+    }),
+
+    raiseOutputWithoutDependency({
+      dependency: '#reverseReferenceList',
+      mode: input.value('empty'),
+      output: input.value({'#uniqueReferencingThing': null}),
+    }),
+
+    {
+      dependencies: ['#reverseReferenceList'],
+      compute: (continuation, {
+        ['#reverseReferenceList']: reverseReferenceList,
+      }) => continuation({
+        ['#uniqueReferencingThing']:
+          reverseReferenceList[0],
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-properties/additionalFiles.js b/src/data/composite/wiki-properties/additionalFiles.js
new file mode 100644
index 0000000..6760527
--- /dev/null
+++ b/src/data/composite/wiki-properties/additionalFiles.js
@@ -0,0 +1,30 @@
+// This is a somewhat more involved data structure - it's for additional
+// or "bonus" files associated with albums or tracks (or anything else).
+// It's got this form:
+//
+//   [
+//     {title: 'Booklet', files: ['Booklet.pdf']},
+//     {
+//       title: 'Wallpaper',
+//       description: 'Cool Wallpaper!',
+//       files: ['1440x900.png', '1920x1080.png']
+//     },
+//     {title: 'Alternate Covers', description: null, files: [...]},
+//     ...
+//   ]
+//
+
+import {isAdditionalFileList} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isAdditionalFileList},
+    expose: {
+      transform: (additionalFiles) =>
+        additionalFiles ?? [],
+    },
+  };
+}
diff --git a/src/data/composite/wiki-properties/additionalNameList.js b/src/data/composite/wiki-properties/additionalNameList.js
new file mode 100644
index 0000000..c5971d4
--- /dev/null
+++ b/src/data/composite/wiki-properties/additionalNameList.js
@@ -0,0 +1,14 @@
+// A list of additional names! These can be used for a variety of purposes,
+// e.g. providing extra searchable titles, localizations, romanizations or
+// original titles, and so on. Each item has a name and, optionally, a
+// descriptive annotation.
+
+import {isAdditionalNameList} from '#validators';
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isAdditionalNameList},
+    expose: {transform: value => value ?? []},
+  };
+}
diff --git a/src/data/composite/wiki-properties/color.js b/src/data/composite/wiki-properties/color.js
new file mode 100644
index 0000000..1bc9888
--- /dev/null
+++ b/src/data/composite/wiki-properties/color.js
@@ -0,0 +1,12 @@
+// A color! This'll be some CSS-ready value.
+
+import {isColor} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isColor},
+  };
+}
diff --git a/src/data/composite/wiki-properties/commentary.js b/src/data/composite/wiki-properties/commentary.js
new file mode 100644
index 0000000..cd6b7ac
--- /dev/null
+++ b/src/data/composite/wiki-properties/commentary.js
@@ -0,0 +1,30 @@
+// Artist commentary! Generally present on tracks and albums.
+
+import {input, templateCompositeFrom} from '#composite';
+import {isCommentary} from '#validators';
+
+import {exitWithoutDependency, exposeDependency}
+  from '#composite/control-flow';
+import {withParsedCommentaryEntries} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `commentary`,
+
+  compose: false,
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input.updateValue({validate: isCommentary}),
+      mode: input.value('falsy'),
+      value: input.value(null),
+    }),
+
+    withParsedCommentaryEntries({
+      from: input.updateValue(),
+    }),
+
+    exposeDependency({
+      dependency: '#parsedCommentaryEntries',
+    }),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/commentatorArtists.js b/src/data/composite/wiki-properties/commentatorArtists.js
new file mode 100644
index 0000000..c5c1476
--- /dev/null
+++ b/src/data/composite/wiki-properties/commentatorArtists.js
@@ -0,0 +1,49 @@
+// List of artists referenced in commentary entries.
+// This is mostly useful for credits and listings on artist pages.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency, exposeDependency}
+  from '#composite/control-flow';
+import {withFlattenedList, withPropertyFromList, withUniqueItemsOnly}
+  from '#composite/data';
+import {withParsedCommentaryEntries} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `commentatorArtists`,
+
+  compose: false,
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: 'commentary',
+      mode: input.value('falsy'),
+      value: input.value([]),
+    }),
+
+    withParsedCommentaryEntries({
+      from: 'commentary',
+    }),
+
+    withPropertyFromList({
+      list: '#parsedCommentaryEntries',
+      property: input.value('artists'),
+    }).outputs({
+      '#parsedCommentaryEntries.artists': '#artistLists',
+    }),
+
+    withFlattenedList({
+      list: '#artistLists',
+    }).outputs({
+      '#flattenedList': '#artists',
+    }),
+
+    withUniqueItemsOnly({
+      list: '#artists',
+    }),
+
+    exposeDependency({
+      dependency: '#artists',
+    }),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/contentString.js b/src/data/composite/wiki-properties/contentString.js
new file mode 100644
index 0000000..b0e8244
--- /dev/null
+++ b/src/data/composite/wiki-properties/contentString.js
@@ -0,0 +1,15 @@
+// String type that's slightly more specific than simpleString. If the
+// property is a generic piece of human-reading content, this adds some
+// useful valiation on top of simpleString - but still check if more
+// particular properties like `name` are more appropriate.
+//
+// This type adapts validation for single- and multiline content.
+
+import {isContentString} from '#validators';
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isContentString},
+  };
+}
diff --git a/src/data/composite/wiki-properties/contribsPresent.js b/src/data/composite/wiki-properties/contribsPresent.js
new file mode 100644
index 0000000..24f302a
--- /dev/null
+++ b/src/data/composite/wiki-properties/contribsPresent.js
@@ -0,0 +1,30 @@
+// Nice 'n simple shorthand for an exposed-only flag which is true when any
+// contributions are present in the specified property.
+
+import {input, templateCompositeFrom} from '#composite';
+import {isContributionList} from '#validators';
+
+import {exposeDependency, withResultOfAvailabilityCheck}
+  from '#composite/control-flow';
+
+export default templateCompositeFrom({
+  annotation: `contribsPresent`,
+
+  compose: false,
+
+  inputs: {
+    contribs: input.staticDependency({
+      validate: isContributionList,
+      acceptsNull: true,
+    }),
+  },
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('contribs'),
+      mode: input.value('empty'),
+    }),
+
+    exposeDependency({dependency: '#availability'}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/contributionList.js b/src/data/composite/wiki-properties/contributionList.js
new file mode 100644
index 0000000..8fde2ca
--- /dev/null
+++ b/src/data/composite/wiki-properties/contributionList.js
@@ -0,0 +1,35 @@
+// Strong 'n sturdy contribution list, rolling a list of references (provided
+// as this property's update value) and the resolved results (as get exposed)
+// into one property. Update value will look something like this:
+//
+//   [
+//     {who: 'Artist Name', what: 'Viola'},
+//     {who: 'artist:john-cena', what: null},
+//     ...
+//   ]
+//
+// ...typically as processed from YAML, spreadsheet, or elsewhere.
+// Exposes as the same, but with the "who" replaced with matches found in
+// artistData - which means this always depends on an `artistData` property
+// also existing on this object!
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {isContributionList} from '#validators';
+
+import {exposeConstant, exposeDependencyOrContinue} from '#composite/control-flow';
+import {withResolvedContribs} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `contributionList`,
+
+  compose: false,
+
+  update: {validate: isContributionList},
+
+  steps: () => [
+    withResolvedContribs({from: input.updateValue()}),
+    exposeDependencyOrContinue({dependency: '#resolvedContribs'}),
+    exposeConstant({value: input.value([])}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/dimensions.js b/src/data/composite/wiki-properties/dimensions.js
new file mode 100644
index 0000000..57a0127
--- /dev/null
+++ b/src/data/composite/wiki-properties/dimensions.js
@@ -0,0 +1,13 @@
+// Plain ol' image dimensions. This is a two-item array of positive integers,
+// corresponding to width and height respectively.
+
+import {isDimensions} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDimensions},
+  };
+}
diff --git a/src/data/composite/wiki-properties/directory.js b/src/data/composite/wiki-properties/directory.js
new file mode 100644
index 0000000..0b2181c
--- /dev/null
+++ b/src/data/composite/wiki-properties/directory.js
@@ -0,0 +1,23 @@
+// The all-encompassing "directory" property, used as the unique identifier for
+// almost any data object. Also corresponds to a part of the URL which pages of
+// such objects are visited at.
+
+import {isDirectory} from '#validators';
+import {getKebabCase} from '#wiki-data';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDirectory},
+    expose: {
+      dependencies: ['name'],
+      transform(directory, {name}) {
+        if (directory === null && name === null) return null;
+        else if (directory === null) return getKebabCase(name);
+        else return directory;
+      },
+    },
+  };
+}
diff --git a/src/data/composite/wiki-properties/duration.js b/src/data/composite/wiki-properties/duration.js
new file mode 100644
index 0000000..827f282
--- /dev/null
+++ b/src/data/composite/wiki-properties/duration.js
@@ -0,0 +1,13 @@
+// Duration! This is a number of seconds, possibly floating point, always
+// at minimum zero.
+
+import {isDuration} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDuration},
+  };
+}
diff --git a/src/data/composite/wiki-properties/externalFunction.js b/src/data/composite/wiki-properties/externalFunction.js
new file mode 100644
index 0000000..c388da6
--- /dev/null
+++ b/src/data/composite/wiki-properties/externalFunction.js
@@ -0,0 +1,11 @@
+// External function. These should only be used as dependencies for other
+// properties, so they're left unexposed.
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true},
+    update: {validate: (t) => typeof t === 'function'},
+  };
+}
diff --git a/src/data/composite/wiki-properties/fileExtension.js b/src/data/composite/wiki-properties/fileExtension.js
new file mode 100644
index 0000000..c926fa8
--- /dev/null
+++ b/src/data/composite/wiki-properties/fileExtension.js
@@ -0,0 +1,13 @@
+// A file extension! Or the default, if provided when calling this.
+
+import {isFileExtension} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function(defaultFileExtension = null) {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isFileExtension},
+    expose: {transform: (value) => value ?? defaultFileExtension},
+  };
+}
diff --git a/src/data/composite/wiki-properties/flag.js b/src/data/composite/wiki-properties/flag.js
new file mode 100644
index 0000000..076e663
--- /dev/null
+++ b/src/data/composite/wiki-properties/flag.js
@@ -0,0 +1,19 @@
+// Straightforward flag descriptor for a variety of property purposes.
+// Provide a default value, true or false!
+
+import {isBoolean} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+// TODO: The description is a lie. This defaults to false. Bad.
+
+export default function(defaultValue = false) {
+  if (typeof defaultValue !== 'boolean') {
+    throw new TypeError(`Always set explicit defaults for flags!`);
+  }
+
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isBoolean, default: defaultValue},
+  };
+}
diff --git a/src/data/composite/wiki-properties/index.js b/src/data/composite/wiki-properties/index.js
new file mode 100644
index 0000000..89cb683
--- /dev/null
+++ b/src/data/composite/wiki-properties/index.js
@@ -0,0 +1,28 @@
+// #composite/wiki-properties
+//
+// Entries here may depend on entries in #composite/control-flow,
+// #composite/data, and #composite/wiki-data.
+
+export {default as additionalFiles} from './additionalFiles.js';
+export {default as additionalNameList} from './additionalNameList.js';
+export {default as color} from './color.js';
+export {default as commentary} from './commentary.js';
+export {default as commentatorArtists} from './commentatorArtists.js';
+export {default as contentString} from './contentString.js';
+export {default as contribsPresent} from './contribsPresent.js';
+export {default as contributionList} from './contributionList.js';
+export {default as dimensions} from './dimensions.js';
+export {default as directory} from './directory.js';
+export {default as duration} from './duration.js';
+export {default as externalFunction} from './externalFunction.js';
+export {default as fileExtension} from './fileExtension.js';
+export {default as flag} from './flag.js';
+export {default as name} from './name.js';
+export {default as referenceList} from './referenceList.js';
+export {default as reverseContributionList} from './reverseContributionList.js';
+export {default as reverseReferenceList} from './reverseReferenceList.js';
+export {default as simpleDate} from './simpleDate.js';
+export {default as simpleString} from './simpleString.js';
+export {default as singleReference} from './singleReference.js';
+export {default as urls} from './urls.js';
+export {default as wikiData} from './wikiData.js';
diff --git a/src/data/composite/wiki-properties/name.js b/src/data/composite/wiki-properties/name.js
new file mode 100644
index 0000000..5146488
--- /dev/null
+++ b/src/data/composite/wiki-properties/name.js
@@ -0,0 +1,11 @@
+// A wiki data object's name! Its directory (i.e. unique identifier) will be
+// computed based on this value if not otherwise specified.
+
+import {isName} from '#validators';
+
+export default function(defaultName) {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isName, default: defaultName},
+  };
+}
diff --git a/src/data/composite/wiki-properties/referenceList.js b/src/data/composite/wiki-properties/referenceList.js
new file mode 100644
index 0000000..af634a6
--- /dev/null
+++ b/src/data/composite/wiki-properties/referenceList.js
@@ -0,0 +1,45 @@
+// Stores and exposes a list of references to other data objects; all items
+// must be references to the same type, which is specified on the class input.
+//
+// See also:
+//  - singleReference
+//  - withResolvedReferenceList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {isThingClass, validateReferenceList} from '#validators';
+
+import {exposeDependency} from '#composite/control-flow';
+import {inputWikiData, withResolvedReferenceList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `referenceList`,
+
+  compose: false,
+
+  inputs: {
+    class: input.staticValue({validate: isThingClass}),
+
+    data: inputWikiData({allowMixedTypes: false}),
+
+    find: input({type: 'function'}),
+  },
+
+  update: ({
+    [input.staticValue('class')]: thingClass,
+  }) => ({
+    validate:
+      validateReferenceList(
+        thingClass[Symbol.for('Thing.referenceType')]),
+  }),
+
+  steps: () => [
+    withResolvedReferenceList({
+      list: input.updateValue(),
+      data: input('data'),
+      find: input('find'),
+    }),
+
+    exposeDependency({dependency: '#resolvedReferenceList'}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/reverseContributionList.js b/src/data/composite/wiki-properties/reverseContributionList.js
new file mode 100644
index 0000000..7f3f9c8
--- /dev/null
+++ b/src/data/composite/wiki-properties/reverseContributionList.js
@@ -0,0 +1,24 @@
+import {input, templateCompositeFrom} from '#composite';
+
+import {exposeDependency} from '#composite/control-flow';
+import {inputWikiData, withReverseContributionList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `reverseContributionList`,
+
+  compose: false,
+
+  inputs: {
+    data: inputWikiData({allowMixedTypes: false}),
+    list: input({type: 'string'}),
+  },
+
+  steps: () => [
+    withReverseContributionList({
+      data: input('data'),
+      list: input('list'),
+    }),
+
+    exposeDependency({dependency: '#reverseContributionList'}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/reverseReferenceList.js b/src/data/composite/wiki-properties/reverseReferenceList.js
new file mode 100644
index 0000000..84ba67d
--- /dev/null
+++ b/src/data/composite/wiki-properties/reverseReferenceList.js
@@ -0,0 +1,30 @@
+// Neat little shortcut for "reversing" the reference lists stored on other
+// things - for example, tracks specify a "referenced tracks" property, and
+// you would use this to compute a corresponding "referenced *by* tracks"
+// property. Naturally, the passed ref list property is of the things in the
+// wiki data provided, not the requesting Thing itself.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exposeDependency} from '#composite/control-flow';
+import {inputWikiData, withReverseReferenceList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `reverseReferenceList`,
+
+  compose: false,
+
+  inputs: {
+    data: inputWikiData({allowMixedTypes: false}),
+    list: input({type: 'string'}),
+  },
+
+  steps: () => [
+    withReverseReferenceList({
+      data: input('data'),
+      list: input('list'),
+    }),
+
+    exposeDependency({dependency: '#reverseReferenceList'}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/simpleDate.js b/src/data/composite/wiki-properties/simpleDate.js
new file mode 100644
index 0000000..f08d832
--- /dev/null
+++ b/src/data/composite/wiki-properties/simpleDate.js
@@ -0,0 +1,14 @@
+// General date type, used as the descriptor for a bunch of properties.
+// This isn't dynamic though - it won't inherit from a date stored on
+// another object, for example.
+
+import {isDate} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDate},
+  };
+}
diff --git a/src/data/composite/wiki-properties/simpleString.js b/src/data/composite/wiki-properties/simpleString.js
new file mode 100644
index 0000000..7bf317a
--- /dev/null
+++ b/src/data/composite/wiki-properties/simpleString.js
@@ -0,0 +1,12 @@
+// General string type. This should probably generally be avoided in favor
+// of more specific validation, but using it makes it easy to find where we
+// might want to improve later, and it's a useful shorthand meanwhile.
+
+import {isString} from '#validators';
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isString},
+  };
+}
diff --git a/src/data/composite/wiki-properties/singleReference.js b/src/data/composite/wiki-properties/singleReference.js
new file mode 100644
index 0000000..db4fc9f
--- /dev/null
+++ b/src/data/composite/wiki-properties/singleReference.js
@@ -0,0 +1,46 @@
+// Stores and exposes one connection, or reference, to another data object.
+// The reference must be to a specific type, which is specified on the class
+// input.
+//
+// See also:
+//  - referenceList
+//  - withResolvedReference
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {isThingClass, validateReference} from '#validators';
+
+import {exposeDependency} from '#composite/control-flow';
+import {inputWikiData, withResolvedReference} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `singleReference`,
+
+  compose: false,
+
+  inputs: {
+    class: input.staticValue({validate: isThingClass}),
+
+    find: input({type: 'function'}),
+
+    data: inputWikiData({allowMixedTypes: false}),
+  },
+
+  update: ({
+    [input.staticValue('class')]: thingClass,
+  }) => ({
+    validate:
+      validateReference(
+        thingClass[Symbol.for('Thing.referenceType')]),
+  }),
+
+  steps: () => [
+    withResolvedReference({
+      ref: input.updateValue(),
+      data: input('data'),
+      find: input('find'),
+    }),
+
+    exposeDependency({dependency: '#resolvedReference'}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/urls.js b/src/data/composite/wiki-properties/urls.js
new file mode 100644
index 0000000..3160a0b
--- /dev/null
+++ b/src/data/composite/wiki-properties/urls.js
@@ -0,0 +1,14 @@
+// A list of URLs! This will always be present on the data object, even if set
+// to an empty array or null.
+
+import {isURL, validateArrayItems} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: validateArrayItems(isURL)},
+    expose: {transform: value => value ?? []},
+  };
+}
diff --git a/src/data/composite/wiki-properties/wikiData.js b/src/data/composite/wiki-properties/wikiData.js
new file mode 100644
index 0000000..3bebed3
--- /dev/null
+++ b/src/data/composite/wiki-properties/wikiData.js
@@ -0,0 +1,27 @@
+// General purpose wiki data constructor, for properties like artistData,
+// trackData, etc.
+
+import {input, templateCompositeFrom} from '#composite';
+import {isThingClass, validateWikiData} from '#validators';
+
+export default templateCompositeFrom({
+  annotation: `wikiData`,
+
+  compose: false,
+
+  inputs: {
+    class: input.staticValue({validate: isThingClass}),
+  },
+
+  update: ({
+    [input.staticValue('class')]: thingClass,
+  }) => ({
+    validate:
+      validateWikiData({
+        referenceType:
+          thingClass[Symbol.for('Thing.referenceType')],
+      }),
+  }),
+
+  steps: () => [],
+});
diff --git a/src/data/language.js b/src/data/language.js
new file mode 100644
index 0000000..a149e19
--- /dev/null
+++ b/src/data/language.js
@@ -0,0 +1,331 @@
+import EventEmitter from 'node:events';
+import {readFile} from 'node:fs/promises';
+import path from 'node:path';
+import {fileURLToPath} from 'node:url';
+
+import chokidar from 'chokidar';
+import he from 'he'; // It stands for "HTML Entities", apparently. Cursed.
+import yaml from 'js-yaml';
+
+import {annotateError, annotateErrorWithFile, showAggregate, withAggregate}
+  from '#aggregate';
+import {externalLinkSpec} from '#external-links';
+import {colors, logWarn} from '#cli';
+import {splitKeys, withEntries} from '#sugar';
+import T from '#things';
+
+const {Language} = T;
+
+export const DEFAULT_STRINGS_FILE = 'strings-default.yaml';
+
+export const internalDefaultStringsFile =
+  path.resolve(
+    path.dirname(fileURLToPath(import.meta.url)),
+    '../',
+    DEFAULT_STRINGS_FILE);
+
+export function processLanguageSpec(spec, {existingCode = null} = {}) {
+  const {
+    'meta.languageCode': code,
+    'meta.languageName': name,
+
+    'meta.languageIntlCode': intlCode = null,
+    'meta.hidden': hidden = false,
+
+    ...strings
+  } = spec;
+
+  withAggregate({message: `Errors validating language spec`}, ({push}) => {
+    if (!code) {
+      push(new Error(`Missing language code`));
+    }
+
+    if (!name) {
+      push(new Error(`Missing language name`));
+    }
+
+    if (code && existingCode && code !== existingCode) {
+      push(new Error(`Language code (${code}) doesn't match previous value\n(You'll have to reload hsmusic to load this)`));
+    }
+  });
+
+  return {code, intlCode, name, hidden, strings};
+}
+
+export function flattenLanguageSpec(spec) {
+  const recursive = (keyPath, value) =>
+    (typeof value === 'object'
+      ? Object.assign({}, ...
+          Object.entries(value)
+            .map(([key, value]) =>
+              (key === '_'
+                ? {[keyPath]: value}
+                : recursive(
+                    (keyPath ? `${keyPath}.${key}` : key),
+                    value))))
+      : {[keyPath]: value});
+
+  return recursive('', spec);
+}
+
+export function unflattenLanguageSpec(flat, reference) {
+  const setNestedProp = (obj, key, value) => {
+    const recursive = (o, k) => {
+      if (k.length === 1) {
+        o[k[0]] = value;
+        return;
+      }
+
+      if (typeof o[k[0]] === 'undefined') {
+        o[k[0]] = {};
+      } else if (typeof o[k[0]] === 'string') {
+        o[k[0]] = {_: o[k[0]]};
+      }
+
+      recursive(o[k[0]], k.slice(1));
+    };
+
+    return recursive(obj, splitKeys(key));
+  };
+
+  const walkEntries = (ownNode, refNode) => {
+    const recursive = (refKeys, ownNode, refNode) => {
+      const [firstKey, ...restKeys] = refKeys;
+
+      if (typeof ownNode[firstKey] === 'undefined') {
+        return undefined;
+      }
+
+      const result =
+        (refKeys.length === 1
+          ? walkEntry(ownNode[firstKey], refNode)
+          : recursive(restKeys, ownNode[firstKey], refNode));
+
+      if (typeof result === 'undefined') {
+        return undefined;
+      }
+
+      if (typeof result === 'string') {
+        delete ownNode[firstKey];
+        return {[firstKey]: result};
+      }
+
+      if (refKeys.length > 1) {
+        return withEntries(result, entries =>
+          entries.map(([key, value]) => [`${firstKey}.${key}`, value]));
+      } else {
+        return {[firstKey]: result};
+      }
+    };
+
+    let mapped;
+
+    for (const [key, value] of Object.entries(refNode)) {
+      const result = recursive(splitKeys(key), ownNode, refNode[key]);
+      if (!result) continue;
+      if (!mapped) mapped = {};
+      Object.assign(mapped, result);
+    }
+
+    return mapped;
+  };
+
+  const walkEntry = (ownNode, refNode) => {
+    if (
+      typeof ownNode === 'object' &&
+      typeof refNode === 'object'
+    ) {
+      return walkEntries(ownNode, refNode);
+    }
+
+    if (
+      typeof ownNode === 'string' &&
+      typeof refNode === 'object' &&
+      typeof refNode._ === 'string'
+    ) {
+      return {_: ownNode};
+    }
+
+    if (
+      typeof ownNode === 'object' &&
+      typeof refNode === 'string' &&
+      typeof ownNode._ === 'string'
+    ) {
+      return ownNode._;
+    }
+
+    if (
+      typeof ownNode === 'string' &&
+      typeof refNode === 'string'
+    ) {
+      return ownNode;
+    }
+
+    return undefined;
+  };
+
+  const clean = node => {
+    if (typeof node === 'string') {
+      return node;
+    }
+
+    const entries = Object.entries(node);
+    if (entries.length === 0) {
+      return undefined;
+    }
+
+    let results;
+    for (const [key, value] of entries) {
+      const cleanValue = clean(value);
+      if (typeof cleanValue === 'undefined') continue;
+      if (!results) results = {};
+      results[key] = cleanValue;
+    }
+
+    return results;
+  };
+
+  const storage = {};
+  for (const [key, value] of Object.entries(flat)) {
+    setNestedProp(storage, key, value);
+  }
+
+  const rootResult = walkEntries(storage, reference);
+  const spec = rootResult ?? {};
+
+  const unmapped = clean(storage);
+  if (unmapped) {
+    spec['meta.unmapped'] = unmapped;
+  }
+
+  return spec;
+}
+
+async function processLanguageSpecFromFile(file, processLanguageSpecOpts) {
+  let contents;
+
+  try {
+    contents = await readFile(file, 'utf-8');
+  } catch (caughtError) {
+    throw annotateError(
+      new Error(`Failed to read language file`, {cause: caughtError}),
+      error => annotateErrorWithFile(error, file));
+  }
+
+  let rawSpec;
+  let parseLanguage;
+
+  try {
+    if (path.extname(file) === '.yaml') {
+      parseLanguage = 'YAML';
+      rawSpec = yaml.load(contents);
+    } else {
+      parseLanguage = 'JSON';
+      rawSpec = JSON.parse(contents);
+    }
+  } catch (caughtError) {
+    throw annotateError(
+      new Error(`Failed to parse language file as valid ${parseLanguage}`, {cause: caughtError}),
+      error => annotateErrorWithFile(error, file));
+  }
+
+  const flattenedSpec = flattenLanguageSpec(rawSpec);
+
+  try {
+    return processLanguageSpec(flattenedSpec, processLanguageSpecOpts);
+  } catch (caughtError) {
+    throw annotateErrorWithFile(caughtError, file);
+  }
+}
+
+export function initializeLanguageObject() {
+  const language = new Language();
+
+  language.escapeHTML = string =>
+    he.encode(string, {useNamedReferences: true});
+
+  language.externalLinkSpec = externalLinkSpec;
+
+  return language;
+}
+
+export async function processLanguageFile(file) {
+  const language = initializeLanguageObject();
+  const properties = await processLanguageSpecFromFile(file);
+  return Object.assign(language, properties);
+}
+
+export function watchLanguageFile(file, {
+  logging = true,
+} = {}) {
+  const basename = path.basename(file);
+
+  const events = new EventEmitter();
+  const language = initializeLanguageObject();
+
+  let emittedReady = false;
+  let successfullyAppliedLanguage = false;
+
+  Object.assign(events, {language, close});
+
+  const watcher = chokidar.watch(file);
+  watcher.on('change', () => handleFileUpdated());
+
+  setImmediate(handleFileUpdated);
+
+  return events;
+
+  async function close() {
+    return watcher.close();
+  }
+
+  function checkReadyConditions() {
+    if (emittedReady) return;
+    if (!successfullyAppliedLanguage) return;
+
+    events.emit('ready');
+    emittedReady = true;
+  }
+
+  async function handleFileUpdated() {
+    let properties;
+
+    try {
+      properties = await processLanguageSpecFromFile(file, {
+        existingCode:
+          (successfullyAppliedLanguage
+            ? language.code
+            : null),
+      });
+    } catch (error) {
+      events.emit('error', error);
+
+      if (logging) {
+        const label =
+          (successfullyAppliedLanguage
+            ? `${language.name} (${language.code})`
+            : basename);
+
+        if (successfullyAppliedLanguage) {
+          logWarn`Failed to load language ${label} - using existing version`;
+        } else {
+          logWarn`Failed to load language ${label} - no prior version loaded`;
+        }
+        showAggregate(error, {showTraces: false});
+      }
+
+      return;
+    }
+
+    Object.assign(language, properties);
+    successfullyAppliedLanguage = true;
+
+    if (logging && emittedReady) {
+      const timestamp = new Date().toLocaleString('en-US', {timeStyle: 'medium'});
+      console.log(colors.green(`[${timestamp}] Updated language ${language.name} (${language.code})`));
+    }
+
+    events.emit('update');
+    checkReadyConditions();
+  }
+}
diff --git a/src/data/patches.js b/src/data/patches.js
index 3ed4fad..feeaf39 100644
--- a/src/data/patches.js
+++ b/src/data/patches.js
@@ -1,370 +1,386 @@
 // --> Patch
 
 export class Patch {
-    static INPUT_NONE = 0;
-    static INPUT_CONSTANT = 1;
-    static INPUT_DIRECT_CONNECTION = 2;
-    static INPUT_MANAGED_CONNECTION = 3;
+  static INPUT_NONE = 0;
+  static INPUT_CONSTANT = 1;
+  static INPUT_DIRECT_CONNECTION = 2;
+  static INPUT_MANAGED_CONNECTION = 3;
 
-    static INPUT_UNAVAILABLE = 0;
-    static INPUT_AVAILABLE = 1;
+  static INPUT_UNAVAILABLE = 0;
+  static INPUT_AVAILABLE = 1;
 
-    static OUTPUT_UNAVAILABLE = 0;
-    static OUTPUT_AVAILABLE = 1;
+  static OUTPUT_UNAVAILABLE = 0;
+  static OUTPUT_AVAILABLE = 1;
 
-    static inputNames = []; inputNames = null;
-    static outputNames = []; outputNames = null;
+  static inputNames = [];
+  inputNames = null;
+  static outputNames = [];
+  outputNames = null;
 
-    manager = null;
-    inputs = Object.create(null);
+  manager = null;
+  inputs = Object.create(null);
 
-    constructor({
-        manager,
+  constructor({
+    manager,
 
-        inputNames,
-        outputNames,
+    inputNames,
+    outputNames,
 
-        inputs,
-    } = {}) {
-        this.inputNames = inputNames ?? this.constructor.inputNames;
-        this.outputNames = outputNames ?? this.constructor.outputNames;
+    inputs,
+  } = {}) {
+    this.inputNames = inputNames ?? this.constructor.inputNames;
+    this.outputNames = outputNames ?? this.constructor.outputNames;
 
-        manager?.addManagedPatch(this);
+    manager?.addManagedPatch(this);
 
-        if (inputs) {
-            Object.assign(this.inputs, inputs);
-        }
-
-        this.initializeInputs();
+    if (inputs) {
+      Object.assign(this.inputs, inputs);
     }
 
-    initializeInputs() {
-        for (const inputName of this.inputNames) {
-            if (!this.inputs[inputName]) {
-                this.inputs[inputName] = [Patch.INPUT_NONE];
-            }
-        }
-    }
+    this.initializeInputs();
+  }
 
-    computeInputs() {
-        const inputs = Object.create(null);
-
-        for (const inputName of this.inputNames) {
-            const input = this.inputs[inputName];
-            switch (input[0]) {
-                case Patch.INPUT_NONE:
-                    inputs[inputName] = [Patch.INPUT_UNAVAILABLE];
-                    break;
-
-                case Patch.INPUT_CONSTANT:
-                    inputs[inputName] = [Patch.INPUT_AVAILABLE, input[1]];
-                    break;
-
-                case Patch.INPUT_DIRECT_CONNECTION: {
-                    const patch = input[1];
-                    const outputName = input[2];
-                    const output = patch.computeOutputs()[outputName];
-                    switch (output[0]) {
-                        case Patch.OUTPUT_UNAVAILABLE:
-                            inputs[inputName] = [Patch.INPUT_UNAVAILABLE];
-                            break;
-                        case Patch.OUTPUT_AVAILABLE:
-                            inputs[inputName] = [Patch.INPUT_AVAILABLE, output[1]];
-                            break;
-                    }
-                    throw new Error('Unreachable');
-                }
-
-                case Patch.INPUT_MANAGED_CONNECTION: {
-                    if (!this.manager) {
-                        inputs[inputName] = [Patch.INPUT_UNAVAILABLE];
-                        break;
-                    }
-
-                    inputs[inputName] = this.manager.getManagedInput(input[1]);
-                    break;
-                }
-            }
+  initializeInputs() {
+    for (const inputName of this.inputNames) {
+      if (!this.inputs[inputName]) {
+        this.inputs[inputName] = [Patch.INPUT_NONE];
+      }
+    }
+  }
+
+  computeInputs() {
+    const inputs = Object.create(null);
+
+    for (const inputName of this.inputNames) {
+      const input = this.inputs[inputName];
+      switch (input[0]) {
+        case Patch.INPUT_NONE:
+          inputs[inputName] = [Patch.INPUT_UNAVAILABLE];
+          break;
+
+        case Patch.INPUT_CONSTANT:
+          inputs[inputName] = [Patch.INPUT_AVAILABLE, input[1]];
+          break;
+
+        case Patch.INPUT_DIRECT_CONNECTION: {
+          const patch = input[1];
+          const outputName = input[2];
+          const output = patch.computeOutputs()[outputName];
+          switch (output[0]) {
+            case Patch.OUTPUT_UNAVAILABLE:
+              inputs[inputName] = [Patch.INPUT_UNAVAILABLE];
+              break;
+            case Patch.OUTPUT_AVAILABLE:
+              inputs[inputName] = [Patch.INPUT_AVAILABLE, output[1]];
+              break;
+          }
+          throw new Error('Unreachable');
         }
 
-        return inputs;
-    }
+        case Patch.INPUT_MANAGED_CONNECTION: {
+          if (!this.manager) {
+            inputs[inputName] = [Patch.INPUT_UNAVAILABLE];
+            break;
+          }
 
-    computeOutputs() {
-        const inputs = this.computeInputs();
-        const outputs = Object.create(null);
-        console.log(`Compute: ${this.constructor.name}`);
-        this.compute(inputs, outputs);
-        return outputs;
+          inputs[inputName] = this.manager.getManagedInput(input[1]);
+          break;
+        }
+      }
     }
 
-    compute(inputs, outputs) {
-        // No-op. Return all outputs as unavailable. This should be overridden
-        // in subclasses.
+    return inputs;
+  }
 
-        for (const outputName of this.constructor.outputNames) {
-            outputs[outputName] = [Patch.OUTPUT_UNAVAILABLE];
-        }
-    }
+  computeOutputs() {
+    const inputs = this.computeInputs();
+    const outputs = Object.create(null);
+    console.log(`Compute: ${this.constructor.name}`);
+    this.compute(inputs, outputs);
+    return outputs;
+  }
 
-    attachToManager(manager) {
-        manager.addManagedPatch(this);
+  compute(inputs, outputs) {
+    // No-op. Return all outputs as unavailable. This should be overridden
+    // in subclasses.
+
+    for (const outputName of this.constructor.outputNames) {
+      outputs[outputName] = [Patch.OUTPUT_UNAVAILABLE];
     }
+  }
 
-    detachFromManager() {
-        if (this.manager) {
-            this.manager.removeManagedPatch(this);
-        }
+  attachToManager(manager) {
+    manager.addManagedPatch(this);
+  }
+
+  detachFromManager() {
+    if (this.manager) {
+      this.manager.removeManagedPatch(this);
     }
+  }
 }
 
 // --> PatchManager
 
 export class PatchManager extends Patch {
-    managedPatches = [];
-    managedInputs = {};
+  managedPatches = [];
+  managedInputs = {};
 
-    #externalInputPatch = null;
-    #externalOutputPatch = null;
+  #externalInputPatch = null;
+  #externalOutputPatch = null;
 
-    constructor(...args) {
-        super(...args);
+  constructor(...args) {
+    super(...args);
 
-        this.#externalInputPatch = new PatchManagerExternalInputPatch({manager: this});
-        this.#externalOutputPatch = new PatchManagerExternalOutputPatch({manager: this});
-    }
-
-    addManagedPatch(patch) {
-        if (patch.manager === this) {
-            return false;
-        }
+    this.#externalInputPatch = new PatchManagerExternalInputPatch({
+      manager: this,
+    });
 
-        patch.detachFromManager();
-        patch.manager = this;
+    this.#externalOutputPatch = new PatchManagerExternalOutputPatch({
+      manager: this,
+    });
+  }
 
-        if (patch.manager === this) {
-            this.managedPatches.push(patch);
-            return true;
-        } else {
-            return false;
-        }
+  addManagedPatch(patch) {
+    if (patch.manager === this) {
+      return false;
     }
 
-    removeManagedPatch(patch) {
-        if (patch.manager !== this) {
-            return false;
-        }
-
-        patch.manager = null;
-
-        if (patch.manager === this) {
-            return false;
-        }
+    patch.detachFromManager();
+    patch.manager = this;
 
-        for (const inputNames of patch.inputNames) {
-            const input = patch.inputs[inputName];
-            if (input[0] === Patch.INPUT_MANAGED_CONNECTION) {
-                this.dropManagedInput(input[1]);
-                patch.inputs[inputName] = [Patch.INPUT_NONE];
-            }
-        }
-
-        this.managedPatches.splice(this.managedPatches.indexOf(patch), 1);
-
-        return true;
+    if (patch.manager === this) {
+      this.managedPatches.push(patch);
+      return true;
+    } else {
+      return false;
     }
+  }
 
-    addManagedInput(patchWithInput, inputName, patchWithOutput, outputName) {
-        if (patchWithInput.manager !== this || patchWithOutput.manager !== this) {
-            throw new Error(`Input and output patches must belong to same manager (this)`);
-        }
-
-        const input = patchWithInput.inputs[inputName];
-        if (input[0] === Patch.INPUT_MANAGED_CONNECTION) {
-            this.managedInputs[input[1]] = [patchWithOutput, outputName, {}];
-        } else {
-            const key = this.getManagedConnectionIdentifier();
-            this.managedInputs[key] = [patchWithOutput, outputName, {}];
-            patchWithInput.inputs[inputName] = [Patch.INPUT_MANAGED_CONNECTION, key];
-        }
-
-        return true;
+  removeManagedPatch(patch) {
+    if (patch.manager !== this) {
+      return false;
     }
 
-    dropManagedInput(identifier) {
-        return delete this.managedInputs[key];
+    patch.manager = null;
+
+    if (patch.manager === this) {
+      return false;
     }
 
-    getManagedInput(identifier) {
-        const connection = this.managedInputs[identifier];
-        const patch = connection[0];
-        const outputName = connection[1];
-        const memory = connection[2];
-        return this.computeManagedInput(patch, outputName, memory);
+    for (const inputName of patch.inputNames) {
+      const input = patch.inputs[inputName];
+      if (input[0] === Patch.INPUT_MANAGED_CONNECTION) {
+        this.dropManagedInput(input[1]);
+        patch.inputs[inputName] = [Patch.INPUT_NONE];
+      }
     }
 
-    computeManagedInput(patch, outputName, memory) {
-        // Override this function in subclasses to alter behavior of the "wire"
-        // used for connecting patches.
+    this.managedPatches.splice(this.managedPatches.indexOf(patch), 1);
 
-        const output = patch.computeOutputs()[outputName];
-        switch (output[0]) {
-            case Patch.OUTPUT_UNAVAILABLE:
-                return [Patch.INPUT_UNAVAILABLE];
-            case Patch.OUTPUT_AVAILABLE:
-                return [Patch.INPUT_AVAILABLE, output[1]];
-        }
-    }
+    return true;
+  }
 
-    #managedConnectionIdentifier = 0;
-    getManagedConnectionIdentifier() {
-        return this.#managedConnectionIdentifier++;
+  addManagedInput(patchWithInput, inputName, patchWithOutput, outputName) {
+    if (patchWithInput.manager !== this || patchWithOutput.manager !== this) {
+      throw new Error(`Input and output patches must belong to same manager (this)`);
     }
 
-    addExternalInput(patchWithInput, patchInputName, managerInputName) {
-        return this.addManagedInput(patchWithInput, patchInputName, this.#externalInputPatch, managerInputName);
+    const input = patchWithInput.inputs[inputName];
+    if (input[0] === Patch.INPUT_MANAGED_CONNECTION) {
+      this.managedInputs[input[1]] = [patchWithOutput, outputName, {}];
+    } else {
+      const key = this.getManagedConnectionIdentifier();
+      this.managedInputs[key] = [patchWithOutput, outputName, {}];
+      patchWithInput.inputs[inputName] = [Patch.INPUT_MANAGED_CONNECTION, key];
     }
 
-    setExternalOutput(managerOutputName, patchWithOutput, patchOutputName) {
-        return this.addManagedInput(this.#externalOutputPatch, managerOutputName, patchWithOutput, patchOutputName);
-    }
-
-    compute(inputs, outputs) {
-        Object.assign(outputs, this.#externalOutputPatch.computeOutputs());
+    return true;
+  }
+
+  dropManagedInput(identifier) {
+    return delete this.managedInputs[identifier];
+  }
+
+  getManagedInput(identifier) {
+    const connection = this.managedInputs[identifier];
+    const patch = connection[0];
+    const outputName = connection[1];
+    const memory = connection[2];
+    return this.computeManagedInput(patch, outputName, memory);
+  }
+
+  computeManagedInput(patch, outputName) {
+    // Override this function in subclasses to alter behavior of the "wire"
+    // used for connecting patches.
+
+    const output = patch.computeOutputs()[outputName];
+    switch (output[0]) {
+      case Patch.OUTPUT_UNAVAILABLE:
+        return [Patch.INPUT_UNAVAILABLE];
+      case Patch.OUTPUT_AVAILABLE:
+        return [Patch.INPUT_AVAILABLE, output[1]];
     }
+  }
+
+  #managedConnectionIdentifier = 0;
+  getManagedConnectionIdentifier() {
+    return this.#managedConnectionIdentifier++;
+  }
+
+  addExternalInput(patchWithInput, patchInputName, managerInputName) {
+    return this.addManagedInput(
+      patchWithInput,
+      patchInputName,
+      this.#externalInputPatch,
+      managerInputName
+    );
+  }
+
+  setExternalOutput(managerOutputName, patchWithOutput, patchOutputName) {
+    return this.addManagedInput(
+      this.#externalOutputPatch,
+      managerOutputName,
+      patchWithOutput,
+      patchOutputName
+    );
+  }
+
+  compute(inputs, outputs) {
+    Object.assign(outputs, this.#externalOutputPatch.computeOutputs());
+  }
 }
 
 class PatchManagerExternalInputPatch extends Patch {
-    constructor({manager, ...rest}) {
-        super({
-            manager,
-            inputNames: manager.inputNames,
-            outputNames: manager.inputNames,
-            ...rest
-        });
-    }
-
-    computeInputs() {
-        return this.manager.computeInputs();
-    }
-
-    compute(inputs, outputs) {
-        for (const name of this.inputNames) {
-            const input = inputs[name];
-            switch (input[0]) {
-                case Patch.INPUT_UNAVAILABLE:
-                    outputs[name] = [Patch.OUTPUT_UNAVAILABLE];
-                    break;
-                case Patch.INPUT_AVAILABLE:
-                    outputs[name] = [Patch.INPUT_AVAILABLE, input[1]];
-                    break;
-            }
-        }
+  constructor({manager, ...rest}) {
+    super({
+      manager,
+      inputNames: manager.inputNames,
+      outputNames: manager.inputNames,
+      ...rest,
+    });
+  }
+
+  computeInputs() {
+    return this.manager.computeInputs();
+  }
+
+  compute(inputs, outputs) {
+    for (const name of this.inputNames) {
+      const input = inputs[name];
+      switch (input[0]) {
+        case Patch.INPUT_UNAVAILABLE:
+          outputs[name] = [Patch.OUTPUT_UNAVAILABLE];
+          break;
+        case Patch.INPUT_AVAILABLE:
+          outputs[name] = [Patch.INPUT_AVAILABLE, input[1]];
+          break;
+      }
     }
+  }
 }
 
 class PatchManagerExternalOutputPatch extends Patch {
-    constructor({manager, ...rest}) {
-        super({
-            manager,
-            inputNames: manager.outputNames,
-            outputNames: manager.outputNames,
-            ...rest
-        });
-    }
-
-    compute(inputs, outputs) {
-        for (const name of this.inputNames) {
-            const input = inputs[name];
-            switch (input[0]) {
-                case Patch.INPUT_UNAVAILABLE:
-                    outputs[name] = [Patch.OUTPUT_UNAVAILABLE];
-                    break;
-                case Patch.INPUT_AVAILABLE:
-                    outputs[name] = [Patch.INPUT_AVAILABLE, input[1]];
-                    break;
-            }
-        }
+  constructor({manager, ...rest}) {
+    super({
+      manager,
+      inputNames: manager.outputNames,
+      outputNames: manager.outputNames,
+      ...rest,
+    });
+  }
+
+  compute(inputs, outputs) {
+    for (const name of this.inputNames) {
+      const input = inputs[name];
+      switch (input[0]) {
+        case Patch.INPUT_UNAVAILABLE:
+          outputs[name] = [Patch.OUTPUT_UNAVAILABLE];
+          break;
+        case Patch.INPUT_AVAILABLE:
+          outputs[name] = [Patch.INPUT_AVAILABLE, input[1]];
+          break;
+      }
     }
+  }
 }
 
 // --> demo
 
 const caches = Symbol();
 const common = Symbol();
-const hsmusic = Symbol();
 
 Patch[caches] = {
-    WireCachedPatchManager: class extends PatchManager {
-        // "Wire" caching for PatchManager: Remembers the last outputs to come
-        // from each patch. As long as the inputs for a patch do not change, its
-        // cached outputs are reused.
-
-        // TODO: This has a unique cache for each managed input. It should
-        // re-use a cache for the same patch and output name. How can we ensure
-        // the cache is dropped when the patch is removed, though? (Spoilers:
-        // probably just override removeManagedPatch)
-        computeManagedInput(patch, outputName, memory) {
-            let cache = true;
-
-            const { previousInputs } = memory;
-            const { inputs } = patch;
-            if (memory.previousInputs) {
-                for (const inputName of patch.inputNames) {
-                    // TODO: This doesn't account for connections whose values
-                    // have changed (analogous to bubbling cache invalidation).
-                    if (inputs[inputName] !== previousInputs[inputName]) {
-                        cache = false;
-                        break;
-                    }
-                }
-            } else {
-                cache = false;
-            }
-
-            if (cache) {
-                return memory.previousOutputs[outputName];
-            }
-
-            const outputs = patch.computeOutputs();
-            memory.previousOutputs = outputs;
-            memory.previousInputs = {...inputs};
-            return outputs[outputName];
+  WireCachedPatchManager: class extends PatchManager {
+    // "Wire" caching for PatchManager: Remembers the last outputs to come
+    // from each patch. As long as the inputs for a patch do not change, its
+    // cached outputs are reused.
+
+    // TODO: This has a unique cache for each managed input. It should
+    // re-use a cache for the same patch and output name. How can we ensure
+    // the cache is dropped when the patch is removed, though? (Spoilers:
+    // probably just override removeManagedPatch)
+    computeManagedInput(patch, outputName, memory) {
+      let cache = true;
+
+      const {previousInputs} = memory;
+      const {inputs} = patch;
+      if (memory.previousInputs) {
+        for (const inputName of patch.inputNames) {
+          // TODO: This doesn't account for connections whose values
+          // have changed (analogous to bubbling cache invalidation).
+          if (inputs[inputName] !== previousInputs[inputName]) {
+            cache = false;
+            break;
+          }
         }
-    },
+      } else {
+        cache = false;
+      }
+
+      if (cache) {
+        return memory.previousOutputs[outputName];
+      }
+
+      const outputs = patch.computeOutputs();
+      memory.previousOutputs = outputs;
+      memory.previousInputs = {...inputs};
+      return outputs[outputName];
+    }
+  },
 };
 
 Patch[common] = {
-    Stringify: class extends Patch {
-        static inputNames = ['value'];
-        static outputNames = ['value'];
-
-        compute(inputs, outputs) {
-            if (inputs.value[0] === Patch.INPUT_AVAILABLE) {
-                outputs.value = [Patch.OUTPUT_AVAILABLE, inputs.value[1].toString()];
-            } else {
-                outputs.value = [Patch.OUTPUT_UNAVAILABLE];
-            }
-        }
-    },
-
-    Echo: class extends Patch {
-        static inputNames = ['value'];
-        static outputNames = ['value'];
-
-        compute(inputs, outputs) {
-            if (inputs.value[0] === Patch.INPUT_AVAILABLE) {
-                outputs.value = [Patch.OUTPUT_AVAILABLE, inputs.value[1]];
-            } else {
-                outputs.value = [Patch.OUTPUT_UNAVAILABLE];
-            }
-        }
-    },
+  Stringify: class extends Patch {
+    static inputNames = ['value'];
+    static outputNames = ['value'];
+
+    compute(inputs, outputs) {
+      if (inputs.value[0] === Patch.INPUT_AVAILABLE) {
+        outputs.value = [Patch.OUTPUT_AVAILABLE, inputs.value[1].toString()];
+      } else {
+        outputs.value = [Patch.OUTPUT_UNAVAILABLE];
+      }
+    }
+  },
+
+  Echo: class extends Patch {
+    static inputNames = ['value'];
+    static outputNames = ['value'];
+
+    compute(inputs, outputs) {
+      if (inputs.value[0] === Patch.INPUT_AVAILABLE) {
+        outputs.value = [Patch.OUTPUT_AVAILABLE, inputs.value[1]];
+      } else {
+        outputs.value = [Patch.OUTPUT_UNAVAILABLE];
+      }
+    }
+  },
 };
 
 const PM = new Patch[caches].WireCachedPatchManager({
-    inputNames: ['externalInput'],
-    outputNames: ['externalOutput'],
+  inputNames: ['externalInput'],
+  outputNames: ['externalOutput'],
 });
 
 const P1 = new Patch[common].Stringify({manager: PM});
diff --git a/src/data/serialize.js b/src/data/serialize.js
index 9d4e888..8cac330 100644
--- a/src/data/serialize.js
+++ b/src/data/serialize.js
@@ -1,22 +1,26 @@
-// serialize-util.js: simple interface and utility functions for converting
+// serialize.js: simple interface and utility functions for converting
 // Things into a directly serializeable format
 
 // Utility functions
 
 export function id(x) {
-    return x;
+  return x;
 }
 
 export function toRef(thing) {
-    return thing?.constructor.getReference(thing);
+  return thing?.constructor.getReference(thing);
 }
 
 export function toRefs(things) {
-    return things?.map(toRef);
+  return things?.map(toRef);
 }
 
 export function toContribRefs(contribs) {
-    return contribs?.map(({ who, what }) => ({who: toRef(who), what}));
+  return contribs?.map(({who, what}) => ({who: toRef(who), what}));
+}
+
+export function toCommentaryRefs(entries) {
+  return entries?.map(({artist, ...props}) => ({artist: toRef(artist), ...props}));
 }
 
 // Interface
@@ -24,15 +28,18 @@ export function toContribRefs(contribs) {
 export const serializeDescriptors = Symbol();
 
 export function serializeThing(thing) {
-    const descriptors = thing.constructor[serializeDescriptors];
-    if (!descriptors) {
-        throw new Error(`Constructor ${thing.constructor.name} does not provide serialize descriptors`);
-    }
+  const descriptors = thing.constructor[serializeDescriptors];
+
+  if (!descriptors) {
+    throw new Error(`Constructor ${thing.constructor.name} does not provide serialize descriptors`);
+  }
 
-    return Object.fromEntries(Object.entries(descriptors)
-        .map(([ property, transform ]) => [property, transform(thing[property])]));
+  return Object.fromEntries(
+    Object.entries(descriptors)
+      .map(([property, transform]) => [property, transform(thing[property])])
+  );
 }
 
 export function serializeThings(things) {
-    return things.map(serializeThing);
+  return things.map(serializeThing);
 }
diff --git a/src/data/thing.js b/src/data/thing.js
new file mode 100644
index 0000000..706e893
--- /dev/null
+++ b/src/data/thing.js
@@ -0,0 +1,77 @@
+// Thing: base class for wiki data types, providing interfaces generally useful
+// to all wiki data objects on top of foundational CacheableObject behavior.
+
+import {inspect} from 'node:util';
+
+import CacheableObject from '#cacheable-object';
+import {colors} from '#cli';
+
+export default class Thing extends CacheableObject {
+  static referenceType = Symbol.for('Thing.referenceType');
+  static friendlyName = Symbol.for('Thing.friendlyName');
+
+  static getPropertyDescriptors = Symbol.for('Thing.getPropertyDescriptors');
+  static getSerializeDescriptors = Symbol.for('Thing.getSerializeDescriptors');
+
+  static findSpecs = Symbol.for('Thing.findSpecs');
+  static yamlDocumentSpec = Symbol.for('Thing.yamlDocumentSpec');
+  static getYamlLoadingSpec = Symbol.for('Thing.getYamlLoadingSpec');
+
+  // Default custom inspect function, which may be overridden by Thing
+  // subclasses. This will be used when displaying aggregate errors and other
+  // command-line logging - it's the place to provide information useful in
+  // identifying the Thing being presented.
+  [inspect.custom]() {
+    const cname = this.constructor.name;
+
+    return (
+      (this.name ? `${cname} ${colors.green(`"${this.name}"`)}` : `${cname}`) +
+      (this.directory ? ` (${colors.blue(Thing.getReference(this))})` : '')
+    );
+  }
+
+  static getReference(thing) {
+    if (!thing.constructor[Thing.referenceType]) {
+      throw TypeError(`Passed Thing is ${thing.constructor.name}, which provides no [Thing.referenceType]`);
+    }
+
+    if (!thing.directory) {
+      throw TypeError(`Passed ${thing.constructor.name} is missing its directory`);
+    }
+
+    return `${thing.constructor[Thing.referenceType]}:${thing.directory}`;
+  }
+
+  static extendDocumentSpec(thingClass, subspec) {
+    const superspec = thingClass[Thing.yamlDocumentSpec];
+
+    const {
+      fields,
+      ignoredFields,
+      invalidFieldCombinations,
+      ...restOfSubspec
+    } = subspec;
+
+    const newFields = Object.keys(fields ?? {});
+
+    return {
+      ...superspec,
+      ...restOfSubspec,
+
+      fields: {
+        ...superspec.fields ?? {},
+        ...fields,
+      },
+
+      ignoredFields:
+        (superspec.ignoredFields ?? [])
+          .filter(field => newFields.includes(field))
+          .concat(ignoredFields ?? []),
+
+      invalidFieldCombinations: [
+        ...superspec.invalidFieldCombinations ?? [],
+        ...invalidFieldCombinations ?? [],
+      ],
+    };
+  }
+}
diff --git a/src/data/things.js b/src/data/things.js
deleted file mode 100644
index daec610..0000000
--- a/src/data/things.js
+++ /dev/null
@@ -1,1720 +0,0 @@
-// things.js: class definitions for various object types used across the wiki,
-// most of which correspond to an output page, such as Track, Album, Artist
-
-import CacheableObject from './cacheable-object.js';
-
-import {
-    isAdditionalFileList,
-    isBoolean,
-    isColor,
-    isCommentary,
-    isCountingNumber,
-    isContributionList,
-    isDate,
-    isDimensions,
-    isDirectory,
-    isDuration,
-    isInstance,
-    isFileExtension,
-    isLanguageCode,
-    isName,
-    isNumber,
-    isURL,
-    isString,
-    isWholeNumber,
-    oneOf,
-    validateArrayItems,
-    validateInstanceOf,
-    validateReference,
-    validateReferenceList,
-} from './validators.js';
-
-import * as S from './serialize.js';
-
-import {
-    getKebabCase,
-    sortByArtDate,
-} from '../util/wiki-data.js';
-
-import find from '../util/find.js';
-
-import { inspect } from 'util';
-import { color } from '../util/cli.js';
-
-// Stub classes (and their exports) at the top of the file - these are
-// referenced later when we actually define static class fields. We deliberately
-// define the classes and set their static fields in two separate steps so that
-// every class coexists from the outset, and can be directly referenced in field
-// definitions later.
-
-// This list also acts as a quick table of contents for this JS file - use
-// ctrl+F or similar to skip to a section.
-
-// -> Thing
-export class Thing extends CacheableObject {}
-
-// -> Album
-export class Album extends Thing {}
-export class TrackGroup extends CacheableObject {}
-
-// -> Track
-export class Track extends Thing {}
-
-// -> Artist
-export class Artist extends Thing {}
-
-// -> Group
-export class Group extends Thing {}
-export class GroupCategory extends CacheableObject {}
-
-// -> ArtTag
-export class ArtTag extends Thing {}
-
-// -> NewsEntry
-export class NewsEntry extends Thing {}
-
-// -> StaticPage
-export class StaticPage extends Thing {}
-
-// -> HomepageLayout
-export class HomepageLayout extends CacheableObject {}
-export class HomepageLayoutRow extends CacheableObject {}
-export class HomepageLayoutAlbumsRow extends HomepageLayoutRow {}
-
-// -> Flash
-export class Flash extends Thing {}
-export class FlashAct extends CacheableObject {}
-
-// -> WikiInfo
-export class WikiInfo extends CacheableObject {}
-
-// -> Language
-export class Language extends CacheableObject {}
-
-// Before initializing property descriptors, set additional independent
-// constants on the classes (which are referenced later).
-
-Thing.referenceType = Symbol('Thing.referenceType');
-
-Album[Thing.referenceType] = 'album';
-Track[Thing.referenceType] = 'track';
-Artist[Thing.referenceType] = 'artist';
-Group[Thing.referenceType] = 'group';
-ArtTag[Thing.referenceType] = 'tag';
-NewsEntry[Thing.referenceType] = 'news-entry';
-StaticPage[Thing.referenceType] = 'static';
-Flash[Thing.referenceType] = 'flash';
-
-// -> Thing: base class for wiki data types, providing wiki-specific utility
-// functions on top of essential CacheableObject behavior.
-
-// Regularly reused property descriptors, for ease of access and generally
-// duplicating less code across wiki data types. These are specialized utility
-// functions, so check each for how its own arguments behave!
-Thing.common = {
-    name: (defaultName) => ({
-        flags: {update: true, expose: true},
-        update: {validate: isName, default: defaultName}
-    }),
-
-    color: () => ({
-        flags: {update: true, expose: true},
-        update: {validate: isColor}
-    }),
-
-    directory: () => ({
-        flags: {update: true, expose: true},
-        update: {validate: isDirectory},
-        expose: {
-            dependencies: ['name'],
-            transform(directory, { name }) {
-                if (directory === null && name === null)
-                    return null;
-                else if (directory === null)
-                    return getKebabCase(name);
-                else
-                    return directory;
-            }
-        }
-    }),
-
-    urls: () => ({
-        flags: {update: true, expose: true},
-        update: {validate: validateArrayItems(isURL)}
-    }),
-
-    // A file extension! Or the default, if provided when calling this.
-    fileExtension: (defaultFileExtension = null) => ({
-        flags: {update: true, expose: true},
-        update: {validate: isFileExtension},
-        expose: {transform: value => value ?? defaultFileExtension}
-    }),
-
-    // Straightforward flag descriptor for a variety of property purposes.
-    // Provide a default value, true or false!
-    flag: (defaultValue = false) => {
-        if (typeof defaultValue !== 'boolean') {
-            throw new TypeError(`Always set explicit defaults for flags!`);
-        }
-
-        return {
-            flags: {update: true, expose: true},
-            update: {validate: isBoolean, default: defaultValue}
-        };
-    },
-
-    // General date type, used as the descriptor for a bunch of properties.
-    // This isn't dynamic though - it won't inherit from a date stored on
-    // another object, for example.
-    simpleDate: () => ({
-        flags: {update: true, expose: true},
-        update: {validate: isDate}
-    }),
-
-    // General string type. This should probably generally be avoided in favor
-    // of more specific validation, but using it makes it easy to find where we
-    // might want to improve later, and it's a useful shorthand meanwhile.
-    simpleString: () => ({
-        flags: {update: true, expose: true},
-        update: {validate: isString}
-    }),
-
-    // External function. These should only be used as dependencies for other
-    // properties, so they're left unexposed.
-    externalFunction: () => ({
-        flags: {update: true},
-        update: {validate: t => typeof t === 'function'}
-    }),
-
-    // Super simple "contributions by reference" list, used for a variety of
-    // properties (Artists, Cover Artists, etc). This is the property which is
-    // externally provided, in the form:
-    //
-    //     [
-    //         {who: 'Artist Name', what: 'Viola'},
-    //         {who: 'artist:john-cena', what: null},
-    //         ...
-    //     ]
-    //
-    // ...processed from YAML, spreadsheet, or any other kind of input.
-    contribsByRef: () => ({
-        flags: {update: true, expose: true},
-        update: {validate: isContributionList}
-    }),
-
-    // Artist commentary! Generally present on tracks and albums.
-    commentary: () => ({
-        flags: {update: true, expose: true},
-        update: {validate: isCommentary}
-    }),
-
-    // This is a somewhat more involved data structure - it's for additional
-    // or "bonus" files associated with albums or tracks (or anything else).
-    // It's got this form:
-    //
-    //     [
-    //         {title: 'Booklet', files: ['Booklet.pdf']},
-    //         {
-    //             title: 'Wallpaper',
-    //             description: 'Cool Wallpaper!',
-    //             files: ['1440x900.png', '1920x1080.png']
-    //         },
-    //         {title: 'Alternate Covers', description: null, files: [...]},
-    //         ...
-    //     ]
-    //
-    additionalFiles: () => ({
-        flags: {update: true, expose: true},
-        update: {validate: isAdditionalFileList}
-    }),
-
-    // A reference list! Keep in mind this is for general references to wiki
-    // objects of (usually) other Thing subclasses, not specifically leitmotif
-    // references in tracks (although that property uses referenceList too!).
-    //
-    // The underlying function validateReferenceList expects a string like
-    // 'artist' or 'track', but this utility keeps from having to hard-code the
-    // string in multiple places by referencing the value saved on the class
-    // instead.
-    referenceList: thingClass => {
-        const { [Thing.referenceType]: referenceType } = thingClass;
-        if (!referenceType) {
-            throw new Error(`The passed constructor ${thingClass.name} doesn't define Thing.referenceType!`);
-        }
-
-        return {
-            flags: {update: true, expose: true},
-            update: {validate: validateReferenceList(referenceType)}
-        };
-    },
-
-    // Corresponding function for a single reference.
-    singleReference: thingClass => {
-        const { [Thing.referenceType]: referenceType } = thingClass;
-        if (!referenceType) {
-            throw new Error(`The passed constructor ${thingClass.name} doesn't define Thing.referenceType!`);
-        }
-
-        return {
-            flags: {update: true, expose: true},
-            update: {validate: validateReference(referenceType)}
-        };
-    },
-
-    // Corresponding dynamic property to referenceList, which takes the values
-    // in the provided property and searches the specified wiki data for
-    // matching actual Thing-subclass objects.
-    dynamicThingsFromReferenceList: (
-        referenceListProperty,
-        thingDataProperty,
-        findFn
-    ) => ({
-        flags: {expose: true},
-
-        expose: {
-            dependencies: [referenceListProperty, thingDataProperty],
-            compute: ({ [referenceListProperty]: refs, [thingDataProperty]: thingData }) => (
-                (refs && thingData
-                    ? (refs
-                        .map(ref => findFn(ref, thingData, {mode: 'quiet'}))
-                        .filter(Boolean))
-                    : [])
-            )
-        }
-    }),
-
-    // Corresponding function for a single reference.
-    dynamicThingFromSingleReference: (
-        singleReferenceProperty,
-        thingDataProperty,
-        findFn
-    ) => ({
-        flags: {expose: true},
-
-        expose: {
-            dependencies: [singleReferenceProperty, thingDataProperty],
-            compute: ({ [singleReferenceProperty]: ref, [thingDataProperty]: thingData }) => (
-                (ref && thingData ? findFn(ref, thingData, {mode: 'quiet'}) : null)
-            )
-        }
-    }),
-
-    // Corresponding dynamic property to contribsByRef, which takes the values
-    // in the provided property and searches the object's artistData for
-    // matching actual Artist objects. The computed structure has the same form
-    // as contribsByRef, but with Artist objects instead of string references:
-    //
-    //     [
-    //         {who: (an Artist), what: 'Viola'},
-    //         {who: (an Artist), what: null},
-    //         ...
-    //     ]
-    //
-    // Contributions whose "who" values don't match anything in artistData are
-    // filtered out. (So if the list is all empty, chances are that either the
-    // reference list is somehow messed up, or artistData isn't being provided
-    // properly.)
-    dynamicContribs: (contribsByRefProperty) => ({
-        flags: {expose: true},
-        expose: {
-            dependencies: ['artistData', contribsByRefProperty],
-            compute: ({ artistData, [contribsByRefProperty]: contribsByRef }) => (
-                ((contribsByRef && artistData)
-                    ? (contribsByRef
-                        .map(({ who: ref, what }) => ({
-                            who: find.artist(ref, artistData),
-                            what
-                        }))
-                        .filter(({ who }) => who))
-                    : [])
-            )
-        }
-    }),
-
-    // Dynamically inherit a contribution list from some other object, if it
-    // hasn't been overridden on this object. This is handy for solo albums
-    // where all tracks have the same artist, for example.
-    //
-    // Note: The arguments of this function aren't currently final! The final
-    // format will look more like (contribsByRef, parentContribsByRef), e.g.
-    // ('artistContribsByRef', '@album/artistContribsByRef').
-    dynamicInheritContribs: (
-        contribsByRefProperty,
-        parentContribsByRefProperty,
-        thingDataProperty,
-        findFn
-    ) => ({
-        flags: {expose: true},
-        expose: {
-            dependencies: [contribsByRefProperty, thingDataProperty, 'artistData'],
-            compute({
-                [Thing.instance]: thing,
-                [contribsByRefProperty]: contribsByRef,
-                [thingDataProperty]: thingData,
-                artistData
-            }) {
-                if (!artistData) return [];
-                const refs = (contribsByRef ?? findFn(thing, thingData, {mode: 'quiet'})?.[parentContribsByRefProperty]);
-                if (!refs) return [];
-                return (refs
-                    .map(({ who: ref, what }) => ({
-                        who: find.artist(ref, artistData),
-                        what
-                    }))
-                    .filter(({ who }) => who));
-            }
-        }
-    }),
-
-    // Neat little shortcut for "reversing" the reference lists stored on other
-    // things - for example, tracks specify a "referenced tracks" property, and
-    // you would use this to compute a corresponding "referenced *by* tracks"
-    // property. Naturally, the passed ref list property is of the things in the
-    // wiki data provided, not the requesting Thing itself.
-    reverseReferenceList: (wikiDataProperty, referencerRefListProperty) => ({
-        flags: {expose: true},
-
-        expose: {
-            dependencies: [wikiDataProperty],
-
-            compute: ({ [wikiDataProperty]: wikiData, [Thing.instance]: thing }) => (
-                (wikiData
-                    ? wikiData.filter(t => t[referencerRefListProperty]?.includes(thing))
-                    : [])
-            )
-        }
-    }),
-
-    // Corresponding function for single references. Note that the return value
-    // is still a list - this is for matching all the objects whose single
-    // reference (in the given property) matches this Thing.
-    reverseSingleReference: (wikiDataProperty, referencerRefListProperty) => ({
-        flags: {expose: true},
-
-        expose: {
-            dependencies: [wikiDataProperty],
-
-            compute: ({ [wikiDataProperty]: wikiData, [Thing.instance]: thing }) => (
-                wikiData?.filter(t => t[referencerRefListProperty] === thing))
-        }
-    }),
-
-    // General purpose wiki data constructor, for properties like artistData,
-    // trackData, etc.
-    wikiData: (thingClass) => ({
-        flags: {update: true},
-        update: {
-            validate: validateArrayItems(validateInstanceOf(thingClass))
-        }
-    }),
-
-    // This one's kinda tricky: it parses artist "references" from the
-    // commentary content, and finds the matching artist for each reference.
-    // This is mostly useful for credits and listings on artist pages.
-    commentatorArtists: () => ({
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['artistData', 'commentary'],
-
-            compute: ({ artistData, commentary }) => (
-                (artistData && commentary
-                    ? Array.from(new Set((Array
-                        .from(commentary
-                            .replace(/<\/?b>/g, '')
-                            .matchAll(/<i>(?<who>.*?):<\/i>/g))
-                        .map(({ groups: {who} }) => find.artist(who, artistData, {mode: 'quiet'})))))
-                    : []))
-        }
-    }),
-};
-
-// Get a reference to a thing (e.g. track:showtime-piano-refrain), using its
-// constructor's [Thing.referenceType] as the prefix. This will throw an error
-// if the thing's directory isn't yet provided/computable.
-Thing.getReference = function(thing) {
-    if (!thing.constructor[Thing.referenceType])
-        throw TypeError(`Passed Thing is ${thing.constructor.name}, which provides no [Thing.referenceType]`);
-
-    if (!thing.directory)
-        throw TypeError(`Passed ${thing.constructor.name} is missing its directory`);
-
-    return `${thing.constructor[Thing.referenceType]}:${thing.directory}`;
-};
-
-// Default custom inspect function, which may be overridden by Thing subclasses.
-// This will be used when displaying aggregate errors and other in command-line
-// logging - it's the place to provide information useful in identifying the
-// Thing being presented.
-Thing.prototype[inspect.custom] = function() {
-    const cname = this.constructor.name;
-
-    return (this.name
-        ? `${cname} ${color.green(`"${this.name}"`)}`
-        : `${cname}`) + (this.directory
-            ? ` (${color.blue(Thing.getReference(this))})`
-            : '');
-};
-
-// -> Album
-
-Album.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Album'),
-    color: Thing.common.color(),
-    directory: Thing.common.directory(),
-    urls: Thing.common.urls(),
-
-    date: Thing.common.simpleDate(),
-    trackArtDate: Thing.common.simpleDate(),
-    dateAddedToWiki: Thing.common.simpleDate(),
-
-    coverArtDate: {
-        flags: {update: true, expose: true},
-
-        update: {validate: isDate},
-
-        expose: {
-            dependencies: ['date'],
-            transform: (coverArtDate, { date }) => coverArtDate ?? date ?? null
-        }
-    },
-
-    artistContribsByRef: Thing.common.contribsByRef(),
-    coverArtistContribsByRef: Thing.common.contribsByRef(),
-    trackCoverArtistContribsByRef: Thing.common.contribsByRef(),
-    wallpaperArtistContribsByRef: Thing.common.contribsByRef(),
-    bannerArtistContribsByRef: Thing.common.contribsByRef(),
-
-    groupsByRef: Thing.common.referenceList(Group),
-    artTagsByRef: Thing.common.referenceList(ArtTag),
-
-    trackGroups: {
-        flags: {update: true, expose: true},
-
-        update: {
-            validate: validateArrayItems(validateInstanceOf(TrackGroup))
-        }
-    },
-
-    coverArtFileExtension: Thing.common.fileExtension('jpg'),
-    trackCoverArtFileExtension: Thing.common.fileExtension('jpg'),
-
-    wallpaperStyle: Thing.common.simpleString(),
-    wallpaperFileExtension: Thing.common.fileExtension('jpg'),
-
-    bannerStyle: Thing.common.simpleString(),
-    bannerFileExtension: Thing.common.fileExtension('jpg'),
-    bannerDimensions: {
-        flags: {update: true, expose: true},
-        update: {validate: isDimensions}
-    },
-
-    hasCoverArt: Thing.common.flag(true),
-    hasTrackArt: Thing.common.flag(true),
-    hasTrackNumbers: Thing.common.flag(true),
-    isMajorRelease: Thing.common.flag(false),
-    isListedOnHomepage: Thing.common.flag(true),
-
-    commentary: Thing.common.commentary(),
-    additionalFiles: Thing.common.additionalFiles(),
-
-    // Update only
-
-    artistData: Thing.common.wikiData(Artist),
-    artTagData: Thing.common.wikiData(ArtTag),
-    groupData: Thing.common.wikiData(Group),
-    trackData: Thing.common.wikiData(Track),
-
-    // Expose only
-
-    artistContribs: Thing.common.dynamicContribs('artistContribsByRef'),
-    coverArtistContribs: Thing.common.dynamicContribs('coverArtistContribsByRef'),
-    trackCoverArtistContribs: Thing.common.dynamicContribs('trackCoverArtistContribsByRef'),
-    wallpaperArtistContribs: Thing.common.dynamicContribs('wallpaperArtistContribsByRef'),
-    bannerArtistContribs: Thing.common.dynamicContribs('bannerArtistContribsByRef'),
-
-    commentatorArtists: Thing.common.commentatorArtists(),
-
-    tracks: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['trackGroups', 'trackData'],
-            compute: ({ trackGroups, trackData }) => (
-                (trackGroups && trackData
-                    ? (trackGroups
-                        .flatMap(group => group.tracksByRef ?? [])
-                        .map(ref => find.track(ref, trackData, {mode: 'quiet'}))
-                        .filter(Boolean))
-                    : [])
-            )
-        }
-    },
-
-    groups: Thing.common.dynamicThingsFromReferenceList('groupsByRef', 'groupData', find.group),
-
-    artTags: Thing.common.dynamicThingsFromReferenceList('artTagsByRef', 'artTagData', find.artTag),
-};
-
-Album[S.serializeDescriptors] = {
-    name: S.id,
-    color: S.id,
-    directory: S.id,
-    urls: S.id,
-
-    date: S.id,
-    coverArtDate: S.id,
-    trackArtDate: S.id,
-    dateAddedToWiki: S.id,
-
-    artistContribs: S.toContribRefs,
-    coverArtistContribs: S.toContribRefs,
-    trackCoverArtistContribs: S.toContribRefs,
-    wallpaperArtistContribs: S.toContribRefs,
-    bannerArtistContribs: S.toContribRefs,
-
-    coverArtFileExtension: S.id,
-    trackCoverArtFileExtension: S.id,
-    wallpaperStyle: S.id,
-    wallpaperFileExtension: S.id,
-    bannerStyle: S.id,
-    bannerFileExtension: S.id,
-    bannerDimensions: S.id,
-
-    hasTrackArt: S.id,
-    isMajorRelease: S.id,
-    isListedOnHomepage: S.id,
-
-    commentary: S.id,
-    additionalFiles: S.id,
-
-    tracks: S.toRefs,
-    groups: S.toRefs,
-    artTags: S.toRefs,
-    commentatorArtists: S.toRefs,
-};
-
-TrackGroup.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Track Group'),
-
-    color: {
-        flags: {update: true, expose: true},
-
-        update: {validate: isColor},
-
-        expose: {
-            dependencies: ['album'],
-
-            transform(color, { album }) {
-                return color ?? album?.color ?? null;
-            }
-        }
-    },
-
-    dateOriginallyReleased: Thing.common.simpleDate(),
-
-    tracksByRef: Thing.common.referenceList(Track),
-
-    isDefaultTrackGroup: Thing.common.flag(false),
-
-    // Update only
-
-    album: {
-        flags: {update: true},
-        update: {validate: validateInstanceOf(Album)}
-    },
-
-    trackData: Thing.common.wikiData(Track),
-
-    // Expose only
-
-    tracks: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['tracksByRef', 'trackData'],
-            compute: ({ tracksByRef, trackData }) => (
-                (tracksByRef && trackData
-                    ? (tracksByRef
-                        .map(ref => find.track(ref, trackData))
-                        .filter(Boolean))
-                    : [])
-            )
-        }
-    },
-
-    startIndex: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['album'],
-            compute: ({ album, [TrackGroup.instance]: trackGroup }) => (album.trackGroups
-                .slice(0, album.trackGroups.indexOf(trackGroup))
-                .reduce((acc, tg) => acc + tg.tracks.length, 0))
-        }
-    },
-};
-
-// -> Track
-
-// This is a quick utility function for now, since the same code is reused in
-// several places. Ideally it wouldn't be - we'd just reuse the `album` property
-// - but support for that hasn't been coded yet :P
-Track.findAlbum = (track, albumData) => {
-    return albumData?.find(album => album.tracks.includes(track));
-};
-
-// Another reused utility function. This one's logic is a bit more complicated.
-Track.hasCoverArt = (track, albumData, coverArtistContribsByRef, hasCoverArt) => {
-    return (
-        hasCoverArt ??
-        (coverArtistContribsByRef?.length > 0 || null) ??
-        Track.findAlbum(track, albumData)?.hasTrackArt ??
-        true);
-};
-
-Track.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Track'),
-    directory: Thing.common.directory(),
-
-    duration: {
-        flags: {update: true, expose: true},
-        update: {validate: isDuration}
-    },
-
-    urls: Thing.common.urls(),
-    dateFirstReleased: Thing.common.simpleDate(),
-
-    hasURLs: Thing.common.flag(true),
-
-    artistContribsByRef: Thing.common.contribsByRef(),
-    contributorContribsByRef: Thing.common.contribsByRef(),
-    coverArtistContribsByRef: Thing.common.contribsByRef(),
-
-    referencedTracksByRef: Thing.common.referenceList(Track),
-    artTagsByRef: Thing.common.referenceList(ArtTag),
-
-    hasCoverArt: {
-        flags: {update: true, expose: true},
-
-        update: {validate: isBoolean},
-
-        expose: {
-            dependencies: ['albumData', 'coverArtistContribsByRef'],
-            transform: (hasCoverArt, { albumData, coverArtistContribsByRef, [Track.instance]: track }) => (
-                Track.hasCoverArt(track, albumData, coverArtistContribsByRef, hasCoverArt))
-        }
-    },
-
-    coverArtFileExtension: {
-        flags: {update: true, expose: true},
-
-        update: {validate: isFileExtension},
-
-        expose: {
-            dependencies: ['albumData', 'coverArtistContribsByRef'],
-            transform: (coverArtFileExtension, { albumData, coverArtistContribsByRef, hasCoverArt, [Track.instance]: track }) => (
-                coverArtFileExtension ??
-                (Track.hasCoverArt(track, albumData, coverArtistContribsByRef, hasCoverArt)
-                    ? Track.findAlbum(track, albumData)?.trackCoverArtFileExtension
-                    : Track.findAlbum(track, albumData)?.coverArtFileExtension) ??
-                'jpg')
-        }
-    },
-
-    // Previously known as: (track).aka
-    originalReleaseTrackByRef: Thing.common.singleReference(Track),
-
-    dataSourceAlbumByRef: Thing.common.singleReference(Album),
-
-    commentary: Thing.common.commentary(),
-    lyrics: Thing.common.simpleString(),
-    additionalFiles: Thing.common.additionalFiles(),
-
-    // Update only
-
-    albumData: Thing.common.wikiData(Album),
-    artistData: Thing.common.wikiData(Artist),
-    artTagData: Thing.common.wikiData(ArtTag),
-    flashData: Thing.common.wikiData(Flash),
-    trackData: Thing.common.wikiData(Track),
-
-    // Expose only
-
-    commentatorArtists: Thing.common.commentatorArtists(),
-
-    album: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['albumData'],
-            compute: ({ [Track.instance]: track, albumData }) => (
-                albumData?.find(album => album.tracks.includes(track)) ?? null)
-        }
-    },
-
-    // Note - this is an internal property used only to help identify a track.
-    // It should not be assumed in general that the album and dataSourceAlbum match
-    // (i.e. a track may dynamically be moved from one album to another, at
-    // which point dataSourceAlbum refers to where it was originally from, and is
-    // not generally relevant information). It's also not guaranteed that
-    // dataSourceAlbum is available (depending on the Track creator to optionally
-    // provide dataSourceAlbumByRef).
-    dataSourceAlbum: Thing.common.dynamicThingFromSingleReference('dataSourceAlbumByRef', 'albumData', find.album),
-
-    date: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['albumData', 'dateFirstReleased'],
-            compute: ({ albumData, dateFirstReleased, [Track.instance]: track }) => (
-                dateFirstReleased ??
-                Track.findAlbum(track, albumData)?.date ??
-                null
-            )
-        }
-    },
-
-    color: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['albumData'],
-
-            compute: ({ albumData, [Track.instance]: track }) => (
-                (Track.findAlbum(track, albumData)?.trackGroups
-                    .find(tg => tg.tracks.includes(track))?.color)
-                ?? null
-            )
-        }
-    },
-
-    coverArtDate: {
-        flags: {update: true, expose: true},
-
-        update: {validate: isDate},
-
-        expose: {
-            dependencies: ['albumData', 'dateFirstReleased'],
-            transform: (coverArtDate, { albumData, dateFirstReleased, [Track.instance]: track }) => (
-                coverArtDate ??
-                dateFirstReleased ??
-                Track.findAlbum(track, albumData)?.trackArtDate ??
-                Track.findAlbum(track, albumData)?.date ??
-                null
-            )
-        }
-    },
-
-    originalReleaseTrack: Thing.common.dynamicThingFromSingleReference('originalReleaseTrackByRef', 'trackData', find.track),
-
-    otherReleases: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['originalReleaseTrackByRef', 'trackData'],
-
-            compute: ({ originalReleaseTrackByRef: t1origRef, trackData, [Track.instance]: t1 }) => {
-                if (!trackData) {
-                    return [];
-                }
-
-                const t1orig = find.track(t1origRef, trackData);
-
-                return [
-                    t1orig,
-                    ...trackData.filter(t2 => {
-                        const { originalReleaseTrack: t2orig } = t2;
-                        return (
-                            t2 !== t1 &&
-                            t2orig &&
-                            (t2orig === t1orig || t2orig === t1)
-                        );
-                    })
-                ].filter(Boolean);
-            }
-        }
-    },
-
-    // Previously known as: (track).artists
-    artistContribs: Thing.common.dynamicInheritContribs('artistContribsByRef', 'artistContribsByRef', 'albumData', Track.findAlbum),
-
-    // Previously known as: (track).contributors
-    contributorContribs: Thing.common.dynamicContribs('contributorContribsByRef'),
-
-    // Previously known as: (track).coverArtists
-    coverArtistContribs: Thing.common.dynamicInheritContribs('coverArtistContribsByRef', 'trackCoverArtistContribsByRef', 'albumData', Track.findAlbum),
-
-    // Previously known as: (track).references
-    referencedTracks: Thing.common.dynamicThingsFromReferenceList('referencedTracksByRef', 'trackData', find.track),
-
-    // Specifically exclude re-releases from this list - while it's useful to
-    // get from a re-release to the tracks it references, re-releases aren't
-    // generally relevant from the perspective of the tracks being referenced.
-    // Filtering them from data here hides them from the corresponding field
-    // on the site (obviously), and has the bonus of not counting them when
-    // counting the number of times a track has been referenced, for use in
-    // the "Tracks - by Times Referenced" listing page (or other data
-    // processing).
-    referencedByTracks: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['trackData'],
-
-            compute: ({ trackData, [Track.instance]: track }) => (trackData
-                ? (trackData
-                    .filter(t => !t.originalReleaseTrack)
-                    .filter(t => t.referencedTracks?.includes(track)))
-                : [])
-        }
-    },
-
-    // Previously known as: (track).flashes
-    featuredInFlashes: Thing.common.reverseReferenceList('flashData', 'featuredTracks'),
-
-    artTags: Thing.common.dynamicThingsFromReferenceList('artTagsByRef', 'artTagData', find.artTag),
-};
-
-Track.prototype[inspect.custom] = function() {
-    const base = Thing.prototype[inspect.custom].apply(this);
-
-    const { album, dataSourceAlbum } = this;
-    const albumName = (album ? album.name : dataSourceAlbum?.name);
-    const albumIndex = albumName && (album ? album.tracks.indexOf(this) : dataSourceAlbum.tracks.indexOf(this));
-    const trackNum = (albumIndex === -1 ? '#?' : `#${albumIndex + 1}`);
-
-    return (albumName
-        ? base + ` (${color.yellow(trackNum)} in ${color.green(albumName)})`
-        : base);
-};
-
-// -> Artist
-
-Artist.filterByContrib = (thingDataProperty, contribsProperty) => ({
-    flags: {expose: true},
-
-    expose: {
-        dependencies: [thingDataProperty],
-
-        compute: ({ [thingDataProperty]: thingData, [Artist.instance]: artist }) => (
-            thingData?.filter(({ [contribsProperty]: contribs }) => (
-                contribs?.some(contrib => contrib.who === artist))))
-    }
-});
-
-Artist.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Artist'),
-    directory: Thing.common.directory(),
-    urls: Thing.common.urls(),
-    contextNotes: Thing.common.simpleString(),
-
-    hasAvatar: Thing.common.flag(false),
-    avatarFileExtension: Thing.common.fileExtension('jpg'),
-
-    aliasNames: {
-        flags: {update: true, expose: true},
-        update: {
-            validate: validateArrayItems(isName)
-        }
-    },
-
-    isAlias: Thing.common.flag(),
-    aliasedArtistRef: Thing.common.singleReference(Artist),
-
-    // Update only
-
-    albumData: Thing.common.wikiData(Album),
-    artistData: Thing.common.wikiData(Artist),
-    flashData: Thing.common.wikiData(Flash),
-    trackData: Thing.common.wikiData(Track),
-
-    // Expose only
-
-    aliasedArtist: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['artistData', 'aliasedArtistRef'],
-            compute: ({ artistData, aliasedArtistRef }) => (
-                (aliasedArtistRef && artistData
-                    ? find.artist(aliasedArtistRef, artistData, {mode: 'quiet'})
-                    : null)
-            )
-        }
-    },
-
-    tracksAsArtist: Artist.filterByContrib('trackData', 'artistContribs'),
-    tracksAsContributor: Artist.filterByContrib('trackData', 'contributorContribs'),
-    tracksAsCoverArtist: Artist.filterByContrib('trackData', 'coverArtistContribs'),
-
-    tracksAsAny: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['trackData'],
-
-            compute: ({ trackData, [Artist.instance]: artist }) => (
-                trackData?.filter(track => (
-                    [
-                        ...track.artistContribs,
-                        ...track.contributorContribs,
-                        ...track.coverArtistContribs
-                    ].some(({ who }) => who === artist))))
-        }
-    },
-
-    tracksAsCommentator: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['trackData'],
-
-            compute: ({ trackData, [Artist.instance]: artist }) => (
-                trackData.filter(({ commentatorArtists }) => commentatorArtists?.includes(artist)))
-        }
-    },
-
-    albumsAsAlbumArtist: Artist.filterByContrib('albumData', 'artistContribs'),
-    albumsAsCoverArtist: Artist.filterByContrib('albumData', 'coverArtistContribs'),
-    albumsAsWallpaperArtist: Artist.filterByContrib('albumData', 'wallpaperArtistContribs'),
-    albumsAsBannerArtist: Artist.filterByContrib('albumData', 'bannerArtistContribs'),
-
-    albumsAsCommentator: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['albumData'],
-
-            compute: ({ albumData, [Artist.instance]: artist }) => (
-                albumData.filter(({ commentatorArtists }) => commentatorArtists?.includes(artist)))
-        }
-    },
-
-    flashesAsContributor: Artist.filterByContrib('flashData', 'contributorContribs'),
-};
-
-Artist[S.serializeDescriptors] = {
-    name: S.id,
-    directory: S.id,
-    urls: S.id,
-    contextNotes: S.id,
-
-    hasAvatar: S.id,
-    avatarFileExtension: S.id,
-
-    aliasNames: S.id,
-
-    tracksAsArtist: S.toRefs,
-    tracksAsContributor: S.toRefs,
-    tracksAsCoverArtist: S.toRefs,
-    tracksAsCommentator: S.toRefs,
-
-    albumsAsAlbumArtist: S.toRefs,
-    albumsAsCoverArtist: S.toRefs,
-    albumsAsWallpaperArtist: S.toRefs,
-    albumsAsBannerArtist: S.toRefs,
-    albumsAsCommentator: S.toRefs,
-
-    flashesAsContributor: S.toRefs,
-};
-
-// -> Group
-
-Group.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Group'),
-    directory: Thing.common.directory(),
-
-    description: Thing.common.simpleString(),
-
-    urls: Thing.common.urls(),
-
-    // Update only
-
-    albumData: Thing.common.wikiData(Album),
-    groupCategoryData: Thing.common.wikiData(GroupCategory),
-
-    // Expose only
-
-    descriptionShort: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['description'],
-            compute: ({ description }) => description.split('<hr class="split">')[0]
-        }
-    },
-
-    albums: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['albumData'],
-            compute: ({ albumData, [Group.instance]: group }) => (
-                albumData?.filter(album => album.groups.includes(group)) ?? [])
-        }
-    },
-
-    color: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['groupCategoryData'],
-
-            compute: ({ groupCategoryData, [Group.instance]: group }) => (
-                groupCategoryData.find(category => category.groups.includes(group))?.color ?? null)
-        }
-    },
-
-    category: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['groupCategoryData'],
-            compute: ({ groupCategoryData, [Group.instance]: group }) => (
-                groupCategoryData.find(category => category.groups.includes(group)) ?? null)
-        }
-    },
-};
-
-GroupCategory.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Group Category'),
-    color: Thing.common.color(),
-
-    groupsByRef: Thing.common.referenceList(Group),
-
-    // Update only
-
-    groupData: Thing.common.wikiData(Group),
-
-    // Expose only
-
-    groups: Thing.common.dynamicThingsFromReferenceList('groupsByRef', 'groupData', find.group),
-};
-
-// -> ArtTag
-
-ArtTag.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Art Tag'),
-    directory: Thing.common.directory(),
-    color: Thing.common.color(),
-    isContentWarning: Thing.common.flag(false),
-
-    // Update only
-
-    albumData: Thing.common.wikiData(Album),
-    trackData: Thing.common.wikiData(Track),
-
-    // Expose only
-
-    // Previously known as: (tag).things
-    taggedInThings: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['albumData', 'trackData'],
-            compute: ({ albumData, trackData, [ArtTag.instance]: artTag }) => (
-                sortByArtDate([...albumData, ...trackData]
-                    .filter(thing => thing.artTags?.includes(artTag))))
-        }
-    }
-};
-
-// -> NewsEntry
-
-NewsEntry.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed News Entry'),
-    directory: Thing.common.directory(),
-    date: Thing.common.simpleDate(),
-
-    content: Thing.common.simpleString(),
-
-    // Expose only
-
-    contentShort: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['content'],
-
-            compute: ({ content }) => content.split('<hr class="split">')[0]
-        }
-    },
-};
-
-// -> StaticPage
-
-StaticPage.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Static Page'),
-
-    nameShort: {
-        flags: {update: true, expose: true},
-        update: {validate: isName},
-
-        expose: {
-            dependencies: ['name'],
-            transform: (value, { name }) => value ?? name
-        }
-    },
-
-    directory: Thing.common.directory(),
-    content: Thing.common.simpleString(),
-    stylesheet: Thing.common.simpleString(),
-    showInNavigationBar: Thing.common.flag(true),
-};
-
-// -> HomepageLayout
-
-HomepageLayout.propertyDescriptors = {
-    // Update & expose
-
-    sidebarContent: Thing.common.simpleString(),
-
-    rows: {
-        flags: {update: true, expose: true},
-
-        update: {
-            validate: validateArrayItems(validateInstanceOf(HomepageLayoutRow))
-        }
-    },
-};
-
-HomepageLayoutRow.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Homepage Row'),
-
-    type: {
-        flags: {update: true, expose: true},
-
-        update: {
-            validate(value) {
-                throw new Error(`'type' property validator must be overridden`);
-            }
-        }
-    },
-
-    color: Thing.common.color(),
-
-    // Update only
-
-    // These aren't necessarily used by every HomepageLayoutRow subclass, but
-    // for convenience of providing this data, every row accepts all wiki data
-    // arrays depended upon by any subclass's behavior.
-    albumData: Thing.common.wikiData(Album),
-    groupData: Thing.common.wikiData(Group),
-};
-
-HomepageLayoutAlbumsRow.propertyDescriptors = {
-    ...HomepageLayoutRow.propertyDescriptors,
-
-    // Update & expose
-
-    type: {
-        flags: {update: true, expose: true},
-        update: {
-            validate(value) {
-                if (value !== 'albums') {
-                    throw new TypeError(`Expected 'albums'`);
-                }
-
-                return true;
-            }
-        }
-    },
-
-    sourceGroupByRef: Thing.common.singleReference(Group),
-    sourceAlbumsByRef: Thing.common.referenceList(Album),
-
-    countAlbumsFromGroup: {
-        flags: {update: true, expose: true},
-        update: {validate: isCountingNumber}
-    },
-
-    actionLinks: {
-        flags: {update: true, expose: true},
-        update: {validate: validateArrayItems(isString)}
-    },
-
-    // Expose only
-
-    sourceGroup: Thing.common.dynamicThingFromSingleReference('sourceGroupByRef', 'groupData', find.group),
-    sourceAlbums: Thing.common.dynamicThingsFromReferenceList('sourceAlbumsByRef', 'albumData', find.album),
-};
-
-// -> Flash
-
-Flash.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Flash'),
-
-    directory: {
-        flags: {update: true, expose: true},
-        update: {validate: isDirectory},
-
-        // Flashes expose directory differently from other Things! Their
-        // default directory is dependent on the page number (or ID), not
-        // the name.
-        expose: {
-            dependencies: ['page'],
-            transform(directory, { page }) {
-                if (directory === null && page === null)
-                    return null;
-                else if (directory === null)
-                    return page;
-                else
-                    return directory;
-            }
-        }
-    },
-
-    page: {
-        flags: {update: true, expose: true},
-        update: {validate: oneOf(isString, isNumber)},
-
-        expose: {
-            transform: value => (value === null ? null : value.toString())
-        }
-    },
-
-    date: Thing.common.simpleDate(),
-
-    coverArtFileExtension: Thing.common.fileExtension('jpg'),
-
-    contributorContribsByRef: Thing.common.contribsByRef(),
-
-    featuredTracksByRef: Thing.common.referenceList(Track),
-
-    urls: Thing.common.urls(),
-
-    // Update only
-
-    artistData: Thing.common.wikiData(Artist),
-    trackData: Thing.common.wikiData(Track),
-    flashActData: Thing.common.wikiData(FlashAct),
-
-    // Expose only
-
-    contributorContribs: Thing.common.dynamicContribs('contributorContribsByRef'),
-
-    featuredTracks: Thing.common.dynamicThingsFromReferenceList('featuredTracksByRef', 'trackData', find.track),
-
-    act: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['flashActData'],
-
-            compute: ({ flashActData, [Flash.instance]: flash }) => (
-                flashActData.find(act => act.flashes.includes(flash)) ?? null)
-        }
-    },
-
-    color: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['flashActData'],
-
-            compute: ({ flashActData, [Flash.instance]: flash }) => (
-                flashActData.find(act => act.flashes.includes(flash))?.color ?? null)
-        }
-    },
-};
-
-Flash[S.serializeDescriptors] = {
-    name: S.id,
-    page: S.id,
-    directory: S.id,
-    date: S.id,
-    contributors: S.toContribRefs,
-    tracks: S.toRefs,
-    urls: S.id,
-    color: S.id,
-};
-
-FlashAct.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Flash Act'),
-    color: Thing.common.color(),
-    anchor: Thing.common.simpleString(),
-    jump: Thing.common.simpleString(),
-    jumpColor: Thing.common.color(),
-
-    flashesByRef: Thing.common.referenceList(Flash),
-
-    // Update only
-
-    flashData: Thing.common.wikiData(Flash),
-
-    // Expose only
-
-    flashes: Thing.common.dynamicThingsFromReferenceList('flashesByRef', 'flashData', find.flash),
-};
-
-// -> WikiInfo
-
-WikiInfo.propertyDescriptors = {
-    // Update & expose
-
-    name: Thing.common.name('Unnamed Wiki'),
-
-    // Displayed in nav bar.
-    nameShort: {
-        flags: {update: true, expose: true},
-        update: {validate: isName},
-
-        expose: {
-            dependencies: ['name'],
-            transform: (value, { name }) => value ?? name
-        }
-    },
-
-    color: Thing.common.color(),
-
-    // One-line description used for <meta rel="description"> tag.
-    description: Thing.common.simpleString(),
-
-    footerContent: Thing.common.simpleString(),
-
-    defaultLanguage: {
-        flags: {update: true, expose: true},
-        update: {validate: isLanguageCode}
-    },
-
-    canonicalBase: {
-        flags: {update: true, expose: true},
-        update: {validate: isURL}
-    },
-
-    divideTrackListsByGroupsByRef: Thing.common.referenceList(Group),
-
-    // Feature toggles
-    enableFlashesAndGames: Thing.common.flag(false),
-    enableListings: Thing.common.flag(false),
-    enableNews: Thing.common.flag(false),
-    enableArtTagUI: Thing.common.flag(false),
-    enableGroupUI: Thing.common.flag(false),
-
-    // Update only
-
-    groupData: Thing.common.wikiData(Group),
-
-    // Expose only
-
-    divideTrackListsByGroups: Thing.common.dynamicThingsFromReferenceList('divideTrackListsByGroupsByRef', 'groupData', find.group),
-};
-
-// -> Language
-
-const intlHelper = (constructor, opts) => ({
-    flags: {expose: true},
-    expose: {
-        dependencies: ['code', 'intlCode'],
-        compute: ({ code, intlCode }) => {
-            const constructCode = intlCode ?? code;
-            if (!constructCode) return null;
-            return Reflect.construct(constructor, [constructCode, opts]);
-        }
-    }
-});
-
-Language.propertyDescriptors = {
-    // Update & expose
-
-    // General language code. This is used to identify the language distinctly
-    // from other languages (similar to how "Directory" operates in many data
-    // objects).
-    code: {
-        flags: {update: true, expose: true},
-        update: {validate: isLanguageCode}
-    },
-
-    // Human-readable name. This should be the language's own native name, not
-    // localized to any other language.
-    name: Thing.common.simpleString(),
-
-    // Language code specific to JavaScript's Internationalization (Intl) API.
-    // Usually this will be the same as the language's general code, but it
-    // may be overridden to provide Intl constructors an alternative value.
-    intlCode: {
-        flags: {update: true, expose: true},
-        update: {validate: isLanguageCode},
-        expose: {
-            dependencies: ['code'],
-            transform: (intlCode, { code }) => intlCode ?? code
-        }
-    },
-
-    // Flag which represents whether or not to hide a language from general
-    // access. If a language is hidden, its portion of the website will still
-    // be built (with all strings localized to the language), but it won't be
-    // included in controls for switching languages or the <link rel=alternate>
-    // tags used for search engine optimization. This flag is intended for use
-    // with languages that are currently in development and not ready for
-    // formal release, or which are just kept hidden as "experimental zones"
-    // for wiki development or content testing.
-    hidden: Thing.common.flag(false),
-
-    // Mapping of translation keys to values (strings). Generally, don't
-    // access this object directly - use methods instead.
-    strings: {
-        flags: {update: true, expose: true},
-        update: {validate: t => typeof t === 'object'},
-        expose: {
-            dependencies: ['inheritedStrings'],
-            transform(strings, { inheritedStrings }) {
-                if (strings || inheritedStrings) {
-                    return {...inheritedStrings ?? {}, ...strings ?? {}};
-                } else {
-                    return null;
-                }
-            }
-        }
-    },
-
-    // May be provided to specify "default" strings, generally (but not
-    // necessarily) inherited from another Language object.
-    inheritedStrings: {
-        flags: {update: true, expose: true},
-        update: {validate: t => typeof t === 'object'}
-    },
-
-    // Update only
-
-    escapeHTML: Thing.common.externalFunction(),
-
-    // Expose only
-
-    intl_date: intlHelper(Intl.DateTimeFormat, {full: true}),
-    intl_number: intlHelper(Intl.NumberFormat),
-    intl_listConjunction: intlHelper(Intl.ListFormat, {type: 'conjunction'}),
-    intl_listDisjunction: intlHelper(Intl.ListFormat, {type: 'disjunction'}),
-    intl_listUnit: intlHelper(Intl.ListFormat, {type: 'unit'}),
-    intl_pluralCardinal: intlHelper(Intl.PluralRules, {type: 'cardinal'}),
-    intl_pluralOrdinal: intlHelper(Intl.PluralRules, {type: 'ordinal'}),
-
-    validKeys: {
-        flags: {expose: true},
-
-        expose: {
-            dependencies: ['strings', 'inheritedStrings'],
-            compute: ({ strings, inheritedStrings }) => Array.from(new Set([
-                ...Object.keys(inheritedStrings ?? {}),
-                ...Object.keys(strings ?? {})
-            ]))
-        }
-    },
-
-    strings_htmlEscaped: {
-        flags: {expose: true},
-        expose: {
-            dependencies: ['strings', 'inheritedStrings', 'escapeHTML'],
-            compute({ strings, inheritedStrings, escapeHTML }) {
-                if (!(strings || inheritedStrings) || !escapeHTML) return null;
-                const allStrings = {...inheritedStrings ?? {}, ...strings ?? {}};
-                return Object.fromEntries(Object.entries(allStrings)
-                    .map(([ k, v ]) => [k, escapeHTML(v)]));
-            }
-        }
-    },
-};
-
-const countHelper = (stringKey, argName = stringKey) => function(value, {unit = false} = {}) {
-    return this.$(
-        (unit
-            ? `count.${stringKey}.withUnit.` + this.getUnitForm(value)
-            : `count.${stringKey}`),
-        {[argName]: this.formatNumber(value)});
-};
-
-Object.assign(Language.prototype, {
-    $(key, args = {}) {
-        return this.formatString(key, args);
-    },
-
-    assertIntlAvailable(property) {
-        if (!this[property]) {
-            throw new Error(`Intl API ${property} unavailable`);
-        }
-    },
-
-    getUnitForm(value) {
-        this.assertIntlAvailable('intl_pluralCardinal');
-        return this.intl_pluralCardinal.select(value);
-    },
-
-    formatString(key, args = {}) {
-        if (this.strings && !this.strings_htmlEscaped) {
-            throw new Error(`HTML-escaped strings unavailable - please ensure escapeHTML function is provided`);
-        }
-
-        return this.formatStringHelper(this.strings_htmlEscaped, key, args);
-    },
-
-    formatStringNoHTMLEscape(key, args = {}) {
-        return this.formatStringHelper(this.strings, key, args);
-    },
-
-    formatStringHelper(strings, key, args = {}) {
-        if (!strings) {
-            throw new Error(`Strings unavailable`);
-        }
-
-        if (!this.validKeys.includes(key)) {
-            throw new Error(`Invalid key ${key} accessed`);
-        }
-
-        const template = strings[key];
-
-        // Convert the keys on the args dict from camelCase to CONSTANT_CASE.
-        // (This isn't an OUTRAGEOUSLY versatile algorithm for doing that, 8ut
-        // like, who cares, dude?) Also, this is an array, 8ecause it's handy
-        // for the iterating we're a8out to do.
-        const processedArgs = Object.entries(args)
-            .map(([ k, v ]) => [k.replace(/[A-Z]/g, '_$&').toUpperCase(), v]);
-
-        // Replacement time! Woot. Reduce comes in handy here!
-        const output = processedArgs.reduce(
-            (x, [ k, v ]) => x.replaceAll(`{${k}}`, v),
-            template);
-
-        // Post-processing: if any expected arguments *weren't* replaced, that
-        // is almost definitely an error.
-        if (output.match(/\{[A-Z_]+\}/)) {
-            throw new Error(`Args in ${key} were missing - output: ${output}`);
-        }
-
-        return output;
-    },
-
-    formatDate(date) {
-        this.assertIntlAvailable('intl_date');
-        return this.intl_date.format(date);
-    },
-
-    formatDateRange(startDate, endDate) {
-        this.assertIntlAvailable('intl_date');
-        return this.intl_date.formatRange(startDate, endDate);
-    },
-
-    formatDuration(secTotal, {approximate = false, unit = false} = {}) {
-        if (secTotal === 0) {
-            return this.formatString('count.duration.missing');
-        }
-
-        const hour = Math.floor(secTotal / 3600);
-        const min = Math.floor((secTotal - hour * 3600) / 60);
-        const sec = Math.floor(secTotal - hour * 3600 - min * 60);
-
-        const pad = val => val.toString().padStart(2, '0');
-
-        const stringSubkey = unit ? '.withUnit' : '';
-
-        const duration = (hour > 0
-            ? this.formatString('count.duration.hours' + stringSubkey, {
-                hours: hour,
-                minutes: pad(min),
-                seconds: pad(sec)
-            })
-            : this.formatString('count.duration.minutes' + stringSubkey, {
-                minutes: min,
-                seconds: pad(sec)
-            }));
-
-        return (approximate
-            ? this.formatString('count.duration.approximate', {duration})
-            : duration);
-    },
-
-    formatIndex(value) {
-        this.assertIntlAvailable('intl_pluralOrdinal');
-        return this.formatString('count.index.' + this.intl_pluralOrdinal.select(value), {index: value});
-    },
-
-    formatNumber(value) {
-        this.assertIntlAvailable('intl_number');
-        return this.intl_number.format(value);
-    },
-
-    formatWordCount(value) {
-        const num = this.formatNumber(value > 1000
-            ? Math.floor(value / 100) / 10
-            : value);
-
-        const words = (value > 1000
-            ? this.formatString('count.words.thousand', {words: num})
-            : this.formatString('count.words', {words: num}));
-
-        return this.formatString('count.words.withUnit.' + this.getUnitForm(value), {words});
-    },
-
-    // Conjunction list: A, B, and C
-    formatConjunctionList(array) {
-        this.assertIntlAvailable('intl_listConjunction');
-        return this.intl_listConjunction.format(array);
-    },
-
-    // Disjunction lists: A, B, or C
-    formatDisjunctionList(array) {
-        this.assertIntlAvailable('intl_listDisjunction');
-        return this.intl_listDisjunction.format(array);
-    },
-
-    // Unit lists: A, B, C
-    formatUnitList(array) {
-        this.assertIntlAvailable('intl_listUnit');
-        return this.intl_listUnit.format(array);
-    },
-
-    // File sizes: 42.5 kB, 127.2 MB, 4.13 GB, 998.82 TB
-    formatFileSize(bytes) {
-        if (!bytes) return '';
-
-        bytes = parseInt(bytes);
-        if (isNaN(bytes)) return '';
-
-        const round = exp => Math.round(bytes / 10 ** (exp - 1)) / 10;
-
-        if (bytes >= 10 ** 12) {
-            return this.formatString('count.fileSize.terabytes', {terabytes: round(12)});
-        } else if (bytes >= 10 ** 9) {
-            return this.formatString('count.fileSize.gigabytes', {gigabytes: round(9)});
-        } else if (bytes >= 10 ** 6) {
-            return this.formatString('count.fileSize.megabytes', {megabytes: round(6)});
-        } else if (bytes >= 10 ** 3) {
-            return this.formatString('count.fileSize.kilobytes', {kilobytes: round(3)});
-        } else {
-            return this.formatString('count.fileSize.bytes', {bytes});
-        }
-    },
-
-    // TODO: These are hard-coded. Is there a better way?
-    countAlbums: countHelper('albums'),
-    countCommentaryEntries: countHelper('commentaryEntries', 'entries'),
-    countContributions: countHelper('contributions'),
-    countCoverArts: countHelper('coverArts'),
-    countTimesReferenced: countHelper('timesReferenced'),
-    countTimesUsed: countHelper('timesUsed'),
-    countTracks: countHelper('tracks'),
-});
diff --git a/src/data/things/album.js b/src/data/things/album.js
new file mode 100644
index 0000000..40cd463
--- /dev/null
+++ b/src/data/things/album.js
@@ -0,0 +1,434 @@
+export const DATA_ALBUM_DIRECTORY = 'album';
+
+import * as path from 'node:path';
+
+import {input} from '#composite';
+import find from '#find';
+import {traverse} from '#node-utils';
+import {sortAlbumsTracksChronologically, sortChronologically} from '#sort';
+import {empty} from '#sugar';
+import Thing from '#thing';
+import {isDate} from '#validators';
+import {parseAdditionalFiles, parseContributors, parseDate, parseDimensions}
+  from '#yaml';
+
+import {exposeDependency, exposeUpdateValueOrContinue}
+  from '#composite/control-flow';
+import {exitWithoutContribs} from '#composite/wiki-data';
+
+import {
+  additionalFiles,
+  commentary,
+  color,
+  commentatorArtists,
+  contribsPresent,
+  contributionList,
+  dimensions,
+  directory,
+  fileExtension,
+  flag,
+  name,
+  referenceList,
+  simpleDate,
+  simpleString,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
+import {withTracks, withTrackSections} from '#composite/things/album';
+
+export class Album extends Thing {
+  static [Thing.referenceType] = 'album';
+
+  static [Thing.getPropertyDescriptors] = ({ArtTag, Artist, Group, Track}) => ({
+    // Update & expose
+
+    name: name('Unnamed Album'),
+    color: color(),
+    directory: directory(),
+    urls: urls(),
+
+    bandcampAlbumIdentifier: simpleString(),
+    bandcampArtworkIdentifier: simpleString(),
+
+    date: simpleDate(),
+    trackArtDate: simpleDate(),
+    dateAddedToWiki: simpleDate(),
+
+    coverArtDate: [
+      exitWithoutContribs({contribs: 'coverArtistContribs'}),
+
+      exposeUpdateValueOrContinue({
+        validate: input.value(isDate),
+      }),
+
+      exposeDependency({dependency: 'date'}),
+    ],
+
+    coverArtFileExtension: [
+      exitWithoutContribs({contribs: 'coverArtistContribs'}),
+      fileExtension('jpg'),
+    ],
+
+    trackCoverArtFileExtension: fileExtension('jpg'),
+
+    wallpaperFileExtension: [
+      exitWithoutContribs({contribs: 'wallpaperArtistContribs'}),
+      fileExtension('jpg'),
+    ],
+
+    bannerFileExtension: [
+      exitWithoutContribs({contribs: 'bannerArtistContribs'}),
+      fileExtension('jpg'),
+    ],
+
+    wallpaperStyle: [
+      exitWithoutContribs({contribs: 'wallpaperArtistContribs'}),
+      simpleString(),
+    ],
+
+    bannerStyle: [
+      exitWithoutContribs({contribs: 'bannerArtistContribs'}),
+      simpleString(),
+    ],
+
+    coverArtDimensions: [
+      exitWithoutContribs({contribs: 'coverArtistContribs'}),
+      dimensions(),
+    ],
+
+    bannerDimensions: [
+      exitWithoutContribs({contribs: 'bannerArtistContribs'}),
+      dimensions(),
+    ],
+
+    hasTrackNumbers: flag(true),
+    isListedOnHomepage: flag(true),
+    isListedInGalleries: flag(true),
+
+    commentary: commentary(),
+    additionalFiles: additionalFiles(),
+
+    trackSections: [
+      withTrackSections(),
+      exposeDependency({dependency: '#trackSections'}),
+    ],
+
+    artistContribs: contributionList(),
+    coverArtistContribs: contributionList(),
+    trackCoverArtistContribs: contributionList(),
+    wallpaperArtistContribs: contributionList(),
+    bannerArtistContribs: contributionList(),
+
+    groups: referenceList({
+      class: input.value(Group),
+      find: input.value(find.group),
+      data: 'groupData',
+    }),
+
+    artTags: [
+      exitWithoutContribs({
+        contribs: 'coverArtistContribs',
+        value: input.value([]),
+      }),
+
+      referenceList({
+        class: input.value(ArtTag),
+        find: input.value(find.artTag),
+        data: 'artTagData',
+      }),
+    ],
+
+    // Update only
+
+    artistData: wikiData({
+      class: input.value(Artist),
+    }),
+
+    artTagData: wikiData({
+      class: input.value(ArtTag),
+    }),
+
+    groupData: wikiData({
+      class: input.value(Group),
+    }),
+
+    // Only the tracks which belong to this album.
+    // Necessary for computing the track list, so provide this statically
+    // or keep it updated.
+    ownTrackData: wikiData({
+      class: input.value(Track),
+    }),
+
+    // Expose only
+
+    commentatorArtists: commentatorArtists(),
+
+    hasCoverArt: contribsPresent({contribs: 'coverArtistContribs'}),
+    hasWallpaperArt: contribsPresent({contribs: 'wallpaperArtistContribs'}),
+    hasBannerArt: contribsPresent({contribs: 'bannerArtistContribs'}),
+
+    tracks: [
+      withTracks(),
+      exposeDependency({dependency: '#tracks'}),
+    ],
+  });
+
+  static [Thing.getSerializeDescriptors] = ({
+    serialize: S,
+  }) => ({
+    name: S.id,
+    color: S.id,
+    directory: S.id,
+    urls: S.id,
+
+    date: S.id,
+    coverArtDate: S.id,
+    trackArtDate: S.id,
+    dateAddedToWiki: S.id,
+
+    artistContribs: S.toContribRefs,
+    coverArtistContribs: S.toContribRefs,
+    trackCoverArtistContribs: S.toContribRefs,
+    wallpaperArtistContribs: S.toContribRefs,
+    bannerArtistContribs: S.toContribRefs,
+
+    coverArtFileExtension: S.id,
+    trackCoverArtFileExtension: S.id,
+    wallpaperStyle: S.id,
+    wallpaperFileExtension: S.id,
+    bannerStyle: S.id,
+    bannerFileExtension: S.id,
+    bannerDimensions: S.id,
+
+    hasTrackArt: S.id,
+    isListedOnHomepage: S.id,
+
+    commentary: S.toCommentaryRefs,
+
+    additionalFiles: S.id,
+
+    tracks: S.toRefs,
+    groups: S.toRefs,
+    artTags: S.toRefs,
+    commentatorArtists: S.toRefs,
+  });
+
+  static [Thing.findSpecs] = {
+    album: {
+      referenceTypes: ['album', 'album-commentary', 'album-gallery'],
+      bindTo: 'albumData',
+    },
+  };
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Album': {property: 'name'},
+      'Directory': {property: 'directory'},
+
+      'Bandcamp Album ID': {
+        property: 'bandcampAlbumIdentifier',
+        transform: String,
+      },
+
+      'Bandcamp Artwork ID': {
+        property: 'bandcampArtworkIdentifier',
+        transform: String,
+      },
+
+      'Date': {
+        property: 'date',
+        transform: parseDate,
+      },
+
+      'Color': {property: 'color'},
+      'URLs': {property: 'urls'},
+
+      'Has Track Numbers': {property: 'hasTrackNumbers'},
+      'Listed on Homepage': {property: 'isListedOnHomepage'},
+      'Listed in Galleries': {property: 'isListedInGalleries'},
+
+      'Cover Art Date': {
+        property: 'coverArtDate',
+        transform: parseDate,
+      },
+
+      'Default Track Cover Art Date': {
+        property: 'trackArtDate',
+        transform: parseDate,
+      },
+
+      'Date Added': {
+        property: 'dateAddedToWiki',
+        transform: parseDate,
+      },
+
+      'Cover Art File Extension': {property: 'coverArtFileExtension'},
+      'Track Art File Extension': {property: 'trackCoverArtFileExtension'},
+
+      'Cover Art Dimensions': {
+        property: 'coverArtDimensions',
+        transform: parseDimensions,
+      },
+
+      'Wallpaper Artists': {
+        property: 'wallpaperArtistContribs',
+        transform: parseContributors,
+      },
+
+      'Wallpaper Style': {property: 'wallpaperStyle'},
+      'Wallpaper File Extension': {property: 'wallpaperFileExtension'},
+
+      'Banner Artists': {
+        property: 'bannerArtistContribs',
+        transform: parseContributors,
+      },
+
+      'Banner Style': {property: 'bannerStyle'},
+      'Banner File Extension': {property: 'bannerFileExtension'},
+
+      'Banner Dimensions': {
+        property: 'bannerDimensions',
+        transform: parseDimensions,
+      },
+
+      'Commentary': {property: 'commentary'},
+
+      'Additional Files': {
+        property: 'additionalFiles',
+        transform: parseAdditionalFiles,
+      },
+
+      'Franchises': {ignore: true},
+
+      'Artists': {
+        property: 'artistContribs',
+        transform: parseContributors,
+      },
+
+      'Cover Artists': {
+        property: 'coverArtistContribs',
+        transform: parseContributors,
+      },
+
+      'Default Track Cover Artists': {
+        property: 'trackCoverArtistContribs',
+        transform: parseContributors,
+      },
+
+      'Groups': {property: 'groups'},
+      'Art Tags': {property: 'artTags'},
+
+      'Review Points': {ignore: true},
+    },
+  };
+
+  static [Thing.getYamlLoadingSpec] = ({
+    documentModes: {headerAndEntries},
+    thingConstructors: {Album, Track, TrackSectionHelper},
+  }) => ({
+    title: `Process album files`,
+
+    files: dataPath =>
+      traverse(path.join(dataPath, DATA_ALBUM_DIRECTORY), {
+        filterFile: name => path.extname(name) === '.yaml',
+        prefixPath: DATA_ALBUM_DIRECTORY,
+      }),
+
+    documentMode: headerAndEntries,
+    headerDocumentThing: Album,
+    entryDocumentThing: document =>
+      ('Section' in document
+        ? TrackSectionHelper
+        : Track),
+
+    save(results) {
+      const albumData = [];
+      const trackData = [];
+
+      for (const {header: album, entries} of results) {
+        // We can't mutate an array once it's set as a property value,
+        // so prepare the track sections that will show up in a track list
+        // all the way before actually applying them. (It's okay to mutate
+        // an individual section before applying it, since those are just
+        // generic objects; they aren't Things in and of themselves.)
+        const trackSections = [];
+        const ownTrackData = [];
+
+        let currentTrackSection = {
+          name: `Default Track Section`,
+          isDefaultTrackSection: true,
+          tracks: [],
+        };
+
+        const albumRef = Thing.getReference(album);
+
+        const closeCurrentTrackSection = () => {
+          if (!empty(currentTrackSection.tracks)) {
+            trackSections.push(currentTrackSection);
+          }
+        };
+
+        for (const entry of entries) {
+          if (entry instanceof TrackSectionHelper) {
+            closeCurrentTrackSection();
+
+            currentTrackSection = {
+              name: entry.name,
+              color: entry.color,
+              dateOriginallyReleased: entry.dateOriginallyReleased,
+              isDefaultTrackSection: false,
+              tracks: [],
+            };
+
+            continue;
+          }
+
+          trackData.push(entry);
+
+          entry.dataSourceAlbum = albumRef;
+
+          ownTrackData.push(entry);
+          currentTrackSection.tracks.push(Thing.getReference(entry));
+        }
+
+        closeCurrentTrackSection();
+
+        albumData.push(album);
+
+        album.trackSections = trackSections;
+        album.ownTrackData = ownTrackData;
+      }
+
+      return {albumData, trackData};
+    },
+
+    sort({albumData, trackData}) {
+      sortChronologically(albumData);
+      sortAlbumsTracksChronologically(trackData);
+    },
+  });
+}
+
+export class TrackSectionHelper extends Thing {
+  static [Thing.friendlyName] = `Track Section`;
+
+  static [Thing.getPropertyDescriptors] = () => ({
+    name: name('Unnamed Track Section'),
+    color: color(),
+    dateOriginallyReleased: simpleDate(),
+    isDefaultTrackGroup: flag(false),
+  })
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Section': {property: 'name'},
+      'Color': {property: 'color'},
+
+      'Date Originally Released': {
+        property: 'dateOriginallyReleased',
+        transform: parseDate,
+      },
+    },
+  };
+}
diff --git a/src/data/things/art-tag.js b/src/data/things/art-tag.js
new file mode 100644
index 0000000..3149b31
--- /dev/null
+++ b/src/data/things/art-tag.js
@@ -0,0 +1,107 @@
+export const ART_TAG_DATA_FILE = 'tags.yaml';
+
+import {input} from '#composite';
+import {sortAlphabetically, sortAlbumsTracksChronologically} from '#sort';
+import Thing from '#thing';
+import {isName} from '#validators';
+
+import {exposeUpdateValueOrContinue} from '#composite/control-flow';
+
+import {
+  color,
+  directory,
+  flag,
+  name,
+  wikiData,
+} from '#composite/wiki-properties';
+
+export class ArtTag extends Thing {
+  static [Thing.referenceType] = 'tag';
+  static [Thing.friendlyName] = `Art Tag`;
+
+  static [Thing.getPropertyDescriptors] = ({Album, Track}) => ({
+    // Update & expose
+
+    name: name('Unnamed Art Tag'),
+    directory: directory(),
+    color: color(),
+    isContentWarning: flag(false),
+
+    nameShort: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isName),
+      }),
+
+      {
+        dependencies: ['name'],
+        compute: ({name}) =>
+          name.replace(/ \([^)]*?\)$/, ''),
+      },
+    ],
+
+    // Update only
+
+    albumData: wikiData({
+      class: input.value(Album),
+    }),
+
+    trackData: wikiData({
+      class: input.value(Track),
+    }),
+
+    // Expose only
+
+    taggedInThings: {
+      flags: {expose: true},
+
+      expose: {
+        dependencies: ['this', 'albumData', 'trackData'],
+        compute: ({this: artTag, albumData, trackData}) =>
+          sortAlbumsTracksChronologically(
+            [...albumData, ...trackData]
+              .filter(({artTags}) => artTags.includes(artTag)),
+            {getDate: thing => thing.coverArtDate ?? thing.date}),
+      },
+    },
+  });
+
+  static [Thing.findSpecs] = {
+    artTag: {
+      referenceTypes: ['tag'],
+      bindTo: 'artTagData',
+
+      getMatchableNames: tag =>
+        (tag.isContentWarning
+          ? [`cw: ${tag.name}`]
+          : [tag.name]),
+    },
+  };
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Tag': {property: 'name'},
+      'Short Name': {property: 'nameShort'},
+      'Directory': {property: 'directory'},
+
+      'Color': {property: 'color'},
+      'Is CW': {property: 'isContentWarning'},
+    },
+  };
+
+  static [Thing.getYamlLoadingSpec] = ({
+    documentModes: {allInOne},
+    thingConstructors: {ArtTag},
+  }) => ({
+    title: `Process art tags file`,
+    file: ART_TAG_DATA_FILE,
+
+    documentMode: allInOne,
+    documentThing: ArtTag,
+
+    save: (results) => ({artTagData: results}),
+
+    sort({artTagData}) {
+      sortAlphabetically(artTagData);
+    },
+  });
+}
diff --git a/src/data/things/artist.js b/src/data/things/artist.js
new file mode 100644
index 0000000..841d652
--- /dev/null
+++ b/src/data/things/artist.js
@@ -0,0 +1,392 @@
+export const ARTIST_DATA_FILE = 'artists.yaml';
+
+import {inspect} from 'node:util';
+
+import CacheableObject from '#cacheable-object';
+import {colors} from '#cli';
+import {input} from '#composite';
+import find from '#find';
+import {sortAlphabetically} from '#sort';
+import {stitchArrays, unique} from '#sugar';
+import Thing from '#thing';
+import {isName, validateArrayItems} from '#validators';
+import {getKebabCase} from '#wiki-data';
+
+import {withReverseContributionList} from '#composite/wiki-data';
+
+import {
+  contentString,
+  directory,
+  fileExtension,
+  flag,
+  name,
+  reverseContributionList,
+  reverseReferenceList,
+  singleReference,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
+export class Artist extends Thing {
+  static [Thing.referenceType] = 'artist';
+  static [Thing.wikiDataArray] = 'artistData';
+
+  static [Thing.getPropertyDescriptors] = ({Album, Flash, Track}) => ({
+    // Update & expose
+
+    name: name('Unnamed Artist'),
+    directory: directory(),
+    urls: urls(),
+
+    contextNotes: contentString(),
+
+    hasAvatar: flag(false),
+    avatarFileExtension: fileExtension('jpg'),
+
+    aliasNames: {
+      flags: {update: true, expose: true},
+      update: {validate: validateArrayItems(isName)},
+      expose: {transform: (names) => names ?? []},
+    },
+
+    isAlias: flag(),
+
+    aliasedArtist: singleReference({
+      class: input.value(Artist),
+      find: input.value(find.artist),
+      data: 'artistData',
+    }),
+
+    // Update only
+
+    albumData: wikiData({
+      class: input.value(Album),
+    }),
+
+    artistData: wikiData({
+      class: input.value(Artist),
+    }),
+
+    flashData: wikiData({
+      class: input.value(Flash),
+    }),
+
+    trackData: wikiData({
+      class: input.value(Track),
+    }),
+
+    // Expose only
+
+    tracksAsArtist: reverseContributionList({
+      data: 'trackData',
+      list: input.value('artistContribs'),
+    }),
+
+    tracksAsContributor: reverseContributionList({
+      data: 'trackData',
+      list: input.value('contributorContribs'),
+    }),
+
+    tracksAsCoverArtist: reverseContributionList({
+      data: 'trackData',
+      list: input.value('coverArtistContribs'),
+    }),
+
+    tracksAsAny: [
+      withReverseContributionList({
+        data: 'trackData',
+        list: input.value('artistContribs'),
+      }).outputs({
+        '#reverseContributionList': '#tracksAsArtist',
+      }),
+
+      withReverseContributionList({
+        data: 'trackData',
+        list: input.value('contributorContribs'),
+      }).outputs({
+        '#reverseContributionList': '#tracksAsContributor',
+      }),
+
+      withReverseContributionList({
+        data: 'trackData',
+        list: input.value('coverArtistContribs'),
+      }).outputs({
+        '#reverseContributionList': '#tracksAsCoverArtist',
+      }),
+
+      {
+        dependencies: [
+          '#tracksAsArtist',
+          '#tracksAsContributor',
+          '#tracksAsCoverArtist',
+        ],
+
+        compute: ({
+          ['#tracksAsArtist']: tracksAsArtist,
+          ['#tracksAsContributor']: tracksAsContributor,
+          ['#tracksAsCoverArtist']: tracksAsCoverArtist,
+        }) =>
+          unique([
+            ...tracksAsArtist,
+            ...tracksAsContributor,
+            ...tracksAsCoverArtist,
+          ]),
+      },
+    ],
+
+    tracksAsCommentator: reverseReferenceList({
+      data: 'trackData',
+      list: input.value('commentatorArtists'),
+    }),
+
+    albumsAsAlbumArtist: reverseContributionList({
+      data: 'albumData',
+      list: input.value('artistContribs'),
+    }),
+
+    albumsAsCoverArtist: reverseContributionList({
+      data: 'albumData',
+      list: input.value('coverArtistContribs'),
+    }),
+
+    albumsAsWallpaperArtist: reverseContributionList({
+      data: 'albumData',
+      list: input.value('wallpaperArtistContribs'),
+    }),
+
+    albumsAsBannerArtist: reverseContributionList({
+      data: 'albumData',
+      list: input.value('bannerArtistContribs'),
+    }),
+
+    albumsAsAny: [
+      withReverseContributionList({
+        data: 'albumData',
+        list: input.value('artistContribs'),
+      }).outputs({
+        '#reverseContributionList': '#albumsAsArtist',
+      }),
+
+      withReverseContributionList({
+        data: 'albumData',
+        list: input.value('coverArtistContribs'),
+      }).outputs({
+        '#reverseContributionList': '#albumsAsCoverArtist',
+      }),
+
+      withReverseContributionList({
+        data: 'albumData',
+        list: input.value('wallpaperArtistContribs'),
+      }).outputs({
+        '#reverseContributionList': '#albumsAsWallpaperArtist',
+      }),
+
+      withReverseContributionList({
+        data: 'albumData',
+        list: input.value('bannerArtistContribs'),
+      }).outputs({
+        '#reverseContributionList': '#albumsAsBannerArtist',
+      }),
+
+      {
+        dependencies: [
+          '#albumsAsArtist',
+          '#albumsAsCoverArtist',
+          '#albumsAsWallpaperArtist',
+          '#albumsAsBannerArtist',
+        ],
+
+        compute: ({
+          ['#albumsAsArtist']: albumsAsArtist,
+          ['#albumsAsCoverArtist']: albumsAsCoverArtist,
+          ['#albumsAsWallpaperArtist']: albumsAsWallpaperArtist,
+          ['#albumsAsBannerArtist']: albumsAsBannerArtist,
+        }) =>
+          unique([
+            ...albumsAsArtist,
+            ...albumsAsCoverArtist,
+            ...albumsAsWallpaperArtist,
+            ...albumsAsBannerArtist,
+          ]),
+      },
+    ],
+
+    albumsAsCommentator: reverseReferenceList({
+      data: 'albumData',
+      list: input.value('commentatorArtists'),
+    }),
+
+    flashesAsContributor: reverseContributionList({
+      data: 'flashData',
+      list: input.value('contributorContribs'),
+    }),
+
+    flashesAsCommentator: reverseReferenceList({
+      data: 'flashData',
+      list: input.value('commentatorArtists'),
+    }),
+  });
+
+  static [Thing.getSerializeDescriptors] = ({
+    serialize: S,
+  }) => ({
+    name: S.id,
+    directory: S.id,
+    urls: S.id,
+    contextNotes: S.id,
+
+    hasAvatar: S.id,
+    avatarFileExtension: S.id,
+
+    aliasNames: S.id,
+
+    tracksAsArtist: S.toRefs,
+    tracksAsContributor: S.toRefs,
+    tracksAsCoverArtist: S.toRefs,
+    tracksAsCommentator: S.toRefs,
+
+    albumsAsAlbumArtist: S.toRefs,
+    albumsAsCoverArtist: S.toRefs,
+    albumsAsWallpaperArtist: S.toRefs,
+    albumsAsBannerArtist: S.toRefs,
+    albumsAsCommentator: S.toRefs,
+
+    flashesAsContributor: S.toRefs,
+  });
+
+  static [Thing.findSpecs] = {
+    artist: {
+      referenceTypes: ['artist', 'artist-gallery'],
+      bindTo: 'artistData',
+
+      include: artist => !artist.isAlias,
+    },
+
+    artistAlias: {
+      referenceTypes: ['artist', 'artist-gallery'],
+      bindTo: 'artistData',
+
+      include: artist => artist.isAlias,
+
+      getMatchableDirectories(artist) {
+        const originalArtist = artist.aliasedArtist;
+
+        // Aliases never match by the same directory as the original.
+        if (artist.directory === originalArtist.directory) {
+          return [];
+        }
+
+        // Aliases never match by the same directory as some *previous* alias
+        // in the original's alias list. This is honestly a bit awkward, but it
+        // avoids artist aliases conflicting with each other when checking for
+        // duplicate directories.
+        for (const aliasName of originalArtist.aliasNames) {
+          // These are trouble. We should be accessing aliases' directories
+          // directly, but artists currently don't expose a reverse reference
+          // list for aliases. (This is pending a cleanup of "reverse reference"
+          // behavior in general.) It doesn't actually cause problems *here*
+          // because alias directories are computed from their names 100% of the
+          // time, but that *is* an assumption this code makes.
+          if (aliasName === artist.name) continue;
+          if (artist.directory === getKebabCase(aliasName)) {
+            return [];
+          }
+        }
+
+        // And, aliases never return just a blank string. This part is pretty
+        // spooky because it doesn't handle two differently named aliases, on
+        // different artists, who have names that are similar *apart* from a
+        // character that's shortened. But that's also so fundamentally scary
+        // that we can't support it properly with existing code, anyway - we
+        // would need to be able to specifically set a directory *on an alias,*
+        // which currently can't be done in YAML data files.
+        if (artist.directory === '') {
+          return [];
+        }
+
+        return [artist.directory];
+      },
+    },
+  };
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Artist': {property: 'name'},
+      'Directory': {property: 'directory'},
+      'URLs': {property: 'urls'},
+      'Context Notes': {property: 'contextNotes'},
+
+      'Has Avatar': {property: 'hasAvatar'},
+      'Avatar File Extension': {property: 'avatarFileExtension'},
+
+      'Aliases': {property: 'aliasNames'},
+
+      'Dead URLs': {ignore: true},
+
+      'Review Points': {ignore: true},
+    },
+  };
+
+  static [Thing.getYamlLoadingSpec] = ({
+    documentModes: {allInOne},
+    thingConstructors: {Artist},
+  }) => ({
+    title: `Process artists file`,
+    file: ARTIST_DATA_FILE,
+
+    documentMode: allInOne,
+    documentThing: Artist,
+
+    save(results) {
+      const artists = results;
+
+      const artistRefs =
+        artists.map(artist => Thing.getReference(artist));
+
+      const artistAliasNames =
+        artists.map(artist => artist.aliasNames);
+
+      const artistAliases =
+        stitchArrays({
+          originalArtistRef: artistRefs,
+          aliasNames: artistAliasNames,
+        }).flatMap(({originalArtistRef, aliasNames}) =>
+            aliasNames.map(name => {
+              const alias = new Artist();
+              alias.name = name;
+              alias.isAlias = true;
+              alias.aliasedArtist = originalArtistRef;
+              return alias;
+            }));
+
+      const artistData = [...artists, ...artistAliases];
+
+      return {artistData};
+    },
+
+    sort({artistData}) {
+      sortAlphabetically(artistData);
+    },
+  });
+
+  [inspect.custom]() {
+    const parts = [];
+
+    parts.push(Thing.prototype[inspect.custom].apply(this));
+
+    if (CacheableObject.getUpdateValue(this, 'isAlias')) {
+      parts.unshift(`${colors.yellow('[alias]')} `);
+
+      let aliasedArtist;
+      try {
+        aliasedArtist = this.aliasedArtist.name;
+      } catch (_error) {
+        aliasedArtist = CacheableObject.getUpdateValue(this, 'aliasedArtist');
+      }
+
+      parts.push(` ${colors.yellow(`[of ${aliasedArtist}]`)}`);
+    }
+
+    return parts.join('');
+  }
+}
diff --git a/src/data/things/flash.js b/src/data/things/flash.js
new file mode 100644
index 0000000..ceed79f
--- /dev/null
+++ b/src/data/things/flash.js
@@ -0,0 +1,370 @@
+export const FLASH_DATA_FILE = 'flashes.yaml';
+
+import {input} from '#composite';
+import find from '#find';
+import {empty} from '#sugar';
+import {sortFlashesChronologically} from '#sort';
+import Thing from '#thing';
+import {anyOf, isColor, isContentString, isDirectory, isNumber, isString}
+  from '#validators';
+import {parseDate, parseContributors} from '#yaml';
+
+import {withPropertyFromObject} from '#composite/data';
+
+import {
+  exposeConstant,
+  exposeDependency,
+  exposeDependencyOrContinue,
+  exposeUpdateValueOrContinue,
+} from '#composite/control-flow';
+
+import {
+  color,
+  commentary,
+  commentatorArtists,
+  contentString,
+  contributionList,
+  directory,
+  fileExtension,
+  name,
+  referenceList,
+  simpleDate,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
+import {withFlashAct} from '#composite/things/flash';
+import {withFlashSide} from '#composite/things/flash-act';
+
+export class Flash extends Thing {
+  static [Thing.referenceType] = 'flash';
+
+  static [Thing.getPropertyDescriptors] = ({Artist, Track, FlashAct}) => ({
+    // Update & expose
+
+    name: name('Unnamed Flash'),
+
+    directory: {
+      flags: {update: true, expose: true},
+      update: {validate: isDirectory},
+
+      // Flashes expose directory differently from other Things! Their
+      // default directory is dependent on the page number (or ID), not
+      // the name.
+      expose: {
+        dependencies: ['page'],
+        transform(directory, {page}) {
+          if (directory === null && page === null) return null;
+          else if (directory === null) return page;
+          else return directory;
+        },
+      },
+    },
+
+    page: {
+      flags: {update: true, expose: true},
+      update: {validate: anyOf(isString, isNumber)},
+
+      expose: {
+        transform: (value) => (value === null ? null : value.toString()),
+      },
+    },
+
+    color: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isColor),
+      }),
+
+      withFlashAct(),
+
+      withPropertyFromObject({
+        object: '#flashAct',
+        property: input.value('color'),
+      }),
+
+      exposeDependency({dependency: '#flashAct.color'}),
+    ],
+
+    date: simpleDate(),
+
+    coverArtFileExtension: fileExtension('jpg'),
+
+    contributorContribs: contributionList(),
+
+    featuredTracks: referenceList({
+      class: input.value(Track),
+      find: input.value(find.track),
+      data: 'trackData',
+    }),
+
+    urls: urls(),
+
+    commentary: commentary(),
+
+    // Update only
+
+    artistData: wikiData({
+      class: input.value(Artist),
+    }),
+
+    trackData: wikiData({
+      class: input.value(Track),
+    }),
+
+    flashActData: wikiData({
+      class: input.value(FlashAct),
+    }),
+
+    // Expose only
+
+    commentatorArtists: commentatorArtists(),
+
+    act: [
+      withFlashAct(),
+      exposeDependency({dependency: '#flashAct'}),
+    ],
+
+    side: [
+      withFlashAct(),
+
+      withPropertyFromObject({
+        object: '#flashAct',
+        property: input.value('side'),
+      }),
+
+      exposeDependency({dependency: '#flashAct.side'}),
+    ],
+  });
+
+  static [Thing.getSerializeDescriptors] = ({
+    serialize: S,
+  }) => ({
+    name: S.id,
+    page: S.id,
+    directory: S.id,
+    date: S.id,
+    contributors: S.toContribRefs,
+    tracks: S.toRefs,
+    urls: S.id,
+    color: S.id,
+  });
+
+  static [Thing.findSpecs] = {
+    flash: {
+      referenceTypes: ['flash'],
+      bindTo: 'flashData',
+    },
+  };
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Flash': {property: 'name'},
+      'Directory': {property: 'directory'},
+      'Page': {property: 'page'},
+      'Color': {property: 'color'},
+      'URLs': {property: 'urls'},
+
+      'Date': {
+        property: 'date',
+        transform: parseDate,
+      },
+
+      'Cover Art File Extension': {property: 'coverArtFileExtension'},
+
+      'Featured Tracks': {property: 'featuredTracks'},
+
+      'Contributors': {
+        property: 'contributorContribs',
+        transform: parseContributors,
+      },
+
+      'Commentary': {property: 'commentary'},
+
+      'Review Points': {ignore: true},
+    },
+  };
+}
+
+export class FlashAct extends Thing {
+  static [Thing.referenceType] = 'flash-act';
+  static [Thing.friendlyName] = `Flash Act`;
+
+  static [Thing.getPropertyDescriptors] = () => ({
+    // Update & expose
+
+    name: name('Unnamed Flash Act'),
+    directory: directory(),
+    color: color(),
+
+    listTerminology: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isContentString),
+      }),
+
+      withFlashSide(),
+
+      withPropertyFromObject({
+        object: '#flashSide',
+        property: input.value('listTerminology'),
+      }),
+
+      exposeDependencyOrContinue({
+        dependency: '#flashSide.listTerminology',
+      }),
+
+      exposeConstant({
+        value: input.value(null),
+      }),
+    ],
+
+    flashes: referenceList({
+      class: input.value(Flash),
+      find: input.value(find.flash),
+      data: 'flashData',
+    }),
+
+    // Update only
+
+    flashData: wikiData({
+      class: input.value(Flash),
+    }),
+
+    flashSideData: wikiData({
+      class: input.value(FlashSide),
+    }),
+
+    // Expose only
+
+    side: [
+      withFlashSide(),
+      exposeDependency({dependency: '#flashSide'}),
+    ],
+  });
+
+  static [Thing.findSpecs] = {
+    flashAct: {
+      referenceTypes: ['flash-act'],
+      bindTo: 'flashActData',
+    },
+  };
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Act': {property: 'name'},
+      'Directory': {property: 'directory'},
+
+      'Color': {property: 'color'},
+      'List Terminology': {property: 'listTerminology'},
+
+      'Review Points': {ignore: true},
+    },
+  };
+}
+
+export class FlashSide extends Thing {
+  static [Thing.referenceType] = 'flash-side';
+  static [Thing.friendlyName] = `Flash Side`;
+
+  static [Thing.getPropertyDescriptors] = () => ({
+    // Update & expose
+
+    name: name('Unnamed Flash Side'),
+    directory: directory(),
+    color: color(),
+    listTerminology: contentString(),
+
+    acts: referenceList({
+      class: input.value(FlashAct),
+      find: input.value(find.flashAct),
+      data: 'flashActData',
+    }),
+
+    // Update only
+
+    flashActData: wikiData({
+      class: input.value(FlashAct),
+    }),
+  });
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Side': {property: 'name'},
+      'Directory': {property: 'directory'},
+      'Color': {property: 'color'},
+      'List Terminology': {property: 'listTerminology'},
+    },
+  };
+
+  static [Thing.findSpecs] = {
+    flashSide: {
+      referenceTypes: ['flash-side'],
+      bindTo: 'flashSideData',
+    },
+  };
+
+  static [Thing.getYamlLoadingSpec] = ({
+    documentModes: {allInOne},
+    thingConstructors: {Flash, FlashAct},
+  }) => ({
+    title: `Process flashes file`,
+    file: FLASH_DATA_FILE,
+
+    documentMode: allInOne,
+    documentThing: document =>
+      ('Side' in document
+        ? FlashSide
+     : 'Act' in document
+        ? FlashAct
+        : Flash),
+
+    save(results) {
+      // JavaScript likes you.
+
+      if (!empty(results) && !(results[0] instanceof FlashSide)) {
+        throw new Error(`Expected a side at top of flash data file`);
+      }
+
+      let index = 0;
+      let thing;
+      for (; thing = results[index]; index++) {
+        const flashSide = thing;
+        const flashActRefs = [];
+
+        if (results[index + 1] instanceof Flash) {
+          throw new Error(`Expected an act to immediately follow a side`);
+        }
+
+        for (
+          index++;
+          (thing = results[index]) && thing instanceof FlashAct;
+          index++
+        ) {
+          const flashAct = thing;
+          const flashRefs = [];
+          for (
+            index++;
+            (thing = results[index]) && thing instanceof Flash;
+            index++
+          ) {
+            flashRefs.push(Thing.getReference(thing));
+          }
+          index--;
+          flashAct.flashes = flashRefs;
+          flashActRefs.push(Thing.getReference(flashAct));
+        }
+        index--;
+        flashSide.acts = flashActRefs;
+      }
+
+      const flashData = results.filter(x => x instanceof Flash);
+      const flashActData = results.filter(x => x instanceof FlashAct);
+      const flashSideData = results.filter(x => x instanceof FlashSide);
+
+      return {flashData, flashActData, flashSideData};
+    },
+
+    sort({flashData}) {
+      sortFlashesChronologically(flashData);
+    },
+  });
+}
diff --git a/src/data/things/group.js b/src/data/things/group.js
new file mode 100644
index 0000000..0dbbbb7
--- /dev/null
+++ b/src/data/things/group.js
@@ -0,0 +1,194 @@
+export const GROUP_DATA_FILE = 'groups.yaml';
+
+import {input} from '#composite';
+import find from '#find';
+import Thing from '#thing';
+
+import {
+  color,
+  contentString,
+  directory,
+  name,
+  referenceList,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
+export class Group extends Thing {
+  static [Thing.referenceType] = 'group';
+
+  static [Thing.getPropertyDescriptors] = ({Album}) => ({
+    // Update & expose
+
+    name: name('Unnamed Group'),
+    directory: directory(),
+
+    description: contentString(),
+
+    urls: urls(),
+
+    featuredAlbums: referenceList({
+      class: input.value(Album),
+      find: input.value(find.album),
+      data: 'albumData',
+    }),
+
+    // Update only
+
+    albumData: wikiData({
+      class: input.value(Album),
+    }),
+
+    groupCategoryData: wikiData({
+      class: input.value(GroupCategory),
+    }),
+
+    // Expose only
+
+    descriptionShort: {
+      flags: {expose: true},
+
+      expose: {
+        dependencies: ['description'],
+        compute: ({description}) =>
+          (description
+            ? description.split('<hr class="split">')[0]
+            : null),
+      },
+    },
+
+    albums: {
+      flags: {expose: true},
+
+      expose: {
+        dependencies: ['this', 'albumData'],
+        compute: ({this: group, albumData}) =>
+          albumData?.filter((album) => album.groups.includes(group)) ?? [],
+      },
+    },
+
+    color: {
+      flags: {expose: true},
+
+      expose: {
+        dependencies: ['this', 'groupCategoryData'],
+        compute: ({this: group, groupCategoryData}) =>
+          groupCategoryData.find((category) => category.groups.includes(group))
+            ?.color,
+      },
+    },
+
+    category: {
+      flags: {expose: true},
+
+      expose: {
+        dependencies: ['this', 'groupCategoryData'],
+        compute: ({this: group, groupCategoryData}) =>
+          groupCategoryData.find((category) => category.groups.includes(group)) ??
+          null,
+      },
+    },
+  });
+
+  static [Thing.findSpecs] = {
+    group: {
+      referenceTypes: ['group', 'group-gallery'],
+      bindTo: 'groupData',
+    },
+  };
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Group': {property: 'name'},
+      'Directory': {property: 'directory'},
+      'Description': {property: 'description'},
+      'URLs': {property: 'urls'},
+
+      'Featured Albums': {property: 'featuredAlbums'},
+
+      'Review Points': {ignore: true},
+    },
+  };
+
+  static [Thing.getYamlLoadingSpec] = ({
+    documentModes: {allInOne},
+    thingConstructors: {Group, GroupCategory},
+  }) => ({
+    title: `Process groups file`,
+    file: GROUP_DATA_FILE,
+
+    documentMode: allInOne,
+    documentThing: document =>
+      ('Category' in document
+        ? GroupCategory
+        : Group),
+
+    save(results) {
+      let groupCategory;
+      let groupRefs = [];
+
+      if (results[0] && !(results[0] instanceof GroupCategory)) {
+        throw new Error(`Expected a category at top of group data file`);
+      }
+
+      for (const thing of results) {
+        if (thing instanceof GroupCategory) {
+          if (groupCategory) {
+            Object.assign(groupCategory, {groups: groupRefs});
+          }
+
+          groupCategory = thing;
+          groupRefs = [];
+        } else {
+          groupRefs.push(Thing.getReference(thing));
+        }
+      }
+
+      if (groupCategory) {
+        Object.assign(groupCategory, {groups: groupRefs});
+      }
+
+      const groupData = results.filter(x => x instanceof Group);
+      const groupCategoryData = results.filter(x => x instanceof GroupCategory);
+
+      return {groupData, groupCategoryData};
+    },
+
+    // Groups aren't sorted at all, always preserving the order in the data
+    // file as-is.
+    sort: null,
+  });
+}
+
+export class GroupCategory extends Thing {
+  static [Thing.referenceType] = 'group-category';
+  static [Thing.friendlyName] = `Group Category`;
+
+  static [Thing.getPropertyDescriptors] = ({Group}) => ({
+    // Update & expose
+
+    name: name('Unnamed Group Category'),
+    directory: directory(),
+
+    color: color(),
+
+    groups: referenceList({
+      class: input.value(Group),
+      find: input.value(find.group),
+      data: 'groupData',
+    }),
+
+    // Update only
+
+    groupData: wikiData({
+      class: input.value(Group),
+    }),
+  });
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Category': {property: 'name'},
+      'Color': {property: 'color'},
+    },
+  };
+}
diff --git a/src/data/things/homepage-layout.js b/src/data/things/homepage-layout.js
new file mode 100644
index 0000000..00d6aef
--- /dev/null
+++ b/src/data/things/homepage-layout.js
@@ -0,0 +1,223 @@
+export const HOMEPAGE_LAYOUT_DATA_FILE = 'homepage.yaml';
+
+import {input} from '#composite';
+import find from '#find';
+import Thing from '#thing';
+
+import {
+  anyOf,
+  is,
+  isCountingNumber,
+  isString,
+  isStringNonEmpty,
+  validateArrayItems,
+  validateInstanceOf,
+  validateReference,
+} from '#validators';
+
+import {exposeDependency} from '#composite/control-flow';
+import {withResolvedReference} from '#composite/wiki-data';
+import {color, contentString, name, referenceList, wikiData}
+  from '#composite/wiki-properties';
+
+export class HomepageLayout extends Thing {
+  static [Thing.friendlyName] = `Homepage Layout`;
+
+  static [Thing.getPropertyDescriptors] = ({HomepageLayoutRow}) => ({
+    // Update & expose
+
+    sidebarContent: contentString(),
+
+    navbarLinks: {
+      flags: {update: true, expose: true},
+      update: {validate: validateArrayItems(isStringNonEmpty)},
+    },
+
+    rows: {
+      flags: {update: true, expose: true},
+
+      update: {
+        validate: validateArrayItems(validateInstanceOf(HomepageLayoutRow)),
+      },
+    },
+  });
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Homepage': {ignore: true},
+
+      'Sidebar Content': {property: 'sidebarContent'},
+      'Navbar Links': {property: 'navbarLinks'},
+    },
+  };
+}
+
+export class HomepageLayoutRow extends Thing {
+  static [Thing.friendlyName] = `Homepage Row`;
+
+  static [Thing.getPropertyDescriptors] = ({Album, Group}) => ({
+    // Update & expose
+
+    name: name('Unnamed Homepage Row'),
+
+    type: {
+      flags: {update: true, expose: true},
+
+      update: {
+        validate() {
+          throw new Error(`'type' property validator must be overridden`);
+        },
+      },
+    },
+
+    color: color(),
+
+    // Update only
+
+    // These wiki data arrays aren't necessarily used by every subclass, but
+    // to the convenience of providing these, the superclass accepts all wiki
+    // data arrays depended upon by any subclass.
+
+    albumData: wikiData({
+      class: input.value(Album),
+    }),
+
+    groupData: wikiData({
+      class: input.value(Group),
+    }),
+  });
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Row': {property: 'name'},
+      'Color': {property: 'color'},
+      'Type': {property: 'type'},
+    },
+  };
+}
+
+export class HomepageLayoutAlbumsRow extends HomepageLayoutRow {
+  static [Thing.friendlyName] = `Homepage Albums Row`;
+
+  static [Thing.getPropertyDescriptors] = (opts, {Album, Group} = opts) => ({
+    ...HomepageLayoutRow[Thing.getPropertyDescriptors](opts),
+
+    // Update & expose
+
+    type: {
+      flags: {update: true, expose: true},
+      update: {
+        validate(value) {
+          if (value !== 'albums') {
+            throw new TypeError(`Expected 'albums'`);
+          }
+
+          return true;
+        },
+      },
+    },
+
+    displayStyle: {
+      flags: {update: true, expose: true},
+
+      update: {
+        validate: is('grid', 'carousel'),
+      },
+
+      expose: {
+        transform: (displayStyle) =>
+          displayStyle ?? 'grid',
+      },
+    },
+
+    sourceGroup: [
+      {
+        flags: {expose: true, update: true, compose: true},
+
+        update: {
+          validate:
+            anyOf(
+              is('new-releases', 'new-additions'),
+              validateReference(Group[Thing.referenceType])),
+        },
+
+        expose: {
+          transform: (value, continuation) =>
+            (value === 'new-releases' || value === 'new-additions'
+              ? value
+              : continuation(value)),
+        },
+      },
+
+      withResolvedReference({
+        ref: input.updateValue(),
+        data: 'groupData',
+        find: input.value(find.group),
+      }),
+
+      exposeDependency({dependency: '#resolvedReference'}),
+    ],
+
+    sourceAlbums: referenceList({
+      class: input.value(Album),
+      find: input.value(find.album),
+      data: 'albumData',
+    }),
+
+    countAlbumsFromGroup: {
+      flags: {update: true, expose: true},
+      update: {validate: isCountingNumber},
+    },
+
+    actionLinks: {
+      flags: {update: true, expose: true},
+      update: {validate: validateArrayItems(isString)},
+    },
+  });
+
+  static [Thing.yamlDocumentSpec] = Thing.extendDocumentSpec(HomepageLayoutRow, {
+    fields: {
+      'Display Style': {property: 'displayStyle'},
+      'Group': {property: 'sourceGroup'},
+      'Count': {property: 'countAlbumsFromGroup'},
+      'Albums': {property: 'sourceAlbums'},
+      'Actions': {property: 'actionLinks'},
+    },
+  });
+
+  static [Thing.getYamlLoadingSpec] = ({
+    documentModes: {headerAndEntries}, // Kludge, see below
+    thingConstructors: {
+      HomepageLayout,
+      HomepageLayoutAlbumsRow,
+    },
+  }) => ({
+    title: `Process homepage layout file`,
+
+    // Kludge: This benefits from the same headerAndEntries style messaging as
+    // albums and tracks (for example), but that document mode is designed to
+    // support multiple files, and only one is actually getting processed here.
+    files: [HOMEPAGE_LAYOUT_DATA_FILE],
+
+    documentMode: headerAndEntries,
+    headerDocumentThing: HomepageLayout,
+    entryDocumentThing: document => {
+      switch (document['Type']) {
+        case 'albums':
+          return HomepageLayoutAlbumsRow;
+        default:
+          throw new TypeError(`No processDocument function for row type ${document['Type']}!`);
+      }
+    },
+
+    save(results) {
+      if (!results[0]) {
+        return;
+      }
+
+      const {header: homepageLayout, entries: rows} = results[0];
+      Object.assign(homepageLayout, {rows});
+      return {homepageLayout};
+    },
+  });
+}
diff --git a/src/data/things/index.js b/src/data/things/index.js
new file mode 100644
index 0000000..3bf8409
--- /dev/null
+++ b/src/data/things/index.js
@@ -0,0 +1,188 @@
+import * as path from 'node:path';
+import {fileURLToPath} from 'node:url';
+
+import {openAggregate, showAggregate} from '#aggregate';
+import {logError} from '#cli';
+import {compositeFrom} from '#composite';
+import * as serialize from '#serialize';
+
+import Thing from '#thing';
+
+import * as albumClasses from './album.js';
+import * as artTagClasses from './art-tag.js';
+import * as artistClasses from './artist.js';
+import * as flashClasses from './flash.js';
+import * as groupClasses from './group.js';
+import * as homepageLayoutClasses from './homepage-layout.js';
+import * as languageClasses from './language.js';
+import * as newsEntryClasses from './news-entry.js';
+import * as staticPageClasses from './static-page.js';
+import * as trackClasses from './track.js';
+import * as wikiInfoClasses from './wiki-info.js';
+
+const allClassLists = {
+  'album.js': albumClasses,
+  'art-tag.js': artTagClasses,
+  'artist.js': artistClasses,
+  'flash.js': flashClasses,
+  'group.js': groupClasses,
+  'homepage-layout.js': homepageLayoutClasses,
+  'language.js': languageClasses,
+  'news-entry.js': newsEntryClasses,
+  'static-page.js': staticPageClasses,
+  'track.js': trackClasses,
+  'wiki-info.js': wikiInfoClasses,
+};
+
+let allClasses = Object.create(null);
+
+// src/data/things/index.js -> src/
+const __dirname = path.dirname(
+  path.resolve(
+    fileURLToPath(import.meta.url),
+    '../..'));
+
+function niceShowAggregate(error, ...opts) {
+  showAggregate(error, {
+    pathToFileURL: (f) => path.relative(__dirname, fileURLToPath(f)),
+    ...opts,
+  });
+}
+
+function errorDuplicateClassNames() {
+  const locationDict = Object.create(null);
+
+  for (const [location, classes] of Object.entries(allClassLists)) {
+    for (const className of Object.keys(classes)) {
+      if (className in locationDict) {
+        locationDict[className].push(location);
+      } else {
+        locationDict[className] = [location];
+      }
+    }
+  }
+
+  let success = true;
+
+  for (const [className, locations] of Object.entries(locationDict)) {
+    if (locations.length === 1) {
+      continue;
+    }
+
+    logError`Thing class name ${`"${className}"`} is defined more than once: ${locations.join(', ')}`;
+    success = false;
+  }
+
+  return success;
+}
+
+function flattenClassLists() {
+  for (const classes of Object.values(allClassLists)) {
+    for (const [name, constructor] of Object.entries(classes)) {
+      if (typeof constructor !== 'function') continue;
+      if (!(constructor.prototype instanceof Thing)) continue;
+      allClasses[name] = constructor;
+    }
+  }
+}
+
+function descriptorAggregateHelper({
+  showFailedClasses,
+  message,
+  op,
+}) {
+  const failureSymbol = Symbol();
+  const aggregate = openAggregate({
+    message,
+    returnOnFail: failureSymbol,
+  });
+
+  const failedClasses = [];
+
+  for (const [name, constructor] of Object.entries(allClasses)) {
+    const result = aggregate.call(op, constructor);
+
+    if (result === failureSymbol) {
+      failedClasses.push(name);
+    }
+  }
+
+  try {
+    aggregate.close();
+    return true;
+  } catch (error) {
+    niceShowAggregate(error);
+    showFailedClasses(failedClasses);
+    return false;
+  }
+}
+
+function evaluatePropertyDescriptors() {
+  const opts = {...allClasses};
+
+  return descriptorAggregateHelper({
+    message: `Errors evaluating Thing class property descriptors`,
+
+    op(constructor) {
+      if (!constructor[Thing.getPropertyDescriptors]) {
+        throw new Error(`Missing [Thing.getPropertyDescriptors] function`);
+      }
+
+      const results = constructor[Thing.getPropertyDescriptors](opts);
+
+      for (const [key, value] of Object.entries(results)) {
+        if (Array.isArray(value)) {
+          results[key] = compositeFrom({
+            annotation: `${constructor.name}.${key}`,
+            compose: false,
+            steps: value,
+          });
+        } else if (value.toResolvedComposition) {
+          results[key] = compositeFrom(value.toResolvedComposition());
+        }
+      }
+
+      constructor.propertyDescriptors = results;
+    },
+
+    showFailedClasses(failedClasses) {
+      logError`Failed to evaluate property descriptors for classes: ${failedClasses.join(', ')}`;
+    },
+  });
+}
+
+function evaluateSerializeDescriptors() {
+  const opts = {...allClasses, serialize};
+
+  return descriptorAggregateHelper({
+    message: `Errors evaluating Thing class serialize descriptors`,
+
+    op(constructor) {
+      if (!constructor[Thing.getSerializeDescriptors]) {
+        return;
+      }
+
+      constructor[serialize.serializeDescriptors] =
+        constructor[Thing.getSerializeDescriptors](opts);
+    },
+
+    showFailedClasses(failedClasses) {
+      logError`Failed to evaluate serialize descriptors for classes: ${failedClasses.join(', ')}`;
+    },
+  });
+}
+
+if (!errorDuplicateClassNames())
+  process.exit(1);
+
+flattenClassLists();
+
+if (!evaluatePropertyDescriptors())
+  process.exit(1);
+
+if (!evaluateSerializeDescriptors())
+  process.exit(1);
+
+Object.assign(allClasses, {Thing});
+
+export default allClasses;
diff --git a/src/data/things/language.js b/src/data/things/language.js
new file mode 100644
index 0000000..dbe1ff3
--- /dev/null
+++ b/src/data/things/language.js
@@ -0,0 +1,730 @@
+import { Temporal, toTemporalInstant } from '@js-temporal/polyfill';
+
+import {withAggregate} from '#aggregate';
+import CacheableObject from '#cacheable-object';
+import {logWarn} from '#cli';
+import * as html from '#html';
+import {empty} from '#sugar';
+import {isLanguageCode} from '#validators';
+import Thing from '#thing';
+
+import {
+  getExternalLinkStringOfStyleFromDescriptors,
+  getExternalLinkStringsFromDescriptors,
+  isExternalLinkContext,
+  isExternalLinkSpec,
+  isExternalLinkStyle,
+} from '#external-links';
+
+import {externalFunction, flag, name} from '#composite/wiki-properties';
+
+export const languageOptionRegex = /{(?<name>[A-Z0-9_]+)}/g;
+
+export class Language extends Thing {
+  static [Thing.getPropertyDescriptors] = () => ({
+    // Update & expose
+
+    // General language code. This is used to identify the language distinctly
+    // from other languages (similar to how "Directory" operates in many data
+    // objects).
+    code: {
+      flags: {update: true, expose: true},
+      update: {validate: isLanguageCode},
+    },
+
+    // Human-readable name. This should be the language's own native name, not
+    // localized to any other language.
+    name: name(`Unnamed Language`),
+
+    // Language code specific to JavaScript's Internationalization (Intl) API.
+    // Usually this will be the same as the language's general code, but it
+    // may be overridden to provide Intl constructors an alternative value.
+    intlCode: {
+      flags: {update: true, expose: true},
+      update: {validate: isLanguageCode},
+      expose: {
+        dependencies: ['code'],
+        transform: (intlCode, {code}) => intlCode ?? code,
+      },
+    },
+
+    // Flag which represents whether or not to hide a language from general
+    // access. If a language is hidden, its portion of the website will still
+    // be built (with all strings localized to the language), but it won't be
+    // included in controls for switching languages or the <link rel=alternate>
+    // tags used for search engine optimization. This flag is intended for use
+    // with languages that are currently in development and not ready for
+    // formal release, or which are just kept hidden as "experimental zones"
+    // for wiki development or content testing.
+    hidden: flag(false),
+
+    // Mapping of translation keys to values (strings). Generally, don't
+    // access this object directly - use methods instead.
+    strings: {
+      flags: {update: true, expose: true},
+      update: {validate: (t) => typeof t === 'object'},
+
+      expose: {
+        dependencies: ['inheritedStrings', 'code'],
+        transform(strings, {inheritedStrings, code}) {
+          if (!strings && !inheritedStrings) return null;
+          if (!inheritedStrings) return strings;
+
+          const validStrings = {
+            ...inheritedStrings,
+            ...strings,
+          };
+
+          const optionsFromTemplate = template =>
+            Array.from(template.matchAll(languageOptionRegex))
+              .map(({groups}) => groups.name);
+
+          for (const [key, providedTemplate] of Object.entries(strings)) {
+            const inheritedTemplate = inheritedStrings[key];
+            if (!inheritedTemplate) continue;
+
+            const providedOptions = optionsFromTemplate(providedTemplate);
+            const inheritedOptions = optionsFromTemplate(inheritedTemplate);
+
+            const missingOptionNames =
+              inheritedOptions.filter(name => !providedOptions.includes(name));
+
+            const misplacedOptionNames =
+              providedOptions.filter(name => !inheritedOptions.includes(name));
+
+            if (!empty(missingOptionNames) || !empty(misplacedOptionNames)) {
+              logWarn`Not using ${code ?? '(no code)'} string ${key}:`;
+              if (!empty(missingOptionNames))
+                logWarn`- Missing options: ${missingOptionNames.join(', ')}`;
+              if (!empty(misplacedOptionNames))
+                logWarn`- Unexpected options: ${misplacedOptionNames.join(', ')}`;
+              validStrings[key] = inheritedStrings[key];
+            }
+          }
+
+          return validStrings;
+        },
+      },
+    },
+
+    // May be provided to specify "default" strings, generally (but not
+    // necessarily) inherited from another Language object.
+    inheritedStrings: {
+      flags: {update: true, expose: true},
+      update: {validate: (t) => typeof t === 'object'},
+    },
+
+    // List of descriptors for providing to external link utilities when using
+    // language.formatExternalLink - refer to util/external-links.js for info.
+    externalLinkSpec: {
+      flags: {update: true, expose: true},
+      update: {validate: isExternalLinkSpec},
+    },
+
+    // Update only
+
+    escapeHTML: externalFunction(),
+
+    // Expose only
+
+    intl_date: this.#intlHelper(Intl.DateTimeFormat, {full: true}),
+    intl_number: this.#intlHelper(Intl.NumberFormat),
+    intl_listConjunction: this.#intlHelper(Intl.ListFormat, {type: 'conjunction'}),
+    intl_listDisjunction: this.#intlHelper(Intl.ListFormat, {type: 'disjunction'}),
+    intl_listUnit: this.#intlHelper(Intl.ListFormat, {type: 'unit'}),
+    intl_pluralCardinal: this.#intlHelper(Intl.PluralRules, {type: 'cardinal'}),
+    intl_pluralOrdinal: this.#intlHelper(Intl.PluralRules, {type: 'ordinal'}),
+
+    validKeys: {
+      flags: {expose: true},
+
+      expose: {
+        dependencies: ['strings', 'inheritedStrings'],
+        compute: ({strings, inheritedStrings}) =>
+          Array.from(
+            new Set([
+              ...Object.keys(inheritedStrings ?? {}),
+              ...Object.keys(strings ?? {}),
+            ])
+          ),
+      },
+    },
+
+    // TODO: This currently isn't used. Is it still needed?
+    strings_htmlEscaped: {
+      flags: {expose: true},
+      expose: {
+        dependencies: ['strings', 'inheritedStrings', 'escapeHTML'],
+        compute({strings, inheritedStrings, escapeHTML}) {
+          if (!(strings || inheritedStrings) || !escapeHTML) return null;
+          const allStrings = {...inheritedStrings, ...strings};
+          return Object.fromEntries(
+            Object.entries(allStrings).map(([k, v]) => [k, escapeHTML(v)])
+          );
+        },
+      },
+    },
+  });
+
+  static #intlHelper (constructor, opts) {
+    return {
+      flags: {expose: true},
+      expose: {
+        dependencies: ['code', 'intlCode'],
+        compute: ({code, intlCode}) => {
+          const constructCode = intlCode ?? code;
+          if (!constructCode) return null;
+          return Reflect.construct(constructor, [constructCode, opts]);
+        },
+      },
+    };
+  }
+
+  $(...args) {
+    return this.formatString(...args);
+  }
+
+  assertIntlAvailable(property) {
+    if (!this[property]) {
+      throw new Error(`Intl API ${property} unavailable`);
+    }
+  }
+
+  getUnitForm(value) {
+    this.assertIntlAvailable('intl_pluralCardinal');
+    return this.intl_pluralCardinal.select(value);
+  }
+
+  formatString(...args) {
+    const hasOptions =
+      typeof args.at(-1) === 'object' &&
+      args.at(-1) !== null;
+
+    const key =
+      (hasOptions ? args.slice(0, -1) : args)
+        .filter(Boolean)
+        .join('.');
+
+    const options =
+      (hasOptions
+        ? args.at(-1)
+        : {});
+
+    if (!this.strings) {
+      throw new Error(`Strings unavailable`);
+    }
+
+    if (!this.validKeys.includes(key)) {
+      throw new Error(`Invalid key ${key} accessed`);
+    }
+
+    // These will be filled up as we iterate over the template, slotting in
+    // each option (if it's present).
+    const missingOptionNames = new Set();
+
+    // And this will have entries deleted as they're encountered in the
+    // template. Leftover entries are misplaced.
+    const optionsMap =
+      new Map(
+        Object.entries(options).map(([name, value]) => [
+          name
+            .replace(/[A-Z]/g, '_$&')
+            .toUpperCase(),
+          value,
+        ]));
+
+    const output = this.#iterateOverTemplate({
+      template: this.strings[key],
+
+      match: languageOptionRegex,
+
+      insert: ({name: optionName}, canceledForming) => {
+        if (optionsMap.has(optionName)) {
+          let optionValue;
+
+          // We'll only need the option's value if we're going to use it as
+          // part of the formed output (see below).
+          if (!canceledForming) {
+            optionValue = optionsMap.get(optionName);
+          }
+
+          // But we always have to delete expected options off the provided
+          // option map, since the leftovers are what will be used to tell
+          // which are misplaced.
+          optionsMap.delete(optionName);
+
+          if (canceledForming) {
+            return undefined;
+          } else {
+            return optionValue;
+          }
+        } else {
+          // We don't need to continue forming the output if we've hit a
+          // missing option name, since the end result of this formatString
+          // call will be a thrown error, and formed output won't be needed.
+          missingOptionNames.add(optionName);
+          return undefined;
+        }
+      },
+    });
+
+    const misplacedOptionNames =
+      Array.from(optionsMap.keys());
+
+    withAggregate({message: `Errors in options for string "${key}"`}, ({push}) => {
+      if (!empty(missingOptionNames)) {
+        const names = Array.from(missingOptionNames).join(`, `);
+        push(new Error(`Missing options: ${names}`));
+      }
+
+      if (!empty(misplacedOptionNames)) {
+        const names = Array.from(misplacedOptionNames).join(`, `);
+        push(new Error(`Unexpected options: ${names}`));
+      }
+    });
+
+    return output;
+  }
+
+  #iterateOverTemplate({
+    template,
+    match: regexp,
+    insert: insertFn,
+  }) {
+    const outputParts = [];
+
+    let canceledForming = false;
+
+    let lastIndex = 0;
+    let partInProgress = '';
+
+    for (const match of template.matchAll(regexp)) {
+      const insertion =
+        insertFn(match.groups, canceledForming);
+
+      if (insertion === undefined) {
+        canceledForming = true;
+      }
+
+      // Don't proceed with forming logic if the insertion function has
+      // indicated that's not needed anymore - but continue iterating over
+      // the rest of the template's matches, so other iteration logic (with
+      // side effects) gets to process everything.
+      if (canceledForming) {
+        continue;
+      }
+
+      partInProgress += template.slice(lastIndex, match.index);
+
+      // Sanitize string arguments in particular. These are taken to come from
+      // (raw) data and may include special characters that aren't meant to be
+      // rendered as HTML markup.
+      const sanitizedInsertion =
+        this.#sanitizeValueForInsertion(insertion);
+
+      if (typeof sanitizedInsertion === 'string') {
+        // Join consecutive strings together.
+        partInProgress += sanitizedInsertion;
+      } else if (
+        sanitizedInsertion instanceof html.Tag &&
+        sanitizedInsertion.contentOnly
+      ) {
+        // Collapse string-only tag contents onto the current string part.
+        partInProgress += sanitizedInsertion.toString();
+      } else {
+        // Push the string part in progress, then the insertion as-is.
+        outputParts.push(partInProgress);
+        outputParts.push(sanitizedInsertion);
+        partInProgress = '';
+      }
+
+      lastIndex = match.index + match[0].length;
+    }
+
+    if (canceledForming) {
+      return undefined;
+    }
+
+    // Tack onto the final partInProgress, which may still have a value by this
+    // point, if the final inserted value was a string. (Otherwise, it'll just
+    // be equal to the remaining template text.)
+    if (lastIndex < template.length) {
+      partInProgress += template.slice(lastIndex);
+    }
+
+    if (partInProgress) {
+      outputParts.push(partInProgress);
+    }
+
+    return this.#wrapSanitized(outputParts);
+  }
+
+  // Processes a value so that it's suitable to be inserted into a template.
+  // For strings, this escapes HTML special characters, displaying them as-are
+  // instead of representing HTML markup. For numbers and booleans, this turns
+  // them into string values, so they never accidentally get caught as falsy
+  // by #html stringification. Everything else - most importantly including
+  // html.Tag objects - gets left as-is, preserving the value exactly as it's
+  // provided.
+  #sanitizeValueForInsertion(value) {
+    const escapeHTML = CacheableObject.getUpdateValue(this, 'escapeHTML');
+    if (!escapeHTML) {
+      throw new Error(`escapeHTML unavailable`);
+    }
+
+    switch (typeof value) {
+      case 'string':
+        return escapeHTML(value);
+
+      case 'number':
+      case 'boolean':
+        return value.toString();
+
+      default:
+        return value;
+    }
+  }
+
+  // Wraps the output of a formatting function in a no-name-nor-attributes
+  // HTML tag, which will indicate to other calls to formatString that this
+  // content is a string *that may contain HTML* and doesn't need to
+  // sanitized any further. It'll still .toString() to just the string
+  // contents, if needed.
+  #wrapSanitized(content) {
+    return html.tags(content, {
+      [html.blessAttributes]: true,
+      [html.joinChildren]: '',
+      [html.noEdgeWhitespace]: true,
+    });
+  }
+
+  // Similar to the above internal methods, but this one is public.
+  // It should be used when embedding content that may not have previously
+  // been sanitized directly into an HTML tag or template's contents.
+  // The templating engine usually handles this on its own, as does passing
+  // a value (sanitized or not) directly for inserting into formatting
+  // functions, but if you used a custom slot validation function (for example,
+  // {validate: v => v.isHTML} instead of {type: 'string'} / {type: 'html'})
+  // and are embedding the contents of the slot as a direct child of another
+  // tag, you should manually sanitize those contents with this function.
+  sanitize(value) {
+    if (typeof value === 'string') {
+      return this.#wrapSanitized(this.#sanitizeValueForInsertion(value));
+    } else {
+      return value;
+    }
+  }
+
+  formatDate(date) {
+    this.assertIntlAvailable('intl_date');
+    return this.intl_date.format(date);
+  }
+
+  formatDateRange(startDate, endDate) {
+    this.assertIntlAvailable('intl_date');
+    return this.intl_date.formatRange(startDate, endDate);
+  }
+
+  formatDateDuration({
+    years: numYears = 0,
+    months: numMonths = 0,
+    days: numDays = 0,
+    approximate = false,
+  }) {
+    let basis;
+
+    const years = this.countYears(numYears, {unit: true});
+    const months = this.countMonths(numMonths, {unit: true});
+    const days = this.countDays(numDays, {unit: true});
+
+    if (numYears && numMonths && numDays)
+      basis = this.formatString('count.dateDuration.yearsMonthsDays', {years, months, days});
+    else if (numYears && numMonths)
+      basis = this.formatString('count.dateDuration.yearsMonths', {years, months});
+    else if (numYears && numDays)
+      basis = this.formatString('count.dateDuration.yearsDays', {years, days});
+    else if (numYears)
+      basis = this.formatString('count.dateDuration.years', {years});
+    else if (numMonths && numDays)
+      basis = this.formatString('count.dateDuration.monthsDays', {months, days});
+    else if (numMonths)
+      basis = this.formatString('count.dateDuration.months', {months});
+    else if (numDays)
+      basis = this.formatString('count.dateDuration.days', {days});
+    else
+      return this.formatString('count.dateDuration.zero');
+
+    if (approximate) {
+      return this.formatString('count.dateDuration.approximate', {
+        duration: basis,
+      });
+    } else {
+      return basis;
+    }
+  }
+
+  formatRelativeDate(currentDate, referenceDate, {
+    considerRoundingDays = false,
+    approximate = true,
+    absolute = true,
+  } = {}) {
+    const currentInstant = toTemporalInstant.apply(currentDate);
+    const referenceInstant = toTemporalInstant.apply(referenceDate);
+
+    const comparison =
+      Temporal.Instant.compare(currentInstant, referenceInstant);
+
+    if (comparison === 0) {
+      return this.formatString('count.dateDuration.same');
+    }
+
+    const currentTDZ = currentInstant.toZonedDateTimeISO('Etc/UTC');
+    const referenceTDZ = referenceInstant.toZonedDateTimeISO('Etc/UTC');
+
+    const earlierTDZ = (comparison === -1 ? currentTDZ : referenceTDZ);
+    const laterTDZ = (comparison === 1 ? currentTDZ : referenceTDZ);
+
+    const {years, months, days} =
+      laterTDZ.since(earlierTDZ, {
+        largestUnit: 'year',
+        smallestUnit:
+          (considerRoundingDays
+            ? (laterTDZ.since(earlierTDZ, {
+                largestUnit: 'year',
+                smallestUnit: 'day',
+              }).years
+                ? 'month'
+                : 'day')
+            : 'day'),
+        roundingMode: 'halfCeil',
+      });
+
+    const duration =
+      this.formatDateDuration({
+        years, months, days,
+        approximate: false,
+      });
+
+    const relative =
+      this.formatString(
+        'count.dateDuration',
+        (approximate && (years || months || days)
+          ? (comparison === -1
+              ? 'approximateEarlier'
+              : 'approximateLater')
+          : (comparison === -1
+              ? 'earlier'
+              : 'later')),
+        {duration});
+
+    if (absolute) {
+      return this.formatString('count.dateDuration.relativeAbsolute', {
+        relative,
+        absolute: this.formatDate(currentDate),
+      });
+    } else {
+      return relative;
+    }
+  }
+
+  formatDuration(secTotal, {approximate = false, unit = false} = {}) {
+    if (secTotal === 0) {
+      return this.formatString('count.duration.missing');
+    }
+
+    const hour = Math.floor(secTotal / 3600);
+    const min = Math.floor((secTotal - hour * 3600) / 60);
+    const sec = Math.floor(secTotal - hour * 3600 - min * 60);
+
+    const pad = (val) => val.toString().padStart(2, '0');
+
+    const stringSubkey = unit ? '.withUnit' : '';
+
+    const duration =
+      hour > 0
+        ? this.formatString('count.duration.hours' + stringSubkey, {
+            hours: hour,
+            minutes: pad(min),
+            seconds: pad(sec),
+          })
+        : this.formatString('count.duration.minutes' + stringSubkey, {
+            minutes: min,
+            seconds: pad(sec),
+          });
+
+    return approximate
+      ? this.formatString('count.duration.approximate', {duration})
+      : duration;
+  }
+
+  formatExternalLink(url, {
+    style = 'platform',
+    context = 'generic',
+  } = {}) {
+    if (!this.externalLinkSpec) {
+      throw new TypeError(`externalLinkSpec unavailable`);
+    }
+
+    isExternalLinkContext(context);
+
+    if (style === 'all') {
+      return getExternalLinkStringsFromDescriptors(url, this.externalLinkSpec, {
+        language: this,
+        context,
+      });
+    }
+
+    isExternalLinkStyle(style);
+
+    const result =
+      getExternalLinkStringOfStyleFromDescriptors(url, style, this.externalLinkSpec, {
+        language: this,
+        context,
+      });
+
+    // It's possible for there to not actually be any string available for the
+    // given URL, style, and context, and we want this to be detectable via
+    // html.blank().
+    return result ?? html.blank();
+  }
+
+  formatIndex(value) {
+    this.assertIntlAvailable('intl_pluralOrdinal');
+    return this.formatString('count.index.' + this.intl_pluralOrdinal.select(value), {index: value});
+  }
+
+  formatNumber(value) {
+    this.assertIntlAvailable('intl_number');
+    return this.intl_number.format(value);
+  }
+
+  formatWordCount(value) {
+    const num = this.formatNumber(
+      value > 1000 ? Math.floor(value / 100) / 10 : value
+    );
+
+    const words =
+      value > 1000
+        ? this.formatString('count.words.thousand', {words: num})
+        : this.formatString('count.words', {words: num});
+
+    return this.formatString('count.words.withUnit.' + this.getUnitForm(value), {words});
+  }
+
+  #formatListHelper(array, processFn) {
+    // Operate on "insertion markers" instead of the actual contents of the
+    // array, because the process function (likely an Intl operation) is taken
+    // to only operate on strings. We'll insert the contents of the array back
+    // at these points afterwards.
+
+    const insertionMarkers =
+      Array.from(
+        {length: array.length},
+        (_item, index) => `<::insertion_${index}>`);
+
+    // Basically the same insertion logic as in formatString. Like there, we
+    // can't assume that insertion markers were kept in the same order as they
+    // were provided, so we'll refer to the marked index. But we don't need to
+    // worry about some of the indices *not* corresponding to a provided source
+    // item, like we do in formatString, so that cuts out a lot of the
+    // validation logic.
+
+    return this.#iterateOverTemplate({
+      template: processFn(insertionMarkers),
+
+      match: /<::insertion_(?<index>[0-9]+)>/g,
+
+      insert: ({index: markerIndex}) => {
+        return array[markerIndex];
+      },
+    });
+  }
+
+  // Conjunction list: A, B, and C
+  formatConjunctionList(array) {
+    this.assertIntlAvailable('intl_listConjunction');
+    return this.#formatListHelper(
+      array,
+      array => this.intl_listConjunction.format(array));
+  }
+
+  // Disjunction lists: A, B, or C
+  formatDisjunctionList(array) {
+    this.assertIntlAvailable('intl_listDisjunction');
+    return this.#formatListHelper(
+      array,
+      array => this.intl_listDisjunction.format(array));
+  }
+
+  // Unit lists: A, B, C
+  formatUnitList(array) {
+    this.assertIntlAvailable('intl_listUnit');
+    return this.#formatListHelper(
+      array,
+      array => this.intl_listUnit.format(array));
+  }
+
+  // Lists without separator: A B C
+  formatListWithoutSeparator(array) {
+    return this.#formatListHelper(
+      array,
+      array => array.join(' '));
+  }
+
+  // File sizes: 42.5 kB, 127.2 MB, 4.13 GB, 998.82 TB
+  formatFileSize(bytes) {
+    if (!bytes) return '';
+
+    bytes = parseInt(bytes);
+    if (isNaN(bytes)) return '';
+
+    const round = (exp) => Math.round(bytes / 10 ** (exp - 1)) / 10;
+
+    if (bytes >= 10 ** 12) {
+      return this.formatString('count.fileSize.terabytes', {
+        terabytes: round(12),
+      });
+    } else if (bytes >= 10 ** 9) {
+      return this.formatString('count.fileSize.gigabytes', {
+        gigabytes: round(9),
+      });
+    } else if (bytes >= 10 ** 6) {
+      return this.formatString('count.fileSize.megabytes', {
+        megabytes: round(6),
+      });
+    } else if (bytes >= 10 ** 3) {
+      return this.formatString('count.fileSize.kilobytes', {
+        kilobytes: round(3),
+      });
+    } else {
+      return this.formatString('count.fileSize.bytes', {bytes});
+    }
+  }
+}
+
+const countHelper = (stringKey, optionName = stringKey) =>
+  function(value, {unit = false} = {}) {
+    return this.formatString(
+      unit
+        ? `count.${stringKey}.withUnit.` + this.getUnitForm(value)
+        : `count.${stringKey}`,
+      {[optionName]: this.formatNumber(value)});
+  };
+
+// TODO: These are hard-coded. Is there a better way?
+Object.assign(Language.prototype, {
+  countAdditionalFiles: countHelper('additionalFiles', 'files'),
+  countAlbums: countHelper('albums'),
+  countArtworks: countHelper('artworks'),
+  countCommentaryEntries: countHelper('commentaryEntries', 'entries'),
+  countContributions: countHelper('contributions'),
+  countCoverArts: countHelper('coverArts'),
+  countDays: countHelper('days'),
+  countFlashes: countHelper('flashes'),
+  countMonths: countHelper('months'),
+  countTimesReferenced: countHelper('timesReferenced'),
+  countTimesUsed: countHelper('timesUsed'),
+  countTracks: countHelper('tracks'),
+  countWeeks: countHelper('weeks'),
+  countYears: countHelper('years'),
+});
diff --git a/src/data/things/news-entry.js b/src/data/things/news-entry.js
new file mode 100644
index 0000000..43d1638
--- /dev/null
+++ b/src/data/things/news-entry.js
@@ -0,0 +1,73 @@
+export const NEWS_DATA_FILE = 'news.yaml';
+
+import {sortChronologically} from '#sort';
+import Thing from '#thing';
+import {parseDate} from '#yaml';
+
+import {contentString, directory, name, simpleDate}
+  from '#composite/wiki-properties';
+
+export class NewsEntry extends Thing {
+  static [Thing.referenceType] = 'news-entry';
+  static [Thing.friendlyName] = `News Entry`;
+
+  static [Thing.getPropertyDescriptors] = () => ({
+    // Update & expose
+
+    name: name('Unnamed News Entry'),
+    directory: directory(),
+    date: simpleDate(),
+
+    content: contentString(),
+
+    // Expose only
+
+    contentShort: {
+      flags: {expose: true},
+
+      expose: {
+        dependencies: ['content'],
+
+        compute: ({content}) => content.split('<hr class="split">')[0],
+      },
+    },
+  });
+
+  static [Thing.findSpecs] = {
+    newsEntry: {
+      referenceTypes: ['news-entry'],
+      bindTo: 'newsData',
+    },
+  };
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Name': {property: 'name'},
+      'Directory': {property: 'directory'},
+
+      'Date': {
+        property: 'date',
+        transform: parseDate,
+      },
+
+      'Content': {property: 'content'},
+    },
+  };
+
+  static [Thing.getYamlLoadingSpec] = ({
+    documentModes: {allInOne},
+    thingConstructors: {NewsEntry},
+  }) => ({
+    title: `Process news data file`,
+    file: NEWS_DATA_FILE,
+
+    documentMode: allInOne,
+    documentThing: NewsEntry,
+
+    save: (results) => ({newsData: results}),
+
+    sort({newsData}) {
+      sortChronologically(newsData, {latestFirst: true});
+    },
+  });
+}
diff --git a/src/data/things/static-page.js b/src/data/things/static-page.js
new file mode 100644
index 0000000..0327497
--- /dev/null
+++ b/src/data/things/static-page.js
@@ -0,0 +1,80 @@
+export const DATA_STATIC_PAGE_DIRECTORY = 'static-page';
+
+import * as path from 'node:path';
+
+import {traverse} from '#node-utils';
+import {sortAlphabetically} from '#sort';
+import Thing from '#thing';
+import {isName} from '#validators';
+
+import {contentString, directory, name, simpleString}
+  from '#composite/wiki-properties';
+
+export class StaticPage extends Thing {
+  static [Thing.referenceType] = 'static';
+  static [Thing.friendlyName] = `Static Page`;
+
+  static [Thing.getPropertyDescriptors] = () => ({
+    // Update & expose
+
+    name: name('Unnamed Static Page'),
+
+    nameShort: {
+      flags: {update: true, expose: true},
+      update: {validate: isName},
+
+      expose: {
+        dependencies: ['name'],
+        transform: (value, {name}) => value ?? name,
+      },
+    },
+
+    directory: directory(),
+    content: contentString(),
+    stylesheet: simpleString(),
+    script: simpleString(),
+  });
+
+  static [Thing.findSpecs] = {
+    staticPage: {
+      referenceTypes: ['static'],
+      bindTo: 'staticPageData',
+    },
+  };
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Name': {property: 'name'},
+      'Short Name': {property: 'nameShort'},
+      'Directory': {property: 'directory'},
+
+      'Style': {property: 'stylesheet'},
+      'Script': {property: 'script'},
+      'Content': {property: 'content'},
+
+      'Review Points': {ignore: true},
+    },
+  };
+
+  static [Thing.getYamlLoadingSpec] = ({
+    documentModes: {onePerFile},
+    thingConstructors: {StaticPage},
+  }) => ({
+    title: `Process static page files`,
+
+    files: dataPath =>
+      traverse(path.join(dataPath, DATA_STATIC_PAGE_DIRECTORY), {
+        filterFile: name => path.extname(name) === '.yaml',
+        prefixPath: DATA_STATIC_PAGE_DIRECTORY,
+      }),
+
+    documentMode: onePerFile,
+    documentThing: StaticPage,
+
+    save: (results) => ({staticPageData: results}),
+
+    sort({staticPageData}) {
+      sortAlphabetically(staticPageData);
+    },
+  });
+}
diff --git a/src/data/things/track.js b/src/data/things/track.js
new file mode 100644
index 0000000..cc49fc2
--- /dev/null
+++ b/src/data/things/track.js
@@ -0,0 +1,566 @@
+import {inspect} from 'node:util';
+
+import CacheableObject from '#cacheable-object';
+import {colors} from '#cli';
+import {input} from '#composite';
+import find from '#find';
+import Thing from '#thing';
+import {isColor, isContributionList, isDate, isFileExtension}
+  from '#validators';
+
+import {
+  parseAdditionalFiles,
+  parseAdditionalNames,
+  parseContributors,
+  parseDate,
+  parseDimensions,
+  parseDuration,
+} from '#yaml';
+
+import {withPropertyFromObject} from '#composite/data';
+import {withResolvedContribs} from '#composite/wiki-data';
+
+import {
+  exitWithoutDependency,
+  exposeConstant,
+  exposeDependency,
+  exposeDependencyOrContinue,
+  exposeUpdateValueOrContinue,
+} from '#composite/control-flow';
+
+import {
+  additionalFiles,
+  additionalNameList,
+  commentary,
+  commentatorArtists,
+  contentString,
+  contributionList,
+  dimensions,
+  directory,
+  duration,
+  flag,
+  name,
+  referenceList,
+  reverseReferenceList,
+  simpleDate,
+  singleReference,
+  simpleString,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
+import {
+  exitWithoutUniqueCoverArt,
+  inferredAdditionalNameList,
+  inheritFromOriginalRelease,
+  sharedAdditionalNameList,
+  trackReverseReferenceList,
+  withAlbum,
+  withAlwaysReferenceByDirectory,
+  withContainingTrackSection,
+  withHasUniqueCoverArt,
+  withOtherReleases,
+  withPropertyFromAlbum,
+} from '#composite/things/track';
+
+export class Track extends Thing {
+  static [Thing.referenceType] = 'track';
+
+  static [Thing.getPropertyDescriptors] = ({Album, ArtTag, Artist, Flash}) => ({
+    // Update & expose
+
+    name: name('Unnamed Track'),
+    directory: directory(),
+
+    additionalNames: additionalNameList(),
+    sharedAdditionalNames: sharedAdditionalNameList(),
+    inferredAdditionalNames: inferredAdditionalNameList(),
+
+    bandcampTrackIdentifier: simpleString(),
+    bandcampArtworkIdentifier: simpleString(),
+
+    duration: duration(),
+    urls: urls(),
+    dateFirstReleased: simpleDate(),
+
+    color: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isColor),
+      }),
+
+      withContainingTrackSection(),
+
+      withPropertyFromObject({
+        object: '#trackSection',
+        property: input.value('color'),
+      }),
+
+      exposeDependencyOrContinue({dependency: '#trackSection.color'}),
+
+      withPropertyFromAlbum({
+        property: input.value('color'),
+      }),
+
+      exposeDependency({dependency: '#album.color'}),
+    ],
+
+    alwaysReferenceByDirectory: [
+      withAlwaysReferenceByDirectory(),
+      exposeDependency({dependency: '#alwaysReferenceByDirectory'}),
+    ],
+
+    // Disables presenting the track as though it has its own unique artwork.
+    // This flag should only be used in select circumstances, i.e. to override
+    // an album's trackCoverArtists. This flag supercedes that property, as well
+    // as the track's own coverArtists.
+    disableUniqueCoverArt: flag(),
+
+    // File extension for track's corresponding media file. This represents the
+    // track's unique cover artwork, if any, and does not inherit the extension
+    // of the album's main artwork. It does inherit trackCoverArtFileExtension,
+    // if present on the album.
+    coverArtFileExtension: [
+      exitWithoutUniqueCoverArt(),
+
+      exposeUpdateValueOrContinue({
+        validate: input.value(isFileExtension),
+      }),
+
+      withPropertyFromAlbum({
+        property: input.value('trackCoverArtFileExtension'),
+      }),
+
+      exposeDependencyOrContinue({dependency: '#album.trackCoverArtFileExtension'}),
+
+      exposeConstant({
+        value: input.value('jpg'),
+      }),
+    ],
+
+    // Date of cover art release. Like coverArtFileExtension, this represents
+    // only the track's own unique cover artwork, if any. This exposes only as
+    // the track's own coverArtDate or its album's trackArtDate, so if neither
+    // is specified, this value is null.
+    coverArtDate: [
+      withHasUniqueCoverArt(),
+
+      exitWithoutDependency({
+        dependency: '#hasUniqueCoverArt',
+        mode: input.value('falsy'),
+      }),
+
+      exposeUpdateValueOrContinue({
+        validate: input.value(isDate),
+      }),
+
+      withPropertyFromAlbum({
+        property: input.value('trackArtDate'),
+      }),
+
+      exposeDependency({dependency: '#album.trackArtDate'}),
+    ],
+
+    coverArtDimensions: [
+      exitWithoutUniqueCoverArt(),
+      dimensions(),
+    ],
+
+    commentary: commentary(),
+
+    lyrics: [
+      inheritFromOriginalRelease({
+        property: input.value('lyrics'),
+      }),
+
+      contentString(),
+    ],
+
+    additionalFiles: additionalFiles(),
+    sheetMusicFiles: additionalFiles(),
+    midiProjectFiles: additionalFiles(),
+
+    originalReleaseTrack: singleReference({
+      class: input.value(Track),
+      find: input.value(find.track),
+      data: 'trackData',
+    }),
+
+    // Internal use only - for directly identifying an album inside a track's
+    // util.inspect display, if it isn't indirectly available (by way of being
+    // included in an album's track list).
+    dataSourceAlbum: singleReference({
+      class: input.value(Album),
+      find: input.value(find.album),
+      data: 'albumData',
+    }),
+
+    artistContribs: [
+      inheritFromOriginalRelease({
+        property: input.value('artistContribs'),
+        notFoundValue: input.value([]),
+      }),
+
+      withResolvedContribs({
+        from: input.updateValue({validate: isContributionList}),
+      }).outputs({
+        '#resolvedContribs': '#artistContribs',
+      }),
+
+      exposeDependencyOrContinue({
+        dependency: '#artistContribs',
+        mode: input.value('empty'),
+      }),
+
+      withPropertyFromAlbum({
+        property: input.value('artistContribs'),
+      }),
+
+      exposeDependency({dependency: '#album.artistContribs'}),
+    ],
+
+    contributorContribs: [
+      inheritFromOriginalRelease({
+        property: input.value('contributorContribs'),
+        notFoundValue: input.value([]),
+      }),
+
+      contributionList(),
+    ],
+
+    // Cover artists aren't inherited from the original release, since it
+    // typically varies by release and isn't defined by the musical qualities
+    // of the track.
+    coverArtistContribs: [
+      exitWithoutUniqueCoverArt({
+        value: input.value([]),
+      }),
+
+      withResolvedContribs({
+        from: input.updateValue({validate: isContributionList}),
+      }).outputs({
+        '#resolvedContribs': '#coverArtistContribs',
+      }),
+
+      exposeDependencyOrContinue({
+        dependency: '#coverArtistContribs',
+        mode: input.value('empty'),
+      }),
+
+      withPropertyFromAlbum({
+        property: input.value('trackCoverArtistContribs'),
+      }),
+
+      exposeDependency({dependency: '#album.trackCoverArtistContribs'}),
+    ],
+
+    referencedTracks: [
+      inheritFromOriginalRelease({
+        property: input.value('referencedTracks'),
+        notFoundValue: input.value([]),
+      }),
+
+      referenceList({
+        class: input.value(Track),
+        find: input.value(find.track),
+        data: 'trackData',
+      }),
+    ],
+
+    sampledTracks: [
+      inheritFromOriginalRelease({
+        property: input.value('sampledTracks'),
+        notFoundValue: input.value([]),
+      }),
+
+      referenceList({
+        class: input.value(Track),
+        find: input.value(find.track),
+        data: 'trackData',
+      }),
+    ],
+
+    artTags: [
+      exitWithoutUniqueCoverArt({
+        value: input.value([]),
+      }),
+
+      referenceList({
+        class: input.value(ArtTag),
+        find: input.value(find.artTag),
+        data: 'artTagData',
+      }),
+    ],
+
+    // Update only
+
+    albumData: wikiData({
+      class: input.value(Album),
+    }),
+
+    artistData: wikiData({
+      class: input.value(Artist),
+    }),
+
+    artTagData: wikiData({
+      class: input.value(ArtTag),
+    }),
+
+    flashData: wikiData({
+      class: input.value(Flash),
+    }),
+
+    trackData: wikiData({
+      class: input.value(Track),
+    }),
+
+    // Expose only
+
+    commentatorArtists: commentatorArtists(),
+
+    album: [
+      withAlbum(),
+      exposeDependency({dependency: '#album'}),
+    ],
+
+    date: [
+      exposeDependencyOrContinue({dependency: 'dateFirstReleased'}),
+
+      withPropertyFromAlbum({
+        property: input.value('date'),
+      }),
+
+      exposeDependency({dependency: '#album.date'}),
+    ],
+
+    hasUniqueCoverArt: [
+      withHasUniqueCoverArt(),
+      exposeDependency({dependency: '#hasUniqueCoverArt'}),
+    ],
+
+    otherReleases: [
+      withOtherReleases(),
+      exposeDependency({dependency: '#otherReleases'}),
+    ],
+
+    referencedByTracks: trackReverseReferenceList({
+      list: input.value('referencedTracks'),
+    }),
+
+    sampledByTracks: trackReverseReferenceList({
+      list: input.value('sampledTracks'),
+    }),
+
+    featuredInFlashes: reverseReferenceList({
+      data: 'flashData',
+      list: input.value('featuredTracks'),
+    }),
+  });
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Track': {property: 'name'},
+      'Directory': {property: 'directory'},
+
+      'Additional Names': {
+        property: 'additionalNames',
+        transform: parseAdditionalNames,
+      },
+
+      'Bandcamp Track ID': {
+        property: 'bandcampTrackIdentifier',
+        transform: String,
+      },
+
+      'Bandcamp Artwork ID': {
+        property: 'bandcampArtworkIdentifier',
+        transform: String,
+      },
+
+      'Duration': {
+        property: 'duration',
+        transform: parseDuration,
+      },
+
+      'Color': {property: 'color'},
+      'URLs': {property: 'urls'},
+
+      'Date First Released': {
+        property: 'dateFirstReleased',
+        transform: parseDate,
+      },
+
+      'Cover Art Date': {
+        property: 'coverArtDate',
+        transform: parseDate,
+      },
+
+      'Cover Art File Extension': {property: 'coverArtFileExtension'},
+
+      'Cover Art Dimensions': {
+        property: 'coverArtDimensions',
+        transform: parseDimensions,
+      },
+
+      'Has Cover Art': {
+        property: 'disableUniqueCoverArt',
+        transform: value =>
+          (typeof value === 'boolean'
+            ? !value
+            : value),
+      },
+
+      'Always Reference By Directory': {property: 'alwaysReferenceByDirectory'},
+
+      'Lyrics': {property: 'lyrics'},
+      'Commentary': {property: 'commentary'},
+
+      'Additional Files': {
+        property: 'additionalFiles',
+        transform: parseAdditionalFiles,
+      },
+
+      'Sheet Music Files': {
+        property: 'sheetMusicFiles',
+        transform: parseAdditionalFiles,
+      },
+
+      'MIDI Project Files': {
+        property: 'midiProjectFiles',
+        transform: parseAdditionalFiles,
+      },
+
+      'Originally Released As': {property: 'originalReleaseTrack'},
+      'Referenced Tracks': {property: 'referencedTracks'},
+      'Sampled Tracks': {property: 'sampledTracks'},
+
+      'Franchises': {ignore: true},
+      'Inherit Franchises': {ignore: true},
+
+      'Artists': {
+        property: 'artistContribs',
+        transform: parseContributors,
+      },
+
+      'Contributors': {
+        property: 'contributorContribs',
+        transform: parseContributors,
+      },
+
+      'Cover Artists': {
+        property: 'coverArtistContribs',
+        transform: parseContributors,
+      },
+
+      'Art Tags': {property: 'artTags'},
+
+      'Review Points': {ignore: true},
+    },
+
+    invalidFieldCombinations: [
+      {message: `Rereleases inherit references from the original`, fields: [
+        'Originally Released As',
+        'Referenced Tracks',
+      ]},
+
+      {message: `Rereleases inherit samples from the original`, fields: [
+        'Originally Released As',
+        'Sampled Tracks',
+      ]},
+
+      {message: `Rereleases inherit artists from the original`, fields: [
+        'Originally Released As',
+        'Artists',
+      ]},
+
+      {message: `Rereleases inherit contributors from the original`, fields: [
+        'Originally Released As',
+        'Contributors',
+      ]},
+
+      {message: `Rereleases inherit lyrics from the original`, fields: [
+        'Originally Released As',
+        'Lyrics',
+      ]},
+
+      {
+        message: ({'Has Cover Art': hasCoverArt}) =>
+          (hasCoverArt
+            ? `"Has Cover Art: true" is inferred from cover artist credits`
+            : `Tracks without cover art must not have cover artist credits`),
+
+        fields: [
+          'Has Cover Art',
+          'Cover Artists',
+        ],
+      },
+    ],
+  };
+
+  static [Thing.findSpecs] = {
+    track: {
+      referenceTypes: ['track'],
+      bindTo: 'trackData',
+
+      getMatchableNames: track =>
+        (track.alwaysReferenceByDirectory
+          ? []
+          : [track.name]),
+    },
+
+    trackOriginalReleasesOnly: {
+      referenceTypes: ['track'],
+      bindTo: 'trackData',
+
+      include: track =>
+        !CacheableObject.getUpdateValue(track, 'originalReleaseTrack'),
+
+      // It's still necessary to check alwaysReferenceByDirectory here, since
+      // it may be set manually (with `Always Reference By Directory: true`),
+      // and these shouldn't be matched by name (as per usual).
+      // See the definition for that property for more information.
+      getMatchableNames: track =>
+        (track.alwaysReferenceByDirectory
+          ? []
+          : [track.name]),
+    },
+  };
+
+  // Track YAML loading is handled in album.js.
+  static [Thing.getYamlLoadingSpec] = null;
+
+  [inspect.custom](depth) {
+    const parts = [];
+
+    parts.push(Thing.prototype[inspect.custom].apply(this));
+
+    if (CacheableObject.getUpdateValue(this, 'originalReleaseTrack')) {
+      parts.unshift(`${colors.yellow('[rerelease]')} `);
+    }
+
+    let album;
+
+    if (depth >= 0) {
+      try {
+        album = this.album;
+      } catch (_error) {
+        // Computing album might crash for any reason, which we don't want to
+        // distract from another error we might be trying to work out at the
+        // moment (for which debugging might involve inspecting this track!).
+      }
+
+      album ??= this.dataSourceAlbum;
+    }
+
+    if (album) {
+      const albumName = album.name;
+      const albumIndex = album.tracks.indexOf(this);
+      const trackNum =
+        (albumIndex === -1
+          ? 'indeterminate position'
+          : `#${albumIndex + 1}`);
+      parts.push(` (${colors.yellow(trackNum)} in ${colors.green(albumName)})`);
+    }
+
+    return parts.join('');
+  }
+}
diff --git a/src/data/things/wiki-info.js b/src/data/things/wiki-info.js
new file mode 100644
index 0000000..316bd3b
--- /dev/null
+++ b/src/data/things/wiki-info.js
@@ -0,0 +1,110 @@
+export const WIKI_INFO_FILE = 'wiki-info.yaml';
+
+import {input} from '#composite';
+import find from '#find';
+import Thing from '#thing';
+import {isColor, isLanguageCode, isName, isURL} from '#validators';
+
+import {contentString, flag, name, referenceList, wikiData}
+  from '#composite/wiki-properties';
+
+export class WikiInfo extends Thing {
+  static [Thing.friendlyName] = `Wiki Info`;
+
+  static [Thing.getPropertyDescriptors] = ({Group}) => ({
+    // Update & expose
+
+    name: name('Unnamed Wiki'),
+
+    // Displayed in nav bar.
+    nameShort: {
+      flags: {update: true, expose: true},
+      update: {validate: isName},
+
+      expose: {
+        dependencies: ['name'],
+        transform: (value, {name}) => value ?? name,
+      },
+    },
+
+    color: {
+      flags: {update: true, expose: true},
+      update: {validate: isColor},
+
+      expose: {
+        transform: color => color ?? '#0088ff',
+      },
+    },
+
+    // One-line description used for <meta rel="description"> tag.
+    description: contentString(),
+
+    footerContent: contentString(),
+
+    defaultLanguage: {
+      flags: {update: true, expose: true},
+      update: {validate: isLanguageCode},
+    },
+
+    canonicalBase: {
+      flags: {update: true, expose: true},
+      update: {validate: isURL},
+    },
+
+    divideTrackListsByGroups: referenceList({
+      class: input.value(Group),
+      find: input.value(find.group),
+      data: 'groupData',
+    }),
+
+    // Feature toggles
+    enableFlashesAndGames: flag(false),
+    enableListings: flag(false),
+    enableNews: flag(false),
+    enableArtTagUI: flag(false),
+    enableGroupUI: flag(false),
+
+    // Update only
+
+    groupData: wikiData({
+      class: input.value(Group),
+    }),
+  });
+
+  static [Thing.yamlDocumentSpec] = {
+    fields: {
+      'Name': {property: 'name'},
+      'Short Name': {property: 'nameShort'},
+      'Color': {property: 'color'},
+      'Description': {property: 'description'},
+      'Footer Content': {property: 'footerContent'},
+      'Default Language': {property: 'defaultLanguage'},
+      'Canonical Base': {property: 'canonicalBase'},
+      'Divide Track Lists By Groups': {property: 'divideTrackListsByGroups'},
+      'Enable Flashes & Games': {property: 'enableFlashesAndGames'},
+      'Enable Listings': {property: 'enableListings'},
+      'Enable News': {property: 'enableNews'},
+      'Enable Art Tag UI': {property: 'enableArtTagUI'},
+      'Enable Group UI': {property: 'enableGroupUI'},
+    },
+  };
+
+  static [Thing.getYamlLoadingSpec] = ({
+    documentModes: {oneDocumentTotal},
+    thingConstructors: {WikiInfo},
+  }) => ({
+    title: `Process wiki info file`,
+    file: WIKI_INFO_FILE,
+
+    documentMode: oneDocumentTotal,
+    documentThing: WikiInfo,
+
+    save(wikiInfo) {
+      if (!wikiInfo) {
+        return;
+      }
+
+      return {wikiInfo};
+    },
+  });
+}
diff --git a/src/data/validators.js b/src/data/validators.js
index 0d325ae..987f806 100644
--- a/src/data/validators.js
+++ b/src/data/validators.js
@@ -1,367 +1,997 @@
-import { withAggregate } from '../util/sugar.js';
+import {inspect as nodeInspect} from 'node:util';
 
-import { color, ENABLE_COLOR, decorateTime } from '../util/cli.js';
-
-import { inspect as nodeInspect } from 'util';
+import {openAggregate, withAggregate} from '#aggregate';
+import {colors, ENABLE_COLOR} from '#cli';
+import {cut, empty, matchMultiline, typeAppearance} from '#sugar';
+import {commentaryRegexCaseInsensitive, commentaryRegexCaseSensitiveOneShot}
+  from '#wiki-data';
 
 function inspect(value) {
-    return nodeInspect(value, {colors: ENABLE_COLOR});
+  return nodeInspect(value, {colors: ENABLE_COLOR});
+}
+
+export function getValidatorCreator(validator) {
+  return validator[Symbol.for(`hsmusic.validator.creator`)] ?? null;
+}
+
+export function getValidatorCreatorMeta(validator) {
+  return validator[Symbol.for(`hsmusic.validator.creatorMeta`)] ?? null;
+}
+
+export function setValidatorCreatorMeta(validator, creator, meta) {
+  validator[Symbol.for(`hsmusic.validator.creator`)] = creator;
+  validator[Symbol.for(`hsmusic.validator.creatorMeta`)] = meta;
+  return validator;
 }
 
 // Basic types (primitives)
 
-function a(noun) {
-    return (/[aeiou]/.test(noun[0]) ? `an ${noun}` : `a ${noun}`);
+export function a(noun) {
+  return /[aeiou]/.test(noun[0]) ? `an ${noun}` : `a ${noun}`;
 }
 
-function isType(value, type) {
+export function validateType(type) {
+  const fn = value => {
     if (typeof value !== type)
-        throw new TypeError(`Expected ${a(type)}, got ${typeof value}`);
+      throw new TypeError(`Expected ${a(type)}, got ${typeAppearance(value)}`);
 
     return true;
-}
+  };
 
-export function isBoolean(value) {
-    return isType(value, 'boolean');
-}
+  setValidatorCreatorMeta(fn, validateType, {type});
 
-export function isNumber(value) {
-    return isType(value, 'number');
+  return fn;
 }
 
+export const isBoolean =
+  validateType('boolean');
+
+export const isFunction =
+  validateType('function');
+
+export const isNumber =
+  validateType('number');
+
+export const isString =
+  validateType('string');
+
+export const isSymbol =
+  validateType('symbol');
+
+// Use isObject instead, which disallows null.
+export const isTypeofObject =
+  validateType('object');
+
 export function isPositive(number) {
-    isNumber(number);
+  isNumber(number);
 
-    if (number <= 0)
-        throw new TypeError(`Expected positive number`);
+  if (number <= 0) throw new TypeError(`Expected positive number`);
 
-    return true;
+  return true;
 }
 
 export function isNegative(number) {
-    isNumber(number);
+  isNumber(number);
 
-    if (number >= 0)
-        throw new TypeError(`Expected negative number`);
+  if (number >= 0) throw new TypeError(`Expected negative number`);
 
-    return true;
+  return true;
 }
 
 export function isPositiveOrZero(number) {
-    isNumber(number);
+  isNumber(number);
 
-    if (number < 0)
-        throw new TypeError(`Expected positive number or zero`);
+  if (number < 0) throw new TypeError(`Expected positive number or zero`);
 
-    return true;
+  return true;
 }
 
 export function isNegativeOrZero(number) {
-    isNumber(number);
+  isNumber(number);
 
-    if (number > 0)
-        throw new TypeError(`Expected negative number or zero`);
+  if (number > 0) throw new TypeError(`Expected negative number or zero`);
 
-    return true;
+  return true;
 }
 
 export function isInteger(number) {
-    isNumber(number);
+  isNumber(number);
 
-    if (number % 1 !== 0)
-        throw new TypeError(`Expected integer`);
+  if (number % 1 !== 0) throw new TypeError(`Expected integer`);
 
-    return true;
+  return true;
 }
 
 export function isCountingNumber(number) {
-    isInteger(number);
-    isPositive(number);
+  isInteger(number);
+  isPositive(number);
 
-    return true;
+  return true;
 }
 
 export function isWholeNumber(number) {
-    isInteger(number);
-    isPositiveOrZero(number);
-
-    return true;
-}
+  isInteger(number);
+  isPositiveOrZero(number);
 
-export function isString(value) {
-    return isType(value, 'string');
+  return true;
 }
 
 export function isStringNonEmpty(value) {
-    isString(value);
+  isString(value);
 
-    if (value.trim().length === 0)
-        throw new TypeError(`Expected non-empty string`);
+  if (value.trim().length === 0)
+    throw new TypeError(`Expected non-empty string`);
 
-    return true;
+  return true;
+}
+
+export function optional(validator) {
+  return value =>
+    value === null ||
+    value === undefined ||
+    validator(value);
 }
 
 // Complex types (non-primitives)
 
 export function isInstance(value, constructor) {
-    isObject(value);
+  isObject(value);
 
-    if (!(value instanceof constructor))
-        throw new TypeError(`Expected ${constructor.name}, got ${value.constructor.name}`);
+  if (!(value instanceof constructor))
+    throw new TypeError(`Expected ${constructor.name}, got ${value.constructor.name}`);
 
-    return true;
+  return true;
 }
 
 export function isDate(value) {
-    return isInstance(value, Date);
+  isInstance(value, Date);
+
+  if (isNaN(value))
+    throw new TypeError(`Expected valid date`);
+
+  return true;
 }
 
 export function isObject(value) {
-    isType(value, 'object');
+  isTypeofObject(value);
 
-    // Note: Please remember that null is always a valid value for properties
-    // held by a CacheableObject. This assertion is exclusively for use in other
-    // contexts.
-    if (value === null)
-        throw new TypeError(`Expected an object, got null`);
+  // Note: Please remember that null is always a valid value for properties
+  // held by a CacheableObject. This assertion is exclusively for use in other
+  // contexts.
+  if (value === null)
+    throw new TypeError(`Expected an object, got null`);
 
-    return true;
+  return true;
 }
 
 export function isArray(value) {
-    if (typeof value !== 'object' || value === null || !Array.isArray(value))
-        throw new TypeError(`Expected an array, got ${value}`);
+  if (typeof value !== 'object' || value === null || !Array.isArray(value))
+    throw new TypeError(`Expected an array, got ${typeAppearance(value)}`);
+
+  return true;
+}
+
+// This one's shaped a bit different from other "is" functions.
+// More like validate functions, it returns a function.
+export function is(...values) {
+  if (Array.isArray(values)) {
+    values = new Set(values);
+  }
+
+  if (values.size === 1) {
+    const expected = Array.from(values)[0];
+
+    return (value) => {
+      if (value !== expected) {
+        throw new TypeError(`Expected ${expected}, got ${value}`);
+      }
+
+      return true;
+    };
+  }
+
+  const fn = (value) => {
+    if (!values.has(value)) {
+      throw new TypeError(`Expected one of ${Array.from(values).join(' ')}, got ${value}`);
+    }
 
     return true;
+  };
+
+  setValidatorCreatorMeta(fn, is, {values});
+
+  return fn;
 }
 
 function validateArrayItemsHelper(itemValidator) {
-    return (item, index) => {
-        try {
-            const value = itemValidator(item);
-
-            if (value !== true) {
-                throw new Error(`Expected validator to return true`);
-            }
-        } catch (error) {
-            error.message = `(index: ${color.green(index)}, item: ${inspect(item)}) ${error.message}`;
-            throw error;
-        }
-    };
+  return (item, index, array) => {
+    try {
+      const value = itemValidator(item, index, array);
+
+      if (value !== true) {
+        throw new Error(`Expected validator to return true`);
+      }
+    } catch (caughtError) {
+      const indexPart = colors.yellow(`zero-index ${index}`)
+      const itemPart = inspect(item);
+      const message = `Error at ${indexPart}: ${itemPart}`;
+      const error = new Error(message, {cause: caughtError});
+      error[Symbol.for('hsmusic.annotateError.indexInSourceArray')] = index;
+      throw error;
+    }
+  };
 }
 
 export function validateArrayItems(itemValidator) {
-    const fn = validateArrayItemsHelper(itemValidator);
+  const helper = validateArrayItemsHelper(itemValidator);
 
-    return array => {
-        isArray(array);
+  return (array) => {
+    isArray(array);
 
-        withAggregate({message: 'Errors validating array items'}, ({ wrap }) => {
-            array.forEach(wrap(fn));
-        });
+    withAggregate({message: 'Errors validating array items'}, ({call}) => {
+      for (let index = 0; index < array.length; index++) {
+        call(helper, array[index], index, array);
+      }
+    });
 
-        return true;
-    };
+    return true;
+  };
+}
+
+export function strictArrayOf(itemValidator) {
+  return validateArrayItems(itemValidator);
+}
+
+export function sparseArrayOf(itemValidator) {
+  return validateArrayItems((item, index, array) => {
+    if (item === false || item === null) {
+      return true;
+    }
+
+    return itemValidator(item, index, array);
+  });
+}
+
+export function looseArrayOf(itemValidator) {
+  return validateArrayItems((item, index, array) => {
+    if (item === false || item === null || item === undefined) {
+      return true;
+    }
+
+    return itemValidator(item, index, array);
+  });
 }
 
 export function validateInstanceOf(constructor) {
-    return object => isInstance(object, constructor);
+  const fn = (object) => isInstance(object, constructor);
+
+  setValidatorCreatorMeta(fn, validateInstanceOf, {constructor});
+
+  return fn;
 }
 
 // Wiki data (primitives & non-primitives)
 
 export function isColor(color) {
-    isStringNonEmpty(color);
+  isStringNonEmpty(color);
 
-    if (color.startsWith('#')) {
-        if (![1 + 3, 1 + 4, 1 + 6, 1 + 8].includes(color.length))
-            throw new TypeError(`Expected #rgb, #rgba, #rrggbb, or #rrggbbaa, got length ${color.length}`);
+  if (color.startsWith('#')) {
+    if (![4, 5, 7, 9].includes(color.length))
+      throw new TypeError(`Expected #rgb, #rgba, #rrggbb, or #rrggbbaa, got length ${color.length}`);
 
-        if (/[^0-9a-fA-F]/.test(color.slice(1)))
-            throw new TypeError(`Expected hexadecimal digits`);
+    if (/[^0-9a-fA-F]/.test(color.slice(1)))
+      throw new TypeError(`Expected hexadecimal digits`);
 
-        return true;
-    }
+    return true;
+  }
 
-    throw new TypeError(`Unknown color format`);
+  throw new TypeError(`Unknown color format`);
 }
 
-export function isCommentary(commentary) {
-    return isString(commentary);
+export function isCommentary(commentaryText) {
+  isContentString(commentaryText);
+
+  const rawMatches =
+    Array.from(commentaryText.matchAll(commentaryRegexCaseInsensitive));
+
+  if (empty(rawMatches)) {
+    throw new TypeError(`Expected at least one commentary heading`);
+  }
+
+  const niceMatches =
+    rawMatches.map(match => ({
+      position: match.index,
+      length: match[0].length,
+    }));
+
+  validateArrayItems(({position, length}, index) => {
+    if (index === 0 && position > 0) {
+      throw new TypeError(`Expected first commentary heading to be at top`);
+    }
+
+    const ownInput = commentaryText.slice(position, position + length);
+    const restOfInput = commentaryText.slice(position + length);
+
+    const upToNextLineBreak =
+      (restOfInput.includes('\n')
+        ? restOfInput.slice(0, restOfInput.indexOf('\n'))
+        : restOfInput);
+
+    if (/\S/.test(upToNextLineBreak)) {
+      throw new TypeError(
+        `Expected commentary heading to occupy entire line, got extra text:\n` +
+        `${colors.green(`"${cut(ownInput, 40)}"`)} (<- heading)\n` +
+        `(extra on same line ->) ${colors.red(`"${cut(upToNextLineBreak, 30)}"`)}\n` +
+        `(Check for missing "|-" in YAML, or a misshapen annotation)`);
+    }
+
+    if (!commentaryRegexCaseSensitiveOneShot.test(ownInput)) {
+      throw new TypeError(
+        `Miscapitalization in commentary heading:\n` +
+        `${colors.red(`"${cut(ownInput, 60)}"`)}\n` +
+        `(Check for ${colors.red(`"<I>"`)} instead of ${colors.green(`"<i>"`)})`);
+    }
+
+    const nextHeading =
+      (index === niceMatches.length - 1
+        ? commentaryText.length
+        : niceMatches[index + 1].position);
+
+    const upToNextHeading =
+      commentaryText.slice(position + length, nextHeading);
+
+    if (!/\S/.test(upToNextHeading)) {
+      throw new TypeError(
+        `Expected commentary entry to have body text, only got a heading`);
+    }
+
+    return true;
+  })(niceMatches);
+
+  return true;
 }
 
 const isArtistRef = validateReference('artist');
 
 export function validateProperties(spec) {
-    const specEntries = Object.entries(spec);
-    const specKeys = Object.keys(spec);
-
-    return object => {
-        isObject(object);
-
-        if (Array.isArray(object))
-            throw new TypeError(`Expected an object, got array`);
-
-        withAggregate({message: `Errors validating object properties`}, ({ call }) => {
-            for (const [ specKey, specValidator ] of specEntries) {
-                call(() => {
-                    const value = object[specKey];
-                    try {
-                        specValidator(value);
-                    } catch (error) {
-                        error.message = `(key: ${color.green(specKey)}, value: ${inspect(value)}) ${error.message}`;
-                        throw error;
-                    }
-                });
-            }
-
-            const unknownKeys = Object.keys(object).filter(key => !specKeys.includes(key));
-            if (unknownKeys.length > 0) {
-                call(() => {
-                    throw new Error(`Unknown keys present (${unknownKeys.length}): [${unknownKeys.join(', ')}]`);
-                });
-            }
-        });
+  const {
+    [validateProperties.validateOtherKeys]: validateOtherKeys = null,
+    [validateProperties.allowOtherKeys]: allowOtherKeys = false,
+  } = spec;
 
-        return true;
-    };
-}
+  const specEntries = Object.entries(spec);
+  const specKeys = Object.keys(spec);
+
+  return (object) => {
+    isObject(object);
 
+    if (Array.isArray(object))
+      throw new TypeError(`Expected an object, got array`);
+
+    withAggregate({message: `Errors validating object properties`}, ({push}) => {
+      const testEntries = specEntries.slice();
+
+      const unknownKeys = Object.keys(object).filter((key) => !specKeys.includes(key));
+      if (validateOtherKeys) {
+        for (const key of unknownKeys) {
+          testEntries.push([key, validateOtherKeys]);
+        }
+      }
+
+      for (const [specKey, specValidator] of testEntries) {
+        const value = object[specKey];
+        try {
+          specValidator(value);
+        } catch (caughtError) {
+          const keyPart = colors.green(specKey);
+          const valuePart = inspect(value);
+          const message = `Error for key ${keyPart}: ${valuePart}`;
+          push(new Error(message, {cause: caughtError}));
+        }
+      }
+
+      if (!validateOtherKeys && !allowOtherKeys && !empty(unknownKeys)) {
+        push(new Error(
+          `Unknown keys present (${unknownKeys.length}): [${unknownKeys.join(', ')}]`));
+      }
+    });
+
+    return true;
+  };
+}
+
+validateProperties.validateOtherKeys = Symbol();
+validateProperties.allowOtherKeys = Symbol();
+
+export const validateAllPropertyValues = (validator) =>
+  validateProperties({
+    [validateProperties.validateOtherKeys]: validator,
+  });
+
+const illeaglInvisibleSpace = {
+  action: 'delete',
+};
+
+const illegalVisibleSpace = {
+  action: 'replace',
+  with: ' ',
+  withAnnotation: `normal space`,
+};
+
+const illegalContentSpec = [
+  {illegal: '\u200b', annotation: `zero-width space`, ...illeaglInvisibleSpace},
+  {illegal: '\u2005', annotation: `four-per-em space`, ...illegalVisibleSpace},
+  {illegal: '\u205f', annotation: `medium mathematical space`, ...illegalVisibleSpace},
+  {illegal: '\xa0', annotation: `non-breaking space`, ...illegalVisibleSpace},
+
+  {
+    action: 'replace',
+    illegal: '<a href',
+    annotation: `HTML-style link`,
+    with: '[...](...)',
+    withAnnotation: `markdown`,
+  },
+];
+
+for (const entry of illegalContentSpec) {
+  entry.test = string =>
+    string.startsWith(entry.illegal);
+
+  if (entry.action === 'replace') {
+    entry.enact = string =>
+      string.replaceAll(entry.illegal, entry.with);
+  }
+}
+
+const illegalContentRegexp =
+  new RegExp(
+    illegalContentSpec
+      .map(entry => entry.illegal)
+      .map(illegal => `${illegal}+`)
+      .join('|'),
+    'g');
+
+const illegalCharactersInContent =
+  illegalContentSpec
+    .map(entry => entry.illegal)
+    .join('');
+
+const legalContentNearEndRegexp =
+  new RegExp(`[^\n${illegalCharactersInContent}]+$`);
+
+const legalContentNearStartRegexp =
+  new RegExp(`^[^\n${illegalCharactersInContent}]+`);
+
+const trimWhitespaceNearBothSidesRegexp =
+  /^ +| +$/gm;
+
+const trimWhitespaceNearEndRegexp =
+  / +$/gm;
+
+export function isContentString(content) {
+  isStringNonEmpty(content);
+
+  const mainAggregate = openAggregate({
+    message: `Errors validating content string`,
+    translucent: 'single',
+  });
+
+  const illegalAggregate = openAggregate({
+    message: `Illegal characters found in content string`,
+  });
+
+  for (const {match, where} of matchMultiline(content, illegalContentRegexp)) {
+    const {annotation, action, ...options} =
+      illegalContentSpec
+        .find(entry => entry.test(match[0]));
+
+    const matchStart = match.index;
+    const matchEnd = match.index + match[0].length;
+
+    const before =
+      content
+        .slice(Math.max(0, matchStart - 3), matchStart)
+        .match(legalContentNearEndRegexp)
+        ?.[0];
+
+    const after =
+      content
+        .slice(matchEnd, Math.min(content.length, matchEnd + 3))
+        .match(legalContentNearStartRegexp)
+        ?.[0];
+
+    const beforePart =
+      before && `"${before}"`;
+
+    const afterPart =
+      after && `"${after}"`;
+
+    const surroundings =
+      (before && after
+        ? `between ${beforePart} and ${afterPart}`
+     : before
+        ? `after ${beforePart}`
+     : after
+        ? `before ${afterPart}`
+        : ``);
+
+    const illegalPart =
+      colors.red(
+        (annotation
+          ? `"${match[0]}" (${annotation})`
+          : `"${match[0]}"`));
+
+    const replacement =
+      (action === 'replace'
+        ? options.enact(match[0])
+        : null);
+
+    const replaceWithPart =
+      (action === 'replace'
+        ? colors.green(
+            (options.withAnnotation
+              ? `"${replacement}" (${options.withAnnotation})`
+              : `"${replacement}"`))
+        : null);
+
+    const actionPart =
+      (action === `delete`
+        ? `Delete ${illegalPart}`
+     : action === 'replace'
+        ? `Replace ${illegalPart} with ${replaceWithPart}`
+        : `Matched ${illegalPart}`);
+
+    const parts = [
+      actionPart,
+      surroundings,
+      `(${where})`,
+    ].filter(Boolean);
+
+    illegalAggregate.push(new TypeError(parts.join(` `)));
+  }
+
+  const isMultiline = content.includes('\n');
+
+  const trimWhitespaceAggregate = openAggregate({
+    message:
+      (isMultiline
+        ? `Whitespace found at end of line`
+        : `Whitespace found at start or end`),
+  });
+
+  const trimWhitespaceRegexp =
+    (isMultiline
+      ? trimWhitespaceNearEndRegexp
+      : trimWhitespaceNearBothSidesRegexp);
+
+  for (
+    const {match, lineNumber, columnNumber, containingLine} of
+    matchMultiline(content, trimWhitespaceRegexp, {
+      formatWhere: false,
+      getContainingLine: true,
+    })
+  ) {
+    const linePart =
+      colors.yellow(`line ${lineNumber + 1}`);
+
+    const where =
+      (match[0].length === containingLine.length
+        ? `as all of ${linePart}`
+     : columnNumber === 0
+        ? (isMultiline
+            ? `at start of ${linePart}`
+            : `at start`)
+        : (isMultiline
+            ? `at end of ${linePart}`
+            : `at end`));
+
+    const whitespacePart =
+      colors.red(`"${match[0]}"`);
+
+    const parts = [
+      `Matched ${whitespacePart}`,
+      where,
+    ];
+
+    trimWhitespaceAggregate.push(new TypeError(parts.join(` `)));
+  }
+
+  mainAggregate.call(() => illegalAggregate.close());
+  mainAggregate.call(() => trimWhitespaceAggregate.close());
+  mainAggregate.close();
+
+  return true;
+}
+
+export function isThingClass(thingClass) {
+  isFunction(thingClass);
+
+  if (!Object.hasOwn(thingClass, Symbol.for('Thing.referenceType'))) {
+    throw new TypeError(`Expected a Thing constructor, missing Thing.referenceType`);
+  }
+
+  return true;
+}
 
 export const isContribution = validateProperties({
-    who: isArtistRef,
-    what: value => value === undefined || value === null || isStringNonEmpty(value),
+  who: isArtistRef,
+  what: optional(isStringNonEmpty),
 });
 
 export const isContributionList = validateArrayItems(isContribution);
 
 export const isAdditionalFile = validateProperties({
-    title: isString,
-    description: value => (value === undefined || value === null || isString(value)),
-    files: validateArrayItems(isString)
+  title: isName,
+  description: optional(isContentString),
+  files: optional(validateArrayItems(isString)),
 });
 
 export const isAdditionalFileList = validateArrayItems(isAdditionalFile);
 
+export const isTrackSection = validateProperties({
+  name: optional(isName),
+  color: optional(isColor),
+  dateOriginallyReleased: optional(isDate),
+  isDefaultTrackSection: optional(isBoolean),
+  tracks: optional(validateReferenceList('track')),
+});
+
+export const isTrackSectionList = validateArrayItems(isTrackSection);
+
 export function isDimensions(dimensions) {
-    isArray(dimensions);
+  isArray(dimensions);
 
-    if (dimensions.length !== 2)
-        throw new TypeError(`Expected 2 item array`);
+  if (dimensions.length !== 2) throw new TypeError(`Expected 2 item array`);
 
+  if (dimensions[0] !== null) {
     isPositive(dimensions[0]);
     isInteger(dimensions[0]);
+  }
+
+  if (dimensions[1] !== null) {
     isPositive(dimensions[1]);
     isInteger(dimensions[1]);
+  }
 
-    return true;
+  return true;
 }
 
 export function isDirectory(directory) {
-    isStringNonEmpty(directory);
+  isStringNonEmpty(directory);
 
-    if (directory.match(/[^a-zA-Z0-9_\-]/))
-        throw new TypeError(`Expected only letters, numbers, dash, and underscore, got "${directory}"`);
+  if (directory.match(/[^a-zA-Z0-9_-]/))
+    throw new TypeError(`Expected only letters, numbers, dash, and underscore, got "${directory}"`);
 
-    return true;
+  return true;
 }
 
 export function isDuration(duration) {
-    isNumber(duration);
-    isPositiveOrZero(duration);
+  isNumber(duration);
+  isPositiveOrZero(duration);
 
-    return true;
+  return true;
 }
 
 export function isFileExtension(string) {
-    isStringNonEmpty(string);
+  isStringNonEmpty(string);
 
-    if (string[0] === '.')
-        throw new TypeError(`Expected no dot (.) at the start of file extension`);
+  if (string[0] === '.')
+    throw new TypeError(`Expected no dot (.) at the start of file extension`);
 
-    if (string.match(/[^a-zA-Z0-9_]/))
-        throw new TypeError(`Expected only alphanumeric and underscore`);
+  if (string.match(/[^a-zA-Z0-9_]/))
+    throw new TypeError(`Expected only alphanumeric and underscore`);
 
-    return true;
+  return true;
 }
 
 export function isLanguageCode(string) {
-    // TODO: This is a stub function because really we don't need a detailed
-    // is-language-code parser right now.
+  // TODO: This is a stub function because really we don't need a detailed
+  // is-language-code parser right now.
 
-    isString(string);
+  isString(string);
 
-    return true;
+  return true;
 }
 
 export function isName(name) {
-    return isString(name);
+  return isContentString(name);
 }
 
 export function isURL(string) {
-    isStringNonEmpty(string);
+  isStringNonEmpty(string);
 
-    new URL(string);
+  new URL(string);
 
-    return true;
+  return true;
 }
 
 export function validateReference(type = 'track') {
-    return ref => {
-        isStringNonEmpty(ref);
+  return (ref) => {
+    isStringNonEmpty(ref);
 
-        const match = ref.trim().match(/^(?:(?<typePart>\S+):(?=\S))?(?<directoryPart>.+)(?<!:)$/);
+    const match = ref
+      .trim()
+      .match(/^(?:(?<typePart>\S+):(?=\S))?(?<directoryPart>.+)(?<!:)$/);
 
-        if (!match)
-            throw new TypeError(`Malformed reference`);
+    if (!match) throw new TypeError(`Malformed reference`);
 
-        const { groups: { typePart, directoryPart } } = match;
+    const {groups: {typePart, directoryPart}} = match;
 
-        if (typePart && typePart !== type)
-            throw new TypeError(`Expected ref to begin with "${type}:", got "${typePart}:"`);
+    if (typePart) {
+      if (typePart !== type)
+        throw new TypeError(`Expected ref to begin with "${type}:", got "${typePart}:"`);
 
-        if (typePart)
-            isDirectory(directoryPart);
+      isDirectory(directoryPart);
+    }
 
-        isName(ref);
+    isName(ref);
 
-        return true;
-    };
+    return true;
+  };
 }
 
 export function validateReferenceList(type = '') {
-    return validateArrayItems(validateReference(type));
+  return validateArrayItems(validateReference(type));
 }
 
+const validateWikiData_cache = {};
+
+export function validateWikiData({
+  referenceType = '',
+  allowMixedTypes = false,
+}) {
+  if (referenceType && allowMixedTypes) {
+    throw new TypeError(`Don't specify both referenceType and allowMixedTypes`);
+  }
+
+  validateWikiData_cache[referenceType] ??= {};
+  validateWikiData_cache[referenceType][allowMixedTypes] ??= new WeakMap();
+
+  const isArrayOfObjects = validateArrayItems(isObject);
+
+  return (array) => {
+    const subcache = validateWikiData_cache[referenceType][allowMixedTypes];
+    if (subcache.has(array)) return subcache.get(array);
+
+    let OK = false;
+
+    try {
+      isArrayOfObjects(array);
+
+      if (empty(array)) {
+        OK = true; return true;
+      }
+
+      const allRefTypes = new Set();
+
+      let foundThing = false;
+      let foundOtherObject = false;
+
+      for (const object of array) {
+        const {[Symbol.for('Thing.referenceType')]: referenceType} = object.constructor;
+
+        if (referenceType === undefined) {
+          foundOtherObject = true;
+
+          // Early-exit if a Thing has been found - nothing more can be learned.
+          if (foundThing) {
+            throw new TypeError(`Expected array of wiki data objects, got mixed items`);
+          }
+        } else {
+          foundThing = true;
+
+          // Early-exit if a non-Thing object has been found - nothing more can
+          // be learned.
+          if (foundOtherObject) {
+            throw new TypeError(`Expected array of wiki data objects, got mixed items`);
+          }
+
+          allRefTypes.add(referenceType);
+        }
+      }
+
+      if (foundOtherObject && !foundThing) {
+        throw new TypeError(`Expected array of wiki data objects, got array of other objects`);
+      }
+
+      if (allRefTypes.size > 1) {
+        if (allowMixedTypes) {
+          OK = true; return true;
+        }
+
+        const types = () => Array.from(allRefTypes).join(', ');
+
+        if (referenceType) {
+          if (allRefTypes.has(referenceType)) {
+            allRefTypes.remove(referenceType);
+            throw new TypeError(`Expected array of only ${referenceType}, also got other types: ${types()}`)
+          } else {
+            throw new TypeError(`Expected array of only ${referenceType}, got other types: ${types()}`);
+          }
+        }
+
+        throw new TypeError(`Expected array of unmixed reference types, got multiple: ${types()}`);
+      }
+
+      const onlyRefType = Array.from(allRefTypes)[0];
+
+      if (referenceType && onlyRefType !== referenceType) {
+        throw new TypeError(`Expected array of ${referenceType}, got array of ${onlyRefType}`)
+      }
+
+      OK = true; return true;
+    } finally {
+      subcache.set(array, OK);
+    }
+  };
+}
+
+export const isAdditionalName = validateProperties({
+  name: isName,
+  annotation: optional(isContentString),
+
+  // TODO: This only allows indicating sourcing from a track.
+  // That's okay for the current limited use of "from", but
+  // could be expanded later.
+  from:
+    // Double TODO: Explicitly allowing both references and
+    // live objects to co-exist is definitely weird, and
+    // altogether questions the way we define validators...
+    optional(anyOf(
+      validateReferenceList('track'),
+      validateWikiData({referenceType: 'track'}))),
+});
+
+export const isAdditionalNameList = validateArrayItems(isAdditionalName);
+
 // Compositional utilities
 
-export function oneOf(...checks) {
-    return value => {
-        const errorMeta = [];
+export function anyOf(...validators) {
+  const validConstants = new Set();
+  const validConstructors = new Set();
+  const validTypes = new Set();
 
-        for (let i = 0, check; check = checks[i]; i++) {
-            try {
-                const result = check(value);
+  const constantValidators = [];
+  const constructorValidators = [];
+  const typeValidators = [];
 
-                if (result !== true) {
-                    throw new Error(`Check returned false`);
-                }
+  const leftoverValidators = [];
 
-                return true;
-            } catch (error) {
-                errorMeta.push([check, i, error]);
-            }
+  for (const validator of validators) {
+    const creator = getValidatorCreator(validator);
+    const creatorMeta = getValidatorCreatorMeta(validator);
+
+    switch (creator) {
+      case is:
+        for (const value of creatorMeta.values) {
+          validConstants.add(value);
         }
 
-        // Don't process error messages until every check has failed.
-        const errors = [];
-        for (const [ check, i, error ] of errorMeta) {
-            error.message = (check.name
-                ? `(#${i} "${check.name}") ${error.message}`
-                : `(#${i}) ${error.message}`);
-            error.check = check;
-            errors.push(error);
+        constantValidators.push(validator);
+        break;
+
+      case validateInstanceOf:
+        validConstructors.add(creatorMeta.constructor);
+        constructorValidators.push(validator);
+        break;
+
+      case validateType:
+        validTypes.add(creatorMeta.type);
+        typeValidators.push(validator);
+        break;
+
+      default:
+        leftoverValidators.push(validator);
+        break;
+    }
+  }
+
+  return (value) => {
+    const errorInfo = [];
+
+    if (validConstants.has(value)) {
+      return true;
+    }
+
+    if (!empty(validTypes)) {
+      if (validTypes.has(typeof value)) {
+        return true;
+      }
+    }
+
+    for (const constructor of validConstructors) {
+      if (value instanceof constructor) {
+        return true;
+      }
+    }
+
+    for (const [i, validator] of leftoverValidators.entries()) {
+      try {
+        const result = validator(value);
+
+        if (result !== true) {
+          throw new Error(`Check returned false`);
         }
-        throw new AggregateError(errors, `Expected one of ${checks.length} possible checks, but none were true`);
-    };
+
+        return true;
+      } catch (error) {
+        errorInfo.push([validator, i, error]);
+      }
+    }
+
+    // Don't process error messages until every validator has failed.
+
+    const errors = [];
+    const prefaceErrorInfo = [];
+
+    let offset = 0;
+
+    if (!empty(validConstants)) {
+      const constants =
+        Array.from(validConstants);
+
+      const gotPart = `, got ${value}`;
+
+      prefaceErrorInfo.push([
+        constantValidators,
+        offset++,
+        new TypeError(
+          `Expected any of ${constants.join(' ')}` + gotPart),
+      ]);
+    }
+
+    if (!empty(validTypes)) {
+      const types =
+        Array.from(validTypes);
+
+      const gotType = typeAppearance(value);
+      const gotPart = `, got ${gotType}`;
+
+      prefaceErrorInfo.push([
+        typeValidators,
+        offset++,
+        new TypeError(
+          `Expected any of ${types.join(', ')}` + gotPart),
+      ]);
+    }
+
+    if (!empty(validConstructors)) {
+      const names =
+        Array.from(validConstructors)
+          .map(constructor => constructor.name);
+
+      const gotName = value?.constructor?.name;
+      const gotPart = (gotName ? `, got ${gotName}` : ``);
+
+      prefaceErrorInfo.push([
+        constructorValidators,
+        offset++,
+        new TypeError(
+          `Expected any of ${names.join(', ')}` + gotPart),
+      ]);
+    }
+
+    for (const info of errorInfo) {
+      info[1] += offset;
+    }
+
+    for (const [validator, i, error] of prefaceErrorInfo.concat(errorInfo)) {
+      error.message =
+        (validator?.name
+          ? `${i + 1}. "${validator.name}": ${error.message}`
+          : `${i + 1}. ${error.message}`);
+
+      error.check =
+        (Array.isArray(validator) && validator.length === 1
+          ? validator[0]
+          : validator);
+
+      errors.push(error);
+    }
+
+    const total = offset + leftoverValidators.length;
+    throw new AggregateError(errors,
+      `Expected any of ${total} possible checks, ` +
+      `but none were true`);
+  };
 }
diff --git a/src/data/yaml.js b/src/data/yaml.js
index 32cf729..86f3014 100644
--- a/src/data/yaml.js
+++ b/src/data/yaml.js
@@ -1,595 +1,494 @@
 // yaml.js - specification for HSMusic YAML data file format and utilities for
-// loading and processing YAML files and documents
+// loading, processing, and validating YAML files and documents
 
-import * as path from 'path';
-import yaml from 'js-yaml';
+import {readFile, stat} from 'node:fs/promises';
+import * as path from 'node:path';
+import {inspect as nodeInspect} from 'node:util';
 
-import { readFile } from 'fs/promises';
-import { inspect as nodeInspect } from 'util';
+import yaml from 'js-yaml';
 
-import {
-    Album,
-    Artist,
-    ArtTag,
-    Flash,
-    FlashAct,
-    Group,
-    GroupCategory,
-    HomepageLayout,
-    HomepageLayoutAlbumsRow,
-    HomepageLayoutRow,
-    NewsEntry,
-    StaticPage,
-    Thing,
-    Track,
-    TrackGroup,
-    WikiInfo,
-} from './things.js';
+import {colors, ENABLE_COLOR, logInfo, logWarn} from '#cli';
+import {sortByName} from '#sort';
+import {atOffset, empty, filterProperties, typeAppearance, withEntries}
+  from '#sugar';
+import Thing from '#thing';
+import thingConstructors from '#things';
 
 import {
-    color,
-    ENABLE_COLOR,
-    logInfo,
-    logWarn,
-} from '../util/cli.js';
+  filterReferenceErrors,
+  reportContentTextErrors,
+  reportDuplicateDirectories,
+} from '#data-checks';
 
 import {
-    decorateErrorWithIndex,
-    mapAggregate,
-    openAggregate,
-    showAggregate,
-    withAggregate,
-} from '../util/sugar.js';
-
-import {
-    sortByDate,
-    sortByName,
-} from '../util/wiki-data.js';
-
-import find, { bindFind } from '../util/find.js';
-import { findFiles } from '../util/io.js';
-
-// --> General supporting stuff
-
-function inspect(value) {
-    return nodeInspect(value, {colors: ENABLE_COLOR});
+  annotateErrorWithFile,
+  decorateErrorWithIndex,
+  decorateErrorWithAnnotation,
+  openAggregate,
+  showAggregate,
+  withAggregate,
+} from '#aggregate';
+
+function inspect(value, opts = {}) {
+  return nodeInspect(value, {colors: ENABLE_COLOR, ...opts});
 }
 
-// --> YAML data repository structure constants
-
-export const WIKI_INFO_FILE = 'wiki-info.yaml';
-export const BUILD_DIRECTIVE_DATA_FILE = 'build-directives.yaml';
-export const HOMEPAGE_LAYOUT_DATA_FILE = 'homepage.yaml';
-export const ARTIST_DATA_FILE = 'artists.yaml';
-export const FLASH_DATA_FILE = 'flashes.yaml';
-export const NEWS_DATA_FILE = 'news.yaml';
-export const ART_TAG_DATA_FILE = 'tags.yaml';
-export const GROUP_DATA_FILE = 'groups.yaml';
-export const STATIC_PAGE_DATA_FILE = 'static-pages.yaml';
-
-export const DATA_ALBUM_DIRECTORY = 'album';
-
-// --> Document processing functions
-
 // General function for inputting a single document (usually loaded from YAML)
 // and outputting an instance of a provided Thing subclass.
 //
 // makeProcessDocument is a factory function: the returned function will take a
 // document and apply the configuration passed to makeProcessDocument in order
 // to construct a Thing subclass.
-function makeProcessDocument(thingClass, {
-    // Optional early step for transforming field values before providing them
-    // to the Thing's update() method. This is useful when the input format
-    // (i.e. values in the document) differ from the format the actual Thing
-    // expects.
-    //
-    // Each key and value are a field name (not an update() property) and a
-    // function which takes the value for that field and returns the value which
-    // will be passed on to update().
-    fieldTransformations = {},
-
-    // Mapping of Thing.update() source properties to field names.
-    //
-    // Note this is property -> field, not field -> property. This is a
-    // shorthand convenience because properties are generally typical
-    // camel-cased JS properties, while fields may contain whitespace and be
-    // more easily represented as quoted strings.
-    propertyFieldMapping,
-
-    // Completely ignored fields. These won't throw an unknown field error if
-    // they're present in a document, but they won't be used for Thing property
-    // generation, either. Useful for stuff that's present in data files but not
-    // yet implemented as part of a Thing's data model!
-    ignoredFields = []
+//
+function makeProcessDocument(thingConstructor, {
+  // The bulk of configuration happens here in the spec's `fields` property.
+  // Each key is a field that's expected on the source document; fields that
+  // don't match one of these keys will cause an error. Values are object
+  // entries describing what to do with the field.
+  //
+  // A field entry's `property` tells what property the value for this field
+  // will be put into, on the respective Thing (subclass) instance.
+  //
+  // A field entry's `transform` optionally allows converting the raw value in
+  // YAML into some other format before providing setting it on the Thing
+  // instance.
+  //
+  // If a field entry has `ignore: true`, it will be completely skipped by the
+  // YAML parser - it won't be validated, read, or loaded into data objects.
+  // This is mainly useful for fields that are purely annotational or are
+  // currently placeholders.
+  //
+  fields: fieldSpecs = {},
+
+  // List of fields which are invalid when coexisting in a document.
+  // Data objects are generally allowing with regards to what properties go
+  // together, allowing for properties to be set separately from each other
+  // instead of complaining about invalid or unused-data cases. But it's
+  // useful to see these kinds of errors when actually validating YAML files!
+  //
+  // Each item of this array should itself be an object with a descriptive
+  // message and a list of fields. Of those fields, none should ever coexist
+  // with any other. For example:
+  //
+  //   [
+  //     {message: '...', fields: ['A', 'B', 'C']},
+  //     {message: '...', fields: ['C', 'D']},
+  //   ]
+  //
+  // ...means A can't coexist with B or C, B can't coexist with A or C, and
+  // C can't coexist iwth A, B, or D - but it's okay for D to coexist with
+  // A or B.
+  //
+  invalidFieldCombinations = [],
 }) {
-    if (!propertyFieldMapping) {
-        throw new Error(`Expected propertyFieldMapping to be provided`);
-    }
-
-    const knownFields = Object.values(propertyFieldMapping);
-
-    // Invert the property-field mapping, since it'll come in handy for
-    // assigning update() source values later.
-    const fieldPropertyMapping = Object.fromEntries(
-        (Object.entries(propertyFieldMapping)
-            .map(([ property, field ]) => [field, property])));
-
-    const decorateErrorWithName = fn => {
-        const nameField = propertyFieldMapping['name'];
-        if (!nameField) return fn;
-
-        return document => {
-            try {
-                return fn(document);
-            } catch (error) {
-                const name = document[nameField];
-                error.message = (name
-                    ? `(name: ${inspect(name)}) ${error.message}`
-                    : `(${color.dim(`no name found`)}) ${error.message}`);
-                throw error;
-            }
-        };
+  if (!thingConstructor) {
+    throw new Error(`Missing Thing class`);
+  }
+
+  if (!fieldSpecs) {
+    throw new Error(`Expected fields to be provided`);
+  }
+
+  const knownFields = Object.keys(fieldSpecs);
+
+  const ignoredFields =
+    Object.entries(fieldSpecs)
+      .filter(([, {ignore}]) => ignore)
+      .map(([field]) => field);
+
+  const propertyToField =
+    withEntries(fieldSpecs, entries => entries
+      .map(([field, {property}]) => [property, field]));
+
+  // TODO: Is this function even necessary??
+  // Aren't we doing basically the same work in the function it's decorating???
+  const decorateErrorWithName = (fn) => {
+    const nameField = propertyToField.name;
+    if (!nameField) return fn;
+
+    return (document) => {
+      try {
+        return fn(document);
+      } catch (error) {
+        const name = document[nameField];
+        error.message = name
+          ? `(name: ${inspect(name)}) ${error.message}`
+          : `(${colors.dim(`no name found`)}) ${error.message}`;
+        throw error;
+      }
     };
-
-    return decorateErrorWithName(document => {
-        const documentEntries = Object.entries(document)
-            .filter(([ field ]) => !ignoredFields.includes(field));
-
-        const unknownFields = documentEntries
-            .map(([ field ]) => field)
-            .filter(field => !knownFields.includes(field));
-
-        if (unknownFields.length) {
-            throw new makeProcessDocument.UnknownFieldsError(unknownFields);
-        }
-
-        const fieldValues = {};
-
-        for (const [ field, value ] of documentEntries) {
-            if (Object.hasOwn(fieldTransformations, field)) {
-                fieldValues[field] = fieldTransformations[field](value);
-            } else {
-                fieldValues[field] = value;
-            }
-        }
-
-        const sourceProperties = {};
-
-        for (const [ field, value ] of Object.entries(fieldValues)) {
-            const property = fieldPropertyMapping[field];
-            sourceProperties[property] = value;
-        }
-
-        const thing = Reflect.construct(thingClass, []);
-
-        withAggregate({message: `Errors applying ${color.green(thingClass.name)} properties`}, ({ call }) => {
-            for (const [ property, value ] of Object.entries(sourceProperties)) {
-                call(() => (thing[property] = value));
-            }
-        });
-
-        return thing;
+  };
+
+  return decorateErrorWithName((document) => {
+    const nameField = propertyToField.name;
+    const namePart =
+      (nameField
+        ? (document[nameField]
+          ? ` named ${colors.green(`"${document[nameField]}"`)}`
+          : ` (name field, "${nameField}", not specified)`)
+        : ``);
+
+    const constructorPart =
+      (thingConstructor[Thing.friendlyName]
+        ? thingConstructor[Thing.friendlyName]
+     : thingConstructor.name
+        ? thingConstructor.name
+        : `document`);
+
+    const aggregate = openAggregate({
+      message: `Errors processing ${constructorPart}` + namePart,
     });
-}
-
-makeProcessDocument.UnknownFieldsError = class UnknownFieldsError extends Error {
-    constructor(fields) {
-        super(`Unknown fields present: ${fields.join(', ')}`);
-        this.fields = fields;
-    }
-};
 
-export const processAlbumDocument = makeProcessDocument(Album, {
-    fieldTransformations: {
-        'Artists': parseContributors,
-        'Cover Artists': parseContributors,
-        'Default Track Cover Artists': parseContributors,
-        'Wallpaper Artists': parseContributors,
-        'Banner Artists': parseContributors,
+    const documentEntries = Object.entries(document)
+      .filter(([field]) => !ignoredFields.includes(field));
 
-        'Date': value => new Date(value),
-        'Date Added': value => new Date(value),
-        'Cover Art Date': value => new Date(value),
-        'Default Track Cover Art Date': value => new Date(value),
+    const skippedFields = new Set();
 
-        'Banner Dimensions': parseDimensions,
+    const unknownFields = documentEntries
+      .map(([field]) => field)
+      .filter((field) => !knownFields.includes(field));
 
-        'Additional Files': parseAdditionalFiles,
-    },
+    if (!empty(unknownFields)) {
+      aggregate.push(new UnknownFieldsError(unknownFields));
 
-    propertyFieldMapping: {
-        name: 'Album',
-
-        color: 'Color',
-        directory: 'Directory',
-        urls: 'URLs',
-
-        artistContribsByRef: 'Artists',
-        coverArtistContribsByRef: 'Cover Artists',
-        trackCoverArtistContribsByRef: 'Default Track Cover Artists',
-
-        coverArtFileExtension: 'Cover Art File Extension',
-        trackCoverArtFileExtension: 'Track Art File Extension',
+      for (const field of unknownFields) {
+        skippedFields.add(field);
+      }
+    }
 
-        wallpaperArtistContribsByRef: 'Wallpaper Artists',
-        wallpaperStyle: 'Wallpaper Style',
-        wallpaperFileExtension: 'Wallpaper File Extension',
+    const presentFields = Object.keys(document);
 
-        bannerArtistContribsByRef: 'Banner Artists',
-        bannerStyle: 'Banner Style',
-        bannerFileExtension: 'Banner File Extension',
-        bannerDimensions: 'Banner Dimensions',
+    const fieldCombinationErrors = [];
 
-        date: 'Date',
-        trackArtDate: 'Default Track Cover Art Date',
-        coverArtDate: 'Cover Art Date',
-        dateAddedToWiki: 'Date Added',
+    for (const {message, fields} of invalidFieldCombinations) {
+      const fieldsPresent =
+        presentFields.filter(field => fields.includes(field));
 
-        hasCoverArt: 'Has Cover Art',
-        hasTrackArt: 'Has Track Art',
-        hasTrackNumbers: 'Has Track Numbers',
-        isMajorRelease: 'Major Release',
-        isListedOnHomepage: 'Listed on Homepage',
+      if (fieldsPresent.length >= 2) {
+        const filteredDocument =
+          filterProperties(
+            document,
+            fieldsPresent,
+            {preserveOriginalOrder: true});
 
-        groupsByRef: 'Groups',
-        artTagsByRef: 'Art Tags',
-        commentary: 'Commentary',
+        fieldCombinationErrors.push(
+          new FieldCombinationError(filteredDocument, message));
 
-        additionalFiles: 'Additional Files',
+        for (const field of Object.keys(filteredDocument)) {
+          skippedFields.add(field);
+        }
+      }
     }
-});
-
-export const processTrackGroupDocument = makeProcessDocument(TrackGroup, {
-    fieldTransformations: {
-        'Date Originally Released': value => new Date(value),
-    },
 
-    propertyFieldMapping: {
-        name: 'Group',
-        color: 'Color',
-        dateOriginallyReleased: 'Date Originally Released',
+    if (!empty(fieldCombinationErrors)) {
+      aggregate.push(new FieldCombinationAggregateError(fieldCombinationErrors));
     }
-});
-
-export const processTrackDocument = makeProcessDocument(Track, {
-    fieldTransformations: {
-        'Duration': getDurationInSeconds,
 
-        'Date First Released': value => new Date(value),
-        'Cover Art Date': value => new Date(value),
+    const fieldValues = {};
 
-        'Artists': parseContributors,
-        'Contributors': parseContributors,
-        'Cover Artists': parseContributors,
+    for (const [field, documentValue] of documentEntries) {
+      if (skippedFields.has(field)) continue;
 
-        'Additional Files': parseAdditionalFiles,
-    },
+      // This variable would like to certify itself as "not into capitalism".
+      let propertyValue =
+        (fieldSpecs[field].transform
+          ? fieldSpecs[field].transform(documentValue)
+          : documentValue);
 
-    propertyFieldMapping: {
-        name: 'Track',
+      // Completely blank items in a YAML list are read as null.
+      // They're handy to have around when filling out a document and shouldn't
+      // be considered an error (or data at all).
+      if (Array.isArray(propertyValue)) {
+        const wasEmpty = empty(propertyValue);
 
-        directory: 'Directory',
-        duration: 'Duration',
-        urls: 'URLs',
+        propertyValue =
+          propertyValue.filter(item => item !== null);
 
-        coverArtDate: 'Cover Art Date',
-        coverArtFileExtension: 'Cover Art File Extension',
-        dateFirstReleased: 'Date First Released',
-        hasCoverArt: 'Has Cover Art',
-        hasURLs: 'Has URLs',
+        const isEmpty = empty(propertyValue);
 
-        referencedTracksByRef: 'Referenced Tracks',
-        artistContribsByRef: 'Artists',
-        contributorContribsByRef: 'Contributors',
-        coverArtistContribsByRef: 'Cover Artists',
-        artTagsByRef: 'Art Tags',
-        originalReleaseTrackByRef: 'Originally Released As',
-
-        commentary: 'Commentary',
-        lyrics: 'Lyrics',
-
-        additionalFiles: 'Additional Files',
-    },
+        // Don't set arrays which are empty as a result of the above filter.
+        // Arrays which were originally empty, i.e. `Field: []`, are still
+        // valid data, but if it's just an array not containing any filled out
+        // items, it should be treated as a placeholder and skipped over.
+        if (isEmpty && !wasEmpty) {
+          propertyValue = null;
+        }
+      }
 
-    ignoredFields: ['Sampled Tracks']
-});
+      fieldValues[field] = propertyValue;
+    }
 
-export const processArtistDocument = makeProcessDocument(Artist, {
-    propertyFieldMapping: {
-        name: 'Artist',
+    const thing = Reflect.construct(thingConstructor, []);
 
-        directory: 'Directory',
-        urls: 'URLs',
-        hasAvatar: 'Has Avatar',
-        avatarFileExtension: 'Avatar File Extension',
+    const fieldValueErrors = [];
 
-        aliasNames: 'Aliases',
+    for (const [field, value] of Object.entries(fieldValues)) {
+      const {property} = fieldSpecs[field];
 
-        contextNotes: 'Context Notes'
-    },
+      try {
+        thing[property] = value;
+      } catch (caughtError) {
+        skippedFields.add(field);
+        fieldValueErrors.push(new FieldValueError(
+          field, value, {cause: caughtError}));
+      }
+    }
 
-    ignoredFields: ['Dead URLs']
-});
+    if (!empty(fieldValueErrors)) {
+      aggregate.push(new FieldValueAggregateError(
+        fieldValueErrors, thingConstructor));
+    }
 
-export const processFlashDocument = makeProcessDocument(Flash, {
-    fieldTransformations: {
-        'Date': value => new Date(value),
+    if (skippedFields.size >= 1) {
+      aggregate.push(
+        new SkippedFieldsSummaryError(
+          filterProperties(
+            document,
+            Array.from(skippedFields),
+            {preserveOriginalOrder: true})));
+    }
 
-        'Contributors': parseContributors,
-    },
+    return {thing, aggregate};
+  });
+}
 
-    propertyFieldMapping: {
-        name: 'Flash',
+export class UnknownFieldsError extends Error {
+  constructor(fields) {
+    super(`Unknown fields ignored: ${fields.map(field => colors.red(field)).join(', ')}`);
+    this.fields = fields;
+  }
+}
 
-        directory: 'Directory',
-        page: 'Page',
-        date: 'Date',
-        coverArtFileExtension: 'Cover Art File Extension',
+export class FieldCombinationAggregateError extends AggregateError {
+  constructor(errors) {
+    super(errors, `Invalid field combinations - all involved fields ignored`);
+  }
+}
 
-        featuredTracksByRef: 'Featured Tracks',
-        contributorContribsByRef: 'Contributors',
-        urls: 'URLs'
-    },
-});
+export class FieldCombinationError extends Error {
+  constructor(fields, message) {
+    const fieldNames = Object.keys(fields);
+
+    const fieldNamesText =
+      fieldNames
+        .map(field => colors.red(field))
+        .join(', ');
+
+    const mainMessage = `Don't combine ${fieldNamesText}`;
+
+    const causeMessage =
+      (typeof message === 'function'
+        ? message(fields)
+     : typeof message === 'string'
+        ? message
+        : null);
+
+    super(mainMessage, {
+      cause:
+        (causeMessage
+          ? new Error(causeMessage)
+          : null),
+    });
 
-export const processFlashActDocument = makeProcessDocument(FlashAct, {
-    propertyFieldMapping: {
-        name: 'Act',
-        color: 'Color',
-        anchor: 'Anchor',
-        jump: 'Jump',
-        jumpColor: 'Jump Color'
-    }
-});
-
-export const processNewsEntryDocument = makeProcessDocument(NewsEntry, {
-    fieldTransformations: {
-        'Date': value => new Date(value)
-    },
-
-    propertyFieldMapping: {
-        name: 'Name',
-        directory: 'Directory',
-        date: 'Date',
-        content: 'Content',
-    }
-});
-
-export const processArtTagDocument = makeProcessDocument(ArtTag, {
-    propertyFieldMapping: {
-        name: 'Tag',
-        directory: 'Directory',
-        color: 'Color',
-        isContentWarning: 'Is CW'
-    }
-});
-
-export const processGroupDocument = makeProcessDocument(Group, {
-    propertyFieldMapping: {
-        name: 'Group',
-        directory: 'Directory',
-        description: 'Description',
-        urls: 'URLs',
-    }
-});
+    this.fields = fields;
+  }
+}
 
-export const processGroupCategoryDocument = makeProcessDocument(GroupCategory, {
-    propertyFieldMapping: {
-        name: 'Category',
-        color: 'Color',
-    }
-});
+export class FieldValueAggregateError extends AggregateError {
+  [Symbol.for('hsmusic.aggregate.translucent')] = true;
 
-export const processStaticPageDocument = makeProcessDocument(StaticPage, {
-    propertyFieldMapping: {
-        name: 'Name',
-        nameShort: 'Short Name',
-        directory: 'Directory',
+  constructor(errors, thingConstructor) {
+    const constructorText =
+      colors.green(thingConstructor.name);
 
-        content: 'Content',
-        stylesheet: 'Style',
+    super(
+      errors,
+      `Errors processing field values for ${constructorText}`);
+  }
+}
 
-        showInNavigationBar: 'Show in Navigation Bar'
-    }
-});
-
-export const processWikiInfoDocument = makeProcessDocument(WikiInfo, {
-    propertyFieldMapping: {
-        name: 'Name',
-        nameShort: 'Short Name',
-        color: 'Color',
-        description: 'Description',
-        footerContent: 'Footer Content',
-        defaultLanguage: 'Default Language',
-        canonicalBase: 'Canonical Base',
-        divideTrackListsByGroupsByRef: 'Divide Track Lists By Groups',
-        enableFlashesAndGames: 'Enable Flashes & Games',
-        enableListings: 'Enable Listings',
-        enableNews: 'Enable News',
-        enableArtTagUI: 'Enable Art Tag UI',
-        enableGroupUI: 'Enable Group UI',
-    }
-});
+export class FieldValueError extends Error {
+  constructor(field, value, options) {
+    const fieldText =
+      colors.green(`"${field}"`);
 
-export const processHomepageLayoutDocument = makeProcessDocument(HomepageLayout, {
-    propertyFieldMapping: {
-        sidebarContent: 'Sidebar Content'
-    },
+    const valueText =
+      inspect(value, {maxStringLength: 40});
 
-    ignoredFields: ['Homepage']
-});
+    super(
+      `Failed to set ${fieldText} field to ${valueText}`,
+      options);
+  }
+}
 
-export function makeProcessHomepageLayoutRowDocument(rowClass, spec) {
-    return makeProcessDocument(rowClass, {
-        ...spec,
+export class SkippedFieldsSummaryError extends Error {
+  constructor(filteredDocument) {
+    const entries = Object.entries(filteredDocument);
+
+    const lines =
+      entries.map(([field, value]) =>
+        ` - ${field}: ` +
+        inspect(value, {maxStringLength: 70})
+          .split('\n')
+          .map((line, index) => index === 0 ? line : `   ${line}`)
+          .join('\n'));
+
+    const numFieldsText =
+      (entries.length === 1
+        ? `1 field`
+        : `${entries.length} fields`);
+
+    super(
+      colors.bright(colors.yellow(`Altogether, skipped ${numFieldsText}:\n`)) +
+      lines.join('\n') + '\n' +
+      colors.bright(colors.yellow(`See above errors for details.`)));
+  }
+}
 
-        propertyFieldMapping: {
-            name: 'Row',
-            color: 'Color',
-            type: 'Type',
-            ...spec.propertyFieldMapping,
-        }
-    });
+export function parseDate(date) {
+  return new Date(date);
 }
 
-export const homepageLayoutRowTypeProcessMapping = {
-    albums: makeProcessHomepageLayoutRowDocument(HomepageLayoutAlbumsRow, {
-        propertyFieldMapping: {
-            sourceGroupByRef: 'Group',
-            countAlbumsFromGroup: 'Count',
-            sourceAlbumsByRef: 'Albums',
-            actionLinks: 'Actions'
-        }
-    })
-};
+export function parseDuration(string) {
+  if (typeof string !== 'string') {
+    return string;
+  }
+
+  const parts = string.split(':').map((n) => parseInt(n));
+  if (parts.length === 3) {
+    return parts[0] * 3600 + parts[1] * 60 + parts[2];
+  } else if (parts.length === 2) {
+    return parts[0] * 60 + parts[1];
+  } else {
+    return 0;
+  }
+}
 
-export function processHomepageLayoutRowDocument(document) {
-    const type = document['Type'];
+export function parseAdditionalFiles(array) {
+  if (!Array.isArray(array)) {
+    // Error will be caught when validating against whatever this value is
+    return array;
+  }
+
+  return array.map((item) => ({
+    title: item['Title'],
+    description: item['Description'] ?? null,
+    files: item['Files'],
+  }));
+}
 
-    const match = Object.entries(homepageLayoutRowTypeProcessMapping)
-        .find(([ key ]) => key === type);
+export const extractAccentRegex =
+  /^(?<main>.*?)(?: \((?<accent>.*)\))?$/;
 
-    if (!match) {
-        throw new TypeError(`No processDocument function for row type ${type}!`);
-    }
+export const extractPrefixAccentRegex =
+  /^(?:\((?<accent>.*)\) )?(?<main>.*?)$/;
 
-    return match[1](document);
-}
+export function parseContributors(contributionStrings) {
+  // If this isn't something we can parse, just return it as-is.
+  // The Thing object's validators will handle the data error better
+  // than we're able to here.
+  if (!Array.isArray(contributionStrings)) {
+    return contributionStrings;
+  }
 
-// --> Utilities shared across document parsing functions
+  return contributionStrings.map(item => {
+    if (typeof item === 'object' && item['Who'])
+      return {who: item['Who'], what: item['What'] ?? null};
 
-export function getDurationInSeconds(string) {
-    if (typeof string === 'number') {
-        return string;
-    }
+    if (typeof item !== 'string') return item;
 
-    if (typeof string !== 'string') {
-        throw new TypeError(`Expected a string or number, got ${string}`);
-    }
+    const match = item.match(extractAccentRegex);
+    if (!match) return item;
 
-    const parts = string.split(':').map(n => parseInt(n))
-    if (parts.length === 3) {
-        return parts[0] * 3600 + parts[1] * 60 + parts[2]
-    } else if (parts.length === 2) {
-        return parts[0] * 60 + parts[1]
-    } else {
-        return 0
-    }
+    return {
+      who: match.groups.main,
+      what: match.groups.accent ?? null,
+    };
+  });
 }
 
-export function parseAdditionalFiles(array) {
-    if (!array) return null;
-    if (!Array.isArray(array)) {
-        // Error will be caught when validating against whatever this value is
-        return array;
-    }
+export function parseAdditionalNames(additionalNameStrings) {
+  if (!Array.isArray(additionalNameStrings)) {
+    return additionalNameStrings;
+  }
 
-    return array.map(item => ({
-        title: item['Title'],
-        description: item['Description'] ?? null,
-        files: item['Files']
-    }));
-}
+  return additionalNameStrings.map(item => {
+    if (typeof item === 'object' && item['Name'])
+      return {name: item['Name'], annotation: item['Annotation'] ?? null};
 
-export function parseCommentary(text) {
-    if (text) {
-        const lines = String(text).split('\n');
-        if (!lines[0].replace(/<\/b>/g, '').includes(':</i>')) {
-            return {error: `An entry is missing commentary citation: "${lines[0].slice(0, 40)}..."`};
-        }
-        return text;
-    } else {
-        return null;
-    }
-}
+    if (typeof item !== 'string') return item;
 
-export function parseContributors(contributors) {
-    if (!contributors) {
-        return null;
-    }
+    const match = item.match(extractAccentRegex);
+    if (!match) return item;
 
-    if (contributors.length === 1 && contributors[0].startsWith('<i>')) {
-        const arr = [];
-        arr.textContent = contributors[0];
-        return arr;
-    }
+    return {
+      name: match.groups.main,
+      annotation: match.groups.accent ?? null,
+    };
+  });
+}
 
-    contributors = contributors.map(contrib => {
-        // 8asically, the format is "Who (What)", or just "Who". 8e sure to
-        // keep in mind that "what" doesn't necessarily have a value!
-        const match = contrib.match(/^(.*?)( \((.*)\))?$/);
-        if (!match) {
-            return contrib;
-        }
-        const who = match[1];
-        const what = match[3] || null;
-        return {who, what};
-    });
+export function parseDimensions(string) {
+  // It's technically possible to pass an array like [30, 40] through here.
+  // That's not really an issue because if it isn't of the appropriate shape,
+  // the Thing object's validators will handle the error.
+  if (typeof string !== 'string') {
+    return string;
+  }
 
-    const badContributor = contributors.find(val => typeof val === 'string');
-    if (badContributor) {
-        return {error: `An entry has an incorrectly formatted contributor, "${badContributor}".`};
-    }
+  const parts = string.split(/[x,* ]+/g);
 
-    if (contributors.length === 1 && contributors[0].who === 'none') {
-        return null;
-    }
+  if (parts.length !== 2) {
+    throw new Error(`Invalid dimensions: ${string} (expected "width & height")`);
+  }
 
-    return contributors;
-}
+  const nums = parts.map((part) => Number(part.trim()));
 
-function parseDimensions(string) {
-    if (!string) {
-        return null;
-    }
+  if (nums.includes(NaN)) {
+    throw new Error(`Invalid dimensions: ${string} (couldn't parse as numbers)`);
+  }
 
-    const parts = string.split(/[x,* ]+/g);
-    if (parts.length !== 2) throw new Error(`Invalid dimensions: ${string} (expected width & height)`);
-    const nums = parts.map(part => Number(part.trim()));
-    if (nums.includes(NaN)) throw new Error(`Invalid dimensions: ${string} (couldn't parse as numbers)`);
-    return nums;
+  return nums;
 }
 
-// --> Data repository loading functions and descriptors
-
 // documentModes: Symbols indicating sets of behavior for loading and processing
 // data files.
 export const documentModes = {
-    // onePerFile: One document per file. Expects files array (or function) and
-    // processDocument function. Obviously, each specified data file should only
-    // contain one YAML document (an error will be thrown otherwise). Calls save
-    // with an array of processed documents (wiki objects).
-    onePerFile: Symbol('Document mode: onePerFile'),
-
-    // headerAndEntries: One or more documents per file; the first document is
-    // treated as a "header" and represents data which pertains to all following
-    // "entry" documents. Expects files array (or function) and
-    // processHeaderDocument and processEntryDocument functions. Calls save with
-    // an array of {header, entries} objects.
-    //
-    // Please note that the final results loaded from each file may be "missing"
-    // data objects corresponding to entry documents if the processEntryDocument
-    // function throws on any entries, resulting in partial data provided to
-    // save() - errors will be caught and thrown in the final buildSteps
-    // aggregate. However, if the processHeaderDocument function fails, all
-    // following documents in the same file will be ignored as well (i.e. an
-    // entire file will be excempt from the save() function's input).
-    headerAndEntries: Symbol('Document mode: headerAndEntries'),
-
-    // allInOne: One or more documents, all contained in one file. Expects file
-    // string (or function) and processDocument function. Calls save with an
-    // array of processed documents (wiki objects).
-    allInOne: Symbol('Document mode: allInOne'),
-
-    // oneDocumentTotal: Just a single document, represented in one file.
-    // Expects file string (or function) and processDocument function. Calls
-    // save with the single processed wiki document (data object).
-    //
-    // Please note that if the single document fails to process, the save()
-    // function won't be called at all, generally resulting in an altogether
-    // missing property from the global wikiData object. This should be caught
-    // and handled externally.
-    oneDocumentTotal: Symbol('Document mode: oneDocumentTotal'),
+  // onePerFile: One document per file. Expects files array (or function) and
+  // processDocument function. Obviously, each specified data file should only
+  // contain one YAML document (an error will be thrown otherwise). Calls save
+  // with an array of processed documents (wiki objects).
+  onePerFile: Symbol('Document mode: onePerFile'),
+
+  // headerAndEntries: One or more documents per file; the first document is
+  // treated as a "header" and represents data which pertains to all following
+  // "entry" documents. Expects files array (or function) and
+  // processHeaderDocument and processEntryDocument functions. Calls save with
+  // an array of {header, entries} objects.
+  //
+  // Please note that the final results loaded from each file may be "missing"
+  // data objects corresponding to entry documents if the processEntryDocument
+  // function throws on any entries, resulting in partial data provided to
+  // save() - errors will be caught and thrown in the final buildSteps
+  // aggregate. However, if the processHeaderDocument function fails, all
+  // following documents in the same file will be ignored as well (i.e. an
+  // entire file will be excempt from the save() function's input).
+  headerAndEntries: Symbol('Document mode: headerAndEntries'),
+
+  // allInOne: One or more documents, all contained in one file. Expects file
+  // string (or function) and processDocument function. Calls save with an
+  // array of processed documents (wiki objects).
+  allInOne: Symbol('Document mode: allInOne'),
+
+  // oneDocumentTotal: Just a single document, represented in one file.
+  // Expects file string (or function) and processDocument function. Calls
+  // save with the single processed wiki document (data object).
+  //
+  // Please note that if the single document fails to process, the save()
+  // function won't be called at all, generally resulting in an altogether
+  // missing property from the global wikiData object. This should be caught
+  // and handled externally.
+  oneDocumentTotal: Symbol('Document mode: oneDocumentTotal'),
 };
 
 // dataSteps: Top-level array of "steps" for loading YAML document files.
@@ -624,670 +523,490 @@ export const documentModes = {
 //   them to each other, setting additional properties, etc). Input argument
 //   format depends on documentMode.
 //
-export const dataSteps = [
-    {
-        title: `Process wiki info file`,
-        file: WIKI_INFO_FILE,
+export const getDataSteps = () => {
+  const steps = [];
 
-        documentMode: documentModes.oneDocumentTotal,
-        processDocument: processWikiInfoDocument,
+  for (const thingConstructor of Object.values(thingConstructors)) {
+    const getSpecFn = thingConstructor[Thing.getYamlLoadingSpec];
+    if (!getSpecFn) continue;
 
-        save(wikiInfo) {
-            if (!wikiInfo) {
-                return;
-            }
+    steps.push(getSpecFn({
+      documentModes,
+      thingConstructors,
+    }));
+  }
 
-            return {wikiInfo};
+  sortByName(steps, {getName: step => step.title});
+
+  return steps;
+};
+
+export async function loadAndProcessDataDocuments({dataPath}) {
+  const processDataAggregate = openAggregate({
+    message: `Errors processing data files`,
+  });
+  const wikiDataResult = {};
+
+  function decorateErrorWithFile(fn) {
+    return decorateErrorWithAnnotation(fn,
+      (caughtError, firstArg) =>
+        annotateErrorWithFile(
+          caughtError,
+          path.relative(
+            dataPath,
+            (typeof firstArg === 'object'
+              ? firstArg.file
+              : firstArg))));
+  }
+
+  function asyncDecorateErrorWithFile(fn) {
+    return decorateErrorWithFile(fn).async;
+  }
+
+  for (const dataStep of getDataSteps()) {
+    await processDataAggregate.nestAsync(
+      {
+        message: `Errors during data step: ${colors.bright(dataStep.title)}`,
+        translucent: true,
+      },
+      async ({call, callAsync, map, mapAsync, push}) => {
+        const {documentMode} = dataStep;
+
+        if (!Object.values(documentModes).includes(documentMode)) {
+          throw new Error(`Invalid documentMode: ${documentMode.toString()}`);
         }
-    },
-
-    {
-        title: `Process album files`,
-        files: async dataPath => (
-            (await findFiles(path.join(dataPath, DATA_ALBUM_DIRECTORY), {
-                filter: f => path.extname(f) === '.yaml',
-                joinParentDirectory: false
-            })).map(file => path.join(DATA_ALBUM_DIRECTORY, file))),
-
-        documentMode: documentModes.headerAndEntries,
-        processHeaderDocument: processAlbumDocument,
-        processEntryDocument(document) {
-            return ('Group' in document
-                ? processTrackGroupDocument(document)
-                : processTrackDocument(document));
-        },
-
-        save(results) {
-            const albumData = [];
-            const trackData = [];
-
-            for (const { header: album, entries } of results) {
-                // We can't mutate an array once it's set as a property
-                // value, so prepare the tracks and track groups that will
-                // show up in a track list all the way before actually
-                // applying them.
-                const trackGroups = [];
-                let currentTracksByRef = null;
-                let currentTrackGroup = null;
-
-                const albumRef = Thing.getReference(album);
-
-                function closeCurrentTrackGroup() {
-                    if (currentTracksByRef) {
-                        let trackGroup;
-
-                        if (currentTrackGroup) {
-                            trackGroup = currentTrackGroup;
-                        } else {
-                            trackGroup = new TrackGroup();
-                            trackGroup.name = `Default Track Group`;
-                            trackGroup.isDefaultTrackGroup = true;
-                        }
-
-                        trackGroup.album = album;
-                        trackGroup.tracksByRef = currentTracksByRef;
-                        trackGroups.push(trackGroup);
-                    }
-                }
 
-                for (const entry of entries) {
-                    if (entry instanceof TrackGroup) {
-                        closeCurrentTrackGroup();
-                        currentTracksByRef = [];
-                        currentTrackGroup = entry;
-                        continue;
-                    }
+        // Hear me out, it's been like 1200 years since I wrote the rest of
+        // this beautifully error-containing code and I don't know how to
+        // integrate this nicely. So I'm just returning the result and the
+        // error that should be thrown. Yes, we're back in callback hell,
+        // just without the callbacks. Thank you.
+        const filterBlankDocuments = documents => {
+          const aggregate = openAggregate({
+            message: `Found blank documents - check for extra '${colors.cyan(`---`)}'`,
+          });
+
+          const filteredDocuments =
+            documents
+              .filter(doc => doc !== null);
+
+          if (filteredDocuments.length !== documents.length) {
+            const blankIndexRangeInfo =
+              documents
+                .map((doc, index) => [doc, index])
+                .filter(([doc]) => doc === null)
+                .map(([doc, index]) => index)
+                .reduce((accumulator, index) => {
+                  if (accumulator.length === 0) {
+                    return [[index, index]];
+                  }
+                  const current = accumulator.at(-1);
+                  const rest = accumulator.slice(0, -1);
+                  if (current[1] === index - 1) {
+                    return rest.concat([[current[0], index]]);
+                  } else {
+                    return accumulator.concat([[index, index]]);
+                  }
+                }, [])
+                .map(([start, end]) => ({
+                  start,
+                  end,
+                  count: end - start + 1,
+                  previous: atOffset(documents, start, -1),
+                  next: atOffset(documents, end, +1),
+                }));
+
+            for (const {start, end, count, previous, next} of blankIndexRangeInfo) {
+              const parts = [];
+
+              if (count === 1) {
+                const range = `#${start + 1}`;
+                parts.push(`${count} document (${colors.yellow(range)}), `);
+              } else {
+                const range = `#${start + 1}-${end + 1}`;
+                parts.push(`${count} documents (${colors.yellow(range)}), `);
+              }
+
+              if (previous === null) {
+                parts.push(`at start of file`);
+              } else if (next === null) {
+                parts.push(`at end of file`);
+              } else {
+                const previousDescription = Object.entries(previous).at(0).join(': ');
+                const nextDescription = Object.entries(next).at(0).join(': ');
+                parts.push(`between "${colors.cyan(previousDescription)}" and "${colors.cyan(nextDescription)}"`);
+              }
+
+              aggregate.push(new Error(parts.join('')));
+            }
+          }
 
-                    trackData.push(entry);
+          return {documents: filteredDocuments, aggregate};
+        };
 
-                    entry.dataSourceAlbumByRef = albumRef;
+        const processDocument = (document, thingClassOrFn) => {
+          const thingClass =
+            (thingClassOrFn.prototype instanceof Thing
+              ? thingClassOrFn
+              : thingClassOrFn(document));
 
-                    const trackRef = Thing.getReference(entry);
-                    if (currentTracksByRef) {
-                        currentTracksByRef.push(trackRef);
-                    } else {
-                        currentTracksByRef = [trackRef];
-                    }
-                }
+          if (typeof thingClass !== 'function') {
+            throw new Error(`Expected a thing class, got ${typeAppearance(thingClass)}`);
+          }
 
-                closeCurrentTrackGroup();
+          if (!(thingClass.prototype instanceof Thing)) {
+            throw new Error(`Expected a thing class, got ${thingClass.name}`);
+          }
 
-                album.trackGroups = trackGroups;
-                albumData.push(album);
-            }
+          const spec = thingClass[Thing.yamlDocumentSpec];
 
-            return {albumData, trackData};
-        }
-    },
-
-    {
-        title: `Process artists file`,
-        file: ARTIST_DATA_FILE,
-
-        documentMode: documentModes.allInOne,
-        processDocument: processArtistDocument,
-
-        save(results) {
-            const artistData = results;
-
-            const artistAliasData = results.flatMap(artist => {
-                const origRef = Thing.getReference(artist);
-                return (artist.aliasNames?.map(name => {
-                    const alias = new Artist();
-                    alias.name = name;
-                    alias.isAlias = true;
-                    alias.aliasedArtistRef = origRef;
-                    alias.artistData = artistData;
-                    return alias;
-                }) ?? []);
-            });
-
-            return {artistData, artistAliasData};
-        }
-    },
-
-    // TODO: WD.wikiInfo.enableFlashesAndGames &&
-    {
-        title: `Process flashes file`,
-        file: FLASH_DATA_FILE,
-
-        documentMode: documentModes.allInOne,
-        processDocument(document) {
-            return ('Act' in document
-                ? processFlashActDocument(document)
-                : processFlashDocument(document));
-        },
-
-        save(results) {
-            let flashAct;
-            let flashesByRef = [];
-
-            if (results[0] && !(results[0] instanceof FlashAct)) {
-                throw new Error(`Expected an act at top of flash data file`);
-            }
+          if (!spec) {
+            throw new Error(`Class "${thingClass.name}" doesn't specify Thing.yamlDocumentSpec`);
+          }
 
-            for (const thing of results) {
-                if (thing instanceof FlashAct) {
-                    if (flashAct) {
-                        Object.assign(flashAct, {flashesByRef});
-                    }
+          // TODO: Making a function to only call it just like that is
+          // obviously pretty jank! It should be created once per data step.
+          const fn = makeProcessDocument(thingClass, spec);
+          return fn(document);
+        };
 
-                    flashAct = thing;
-                    flashesByRef = [];
+        if (
+          documentMode === documentModes.allInOne ||
+          documentMode === documentModes.oneDocumentTotal
+        ) {
+          if (!dataStep.file) {
+            throw new Error(`Expected 'file' property for ${documentMode.toString()}`);
+          }
+
+          const file = path.join(
+            dataPath,
+            typeof dataStep.file === 'function'
+              ? await callAsync(dataStep.file, dataPath)
+              : dataStep.file);
+
+          const statResult = await callAsync(() =>
+            stat(file).then(
+              () => true,
+              error => {
+                if (error.code === 'ENOENT') {
+                  return false;
                 } else {
-                    flashesByRef.push(Thing.getReference(thing));
+                  throw error;
                 }
-            }
+              }));
 
-            if (flashAct) {
-                Object.assign(flashAct, {flashesByRef});
-            }
+          if (statResult === false) {
+            const saveResult = call(dataStep.save, {
+              [documentModes.allInOne]: [],
+              [documentModes.oneDocumentTotal]: {},
+            }[documentMode]);
 
-            const flashData = results.filter(x => x instanceof Flash);
-            const flashActData = results.filter(x => x instanceof FlashAct);
+            if (!saveResult) return;
 
-            return {flashData, flashActData};
-        }
-    },
+            Object.assign(wikiDataResult, saveResult);
 
-    {
-        title: `Process groups file`,
-        file: GROUP_DATA_FILE,
+            return;
+          }
 
-        documentMode: documentModes.allInOne,
-        processDocument(document) {
-            return ('Category' in document
-                ? processGroupCategoryDocument(document)
-                : processGroupDocument(document));
-        },
+          const readResult = await callAsync(readFile, file, 'utf-8');
 
-        save(results) {
-            let groupCategory;
-            let groupsByRef = [];
+          if (!readResult) {
+            return;
+          }
 
-            if (results[0] && !(results[0] instanceof GroupCategory)) {
-                throw new Error(`Expected a category at top of group data file`);
-            }
+          let processResults;
 
-            for (const thing of results) {
-                if (thing instanceof GroupCategory) {
-                    if (groupCategory) {
-                        Object.assign(groupCategory, {groupsByRef});
-                    }
+          switch (documentMode) {
+            case documentModes.oneDocumentTotal: {
+              const yamlResult = call(yaml.load, readResult);
 
-                    groupCategory = thing;
-                    groupsByRef = [];
-                } else {
-                    groupsByRef.push(Thing.getReference(thing));
-                }
-            }
+              if (!yamlResult) {
+                processResults = null;
+                break;
+              }
 
-            if (groupCategory) {
-                Object.assign(groupCategory, {groupsByRef});
-            }
+              const {thing, aggregate} =
+                processDocument(yamlResult, dataStep.documentThing);
 
-            const groupData = results.filter(x => x instanceof Group);
-            const groupCategoryData = results.filter(x => x instanceof GroupCategory);
+              processResults = thing;
 
-            return {groupData, groupCategoryData};
-        }
-    },
+              call(() => aggregate.close());
 
-    {
-        title: `Process homepage layout file`,
-        files: [HOMEPAGE_LAYOUT_DATA_FILE],
+              break;
+            }
 
-        documentMode: documentModes.headerAndEntries,
-        processHeaderDocument: processHomepageLayoutDocument,
-        processEntryDocument: processHomepageLayoutRowDocument,
+            case documentModes.allInOne: {
+              const yamlResults = call(yaml.loadAll, readResult);
 
-        save(results) {
-            if (!results[0]) {
+              if (!yamlResults) {
+                processResults = [];
                 return;
-            }
+              }
 
-            const { header: homepageLayout, entries: rows } = results[0];
-            Object.assign(homepageLayout, {rows});
-            return {homepageLayout};
-        }
-    },
+              const {documents, aggregate: filterAggregate} =
+                filterBlankDocuments(yamlResults);
 
-    // TODO: WD.wikiInfo.enableNews &&
-    {
-        title: `Process news data file`,
-        file: NEWS_DATA_FILE,
+              call(filterAggregate.close);
 
-        documentMode: documentModes.allInOne,
-        processDocument: processNewsEntryDocument,
+              processResults = [];
 
-        save(newsData) {
-            sortByDate(newsData);
-            newsData.reverse();
+              map(documents, decorateErrorWithIndex(document => {
+                const {thing, aggregate} =
+                  processDocument(document, dataStep.documentThing);
 
-            return {newsData};
-        }
-    },
+                processResults.push(thing);
+                aggregate.close();
+              }), {message: `Errors processing documents`});
 
-    {
-        title: `Process art tags file`,
-        file: ART_TAG_DATA_FILE,
+              break;
+            }
+          }
 
-        documentMode: documentModes.allInOne,
-        processDocument: processArtTagDocument,
+          if (!processResults) return;
 
-        save(artTagData) {
-            artTagData.sort(sortByName);
+          const saveResult = call(dataStep.save, processResults);
 
-            return {artTagData};
-        }
-    },
+          if (!saveResult) return;
 
-    {
-        title: `Process static pages file`,
-        file: STATIC_PAGE_DATA_FILE,
+          Object.assign(wikiDataResult, saveResult);
 
-        documentMode: documentModes.allInOne,
-        processDocument: processStaticPageDocument,
+          return;
+        }
 
-        save(staticPageData) {
-            return {staticPageData};
+        if (!dataStep.files) {
+          throw new Error(`Expected 'files' property for ${documentMode.toString()}`);
         }
-    },
-];
 
-export async function loadAndProcessDataDocuments({
-    dataPath,
-}) {
-    const processDataAggregate = openAggregate({message: `Errors processing data files`});
-    const wikiDataResult = {};
+        const filesFromDataStep =
+          (typeof dataStep.files === 'function'
+            ? await callAsync(() =>
+                dataStep.files(dataPath).then(
+                  files => files,
+                  error => {
+                    if (error.code === 'ENOENT') {
+                      return [];
+                    } else {
+                      throw error;
+                    }
+                  }))
+            : dataStep.files);
 
-    function decorateErrorWithFile(fn) {
-        return (x, index, array) => {
+        const filesUnderDataPath =
+          filesFromDataStep
+            .map(file => path.join(dataPath, file));
+
+        const yamlResults = [];
+
+        await mapAsync(filesUnderDataPath, {message: `Errors loading data files`},
+          asyncDecorateErrorWithFile(async file => {
+            let contents;
             try {
-                return fn(x, index, array);
-            } catch (error) {
-                error.message += (
-                    (error.message.includes('\n') ? '\n' : ' ') +
-                    `(file: ${color.bright(color.blue(path.relative(dataPath, x.file)))})`
-                );
-                throw error;
+              contents = await readFile(file, 'utf-8');
+            } catch (caughtError) {
+              throw new Error(`Failed to read data file`, {cause: caughtError});
             }
-        };
-    }
 
-    for (const dataStep of dataSteps) {
-        await processDataAggregate.nestAsync(
-            {message: `Errors during data step: ${dataStep.title}`},
-            async ({call, callAsync, map, mapAsync, nest}) => {
-                const { documentMode } = dataStep;
+            let documents;
+            try {
+              documents = yaml.loadAll(contents);
+            } catch (caughtError) {
+              throw new Error(`Failed to parse valid YAML`, {cause: caughtError});
+            }
 
-                if (!(Object.values(documentModes).includes(documentMode))) {
-                    throw new Error(`Invalid documentMode: ${documentMode.toString()}`);
-                }
+            const {documents: filteredDocuments, aggregate: filterAggregate} =
+              filterBlankDocuments(documents);
 
-                if (documentMode === documentModes.allInOne || documentMode === documentModes.oneDocumentTotal) {
-                    if (!dataStep.file) {
-                        throw new Error(`Expected 'file' property for ${documentMode.toString()}`);
-                    }
+            try {
+              filterAggregate.close();
+            } catch (caughtError) {
+              // Blank documents aren't a critical error, they're just something
+              // that should be noted - the (filtered) documents still get pushed.
+              const pathToFile = path.relative(dataPath, file);
+              annotateErrorWithFile(caughtError, pathToFile);
+              push(caughtError);
+            }
 
-                    const file = path.join(dataPath,
-                        (typeof dataStep.file === 'function'
-                            ? await callAsync(dataStep.file, dataPath)
-                            : dataStep.file));
+            yamlResults.push({file, documents: filteredDocuments});
+          }));
 
-                    const readResult = await callAsync(readFile, file, 'utf-8');
+        const processResults = [];
 
-                    if (!readResult) {
-                        return;
-                    }
+        switch (documentMode) {
+          case documentModes.headerAndEntries:
+            map(yamlResults, {message: `Errors processing documents in data files`, translucent: true},
+              decorateErrorWithFile(({documents}) => {
+                const headerDocument = documents[0];
+                const entryDocuments = documents.slice(1).filter(Boolean);
 
-                    const yamlResult = (documentMode === documentModes.oneDocumentTotal
-                        ? call(yaml.load, readResult)
-                        : call(yaml.loadAll, readResult));
+                if (!headerDocument)
+                  throw new Error(`Missing header document (empty file or erroneously starting with "---"?)`);
 
-                    if (!yamlResult) {
-                        return;
-                    }
+                withAggregate({message: `Errors processing documents`}, ({push}) => {
+                  const {thing: headerObject, aggregate: headerAggregate} =
+                    processDocument(headerDocument, dataStep.headerDocumentThing);
 
-                    let processResults;
+                  try {
+                    headerAggregate.close();
+                  } catch (caughtError) {
+                    caughtError.message = `(${colors.yellow(`header`)}) ${caughtError.message}`;
+                    push(caughtError);
+                  }
 
-                    if (documentMode === documentModes.oneDocumentTotal) {
-                        nest({message: `Errors processing document`}, ({ call }) => {
-                            processResults = call(dataStep.processDocument, yamlResult);
-                        });
-                    } else {
-                        const { result, aggregate } = mapAggregate(
-                            yamlResult,
-                            decorateErrorWithIndex(dataStep.processDocument),
-                            {message: `Errors processing documents`}
-                        );
-                        processResults = result;
-                        call(aggregate.close);
-                    }
+                  const entryObjects = [];
 
-                    if (!processResults) return;
+                  for (let index = 0; index < entryDocuments.length; index++) {
+                    const entryDocument = entryDocuments[index];
 
-                    const saveResult = call(dataStep.save, processResults);
+                    const {thing: entryObject, aggregate: entryAggregate} =
+                      processDocument(entryDocument, dataStep.entryDocumentThing);
 
-                    if (!saveResult) return;
+                    entryObjects.push(entryObject);
 
-                    Object.assign(wikiDataResult, saveResult);
+                    try {
+                      entryAggregate.close();
+                    } catch (caughtError) {
+                      caughtError.message = `(${colors.yellow(`entry #${index + 1}`)}) ${caughtError.message}`;
+                      push(caughtError);
+                    }
+                  }
 
-                    return;
-                }
+                  processResults.push({
+                    header: headerObject,
+                    entries: entryObjects,
+                  });
+                });
+              }));
+            break;
 
-                if (!dataStep.files) {
-                    throw new Error(`Expected 'files' property for ${documentMode.toString()}`);
-                }
+          case documentModes.onePerFile:
+            map(yamlResults, {message: `Errors processing data files as valid documents`},
+              decorateErrorWithFile(({documents}) => {
+                if (documents.length > 1)
+                  throw new Error(`Only expected one document to be present per file, got ${documents.length} here`);
 
-                const files = (
-                    (typeof dataStep.files === 'function'
-                        ? await callAsync(dataStep.files, dataPath)
-                        : dataStep.files)
-                    .map(file => path.join(dataPath, file)));
-
-                const readResults = await mapAsync(
-                    files,
-                    file => (readFile(file, 'utf-8')
-                        .then(contents => ({file, contents}))),
-                    {message: `Errors reading data files`});
-
-                const yamlResults = map(
-                    readResults,
-                    decorateErrorWithFile(
-                        ({ file, contents }) => ({file, documents: yaml.loadAll(contents)})),
-                    {message: `Errors parsing data files as valid YAML`});
-
-                let processResults;
-
-                if (documentMode === documentModes.headerAndEntries) {
-                    nest({message: `Errors processing data files as valid documents`}, ({ call, map }) => {
-                        processResults = [];
-
-                        yamlResults.forEach(({ file, documents }) => {
-                            const [ headerDocument, ...entryDocuments ] = documents;
-
-                            const header = call(
-                                decorateErrorWithFile(
-                                    ({ document }) => dataStep.processHeaderDocument(document)),
-                                {file, document: headerDocument});
-
-                            // Don't continue processing files whose header
-                            // document is invalid - the entire file is excempt
-                            // from data in this case.
-                            if (!header) {
-                                return;
-                            }
-
-                            const entries = map(
-                                entryDocuments.map(document => ({file, document})),
-                                decorateErrorWithFile(
-                                    decorateErrorWithIndex(
-                                        ({ document }) => dataStep.processEntryDocument(document))),
-                                {message: `Errors processing entry documents`});
-
-                            // Entries may be incomplete (i.e. any errored
-                            // documents won't have a processed output
-                            // represented here) - this is intentional! By
-                            // principle, partial output is preferred over
-                            // erroring an entire file.
-                            processResults.push({header, entries});
-                        });
-                    });
-                }
+                if (empty(documents) || !documents[0])
+                  throw new Error(`Expected a document, this file is empty`);
 
-                if (documentMode === documentModes.onePerFile) {
-                    nest({message: `Errors processing data files as valid documents`}, ({ call, map }) => {
-                        processResults = [];
-
-                        yamlResults.forEach(({ file, documents }) => {
-                            if (documents.length > 1) {
-                                call(decorateErrorWithFile(() => {
-                                    throw new Error(`Only expected one document to be present per file`);
-                                }));
-                                return;
-                            }
-
-                            const result = call(
-                                decorateErrorWithFile(
-                                    ({ document }) => dataStep.processDocument(document)),
-                                {file, document: documents[0]});
-
-                            if (!result) {
-                                return;
-                            }
-
-                            processResults.push(result);
-                        });
-                    });
-                }
+                const {thing, aggregate} =
+                  processDocument(documents[0], dataStep.documentThing);
 
-                const saveResult = call(dataStep.save, processResults);
+                processResults.push(thing);
+                aggregate.close();
+              }));
+            break;
+        }
 
-                if (!saveResult) return;
+        const saveResult = call(dataStep.save, processResults);
 
-                Object.assign(wikiDataResult, saveResult);
-            });
-    }
+        if (!saveResult) return;
 
-    return {
-        aggregate: processDataAggregate,
-        result: wikiDataResult
-    };
+        Object.assign(wikiDataResult, saveResult);
+      }
+    );
+  }
+
+  return {
+    aggregate: processDataAggregate,
+    result: wikiDataResult,
+  };
 }
 
 // Data linking! Basically, provide (portions of) wikiData to the Things which
 // require it - they'll expose dynamically computed properties as a result (many
-// of which are required for page HTML generation).
+// of which are required for page HTML generation and other expected behavior).
 export function linkWikiDataArrays(wikiData) {
-    function assignWikiData(things, ...keys) {
-        for (let i = 0; i < things.length; i++) {
-            for (let j = 0; j < keys.length; j++) {
-                const key = keys[j];
-                things[i][key] = wikiData[key];
-            }
-        }
+  const linkWikiDataSpec = new Map([
+    [wikiData.albumData, [
+      'artTagData',
+      'artistData',
+      'groupData',
+    ]],
+
+    [wikiData.artTagData, [
+      'albumData',
+      'trackData',
+    ]],
+
+    [wikiData.artistData, [
+      'albumData',
+      'artistData',
+      'flashData',
+      'trackData',
+    ]],
+
+    [wikiData.flashData, [
+      'artistData',
+      'flashActData',
+      'trackData',
+    ]],
+
+    [wikiData.flashActData, [
+      'flashData',
+      'flashSideData',
+    ]],
+
+    [wikiData.flashSideData, [
+      'flashActData',
+    ]],
+
+    [wikiData.groupData, [
+      'albumData',
+      'groupCategoryData',
+    ]],
+
+    [wikiData.groupCategoryData, [
+      'groupData',
+    ]],
+
+    [wikiData.homepageLayout?.rows, [
+      'albumData',
+      'groupData',
+    ]],
+
+    [wikiData.trackData, [
+      'albumData',
+      'artTagData',
+      'artistData',
+      'flashData',
+      'trackData',
+    ]],
+
+    [[wikiData.wikiInfo], [
+      'groupData',
+    ]],
+  ]);
+
+  for (const [things, keys] of linkWikiDataSpec.entries()) {
+    if (things === undefined) continue;
+    for (const thing of things) {
+      if (thing === undefined) continue;
+      for (const key of keys) {
+        if (!(key in wikiData)) continue;
+        thing[key] = wikiData[key];
+      }
     }
-
-    const WD = wikiData;
-
-    assignWikiData([WD.wikiInfo], 'groupData');
-
-    assignWikiData(WD.albumData, 'artistData', 'artTagData', 'groupData', 'trackData');
-    WD.albumData.forEach(album => assignWikiData(album.trackGroups, 'trackData'));
-
-    assignWikiData(WD.trackData, 'albumData', 'artistData', 'artTagData', 'flashData', 'trackData');
-    assignWikiData(WD.artistData, 'albumData', 'artistData', 'flashData', 'trackData');
-    assignWikiData(WD.groupData, 'albumData', 'groupCategoryData');
-    assignWikiData(WD.groupCategoryData, 'groupData');
-    assignWikiData(WD.flashData, 'artistData', 'flashActData', 'trackData');
-    assignWikiData(WD.flashActData, 'flashData');
-    assignWikiData(WD.artTagData, 'albumData', 'trackData');
-    assignWikiData(WD.homepageLayout.rows, 'albumData', 'groupData');
+  }
 }
 
 export function sortWikiDataArrays(wikiData) {
-    Object.assign(wikiData, {
-        albumData: sortByDate(wikiData.albumData.slice()),
-        trackData: sortByDate(wikiData.trackData.slice())
-    });
-
-    // Re-link data arrays, so that every object has the new, sorted versions.
-    // Note that the sorting step deliberately creates new arrays (mutating
-    // slices instead of the original arrays) - this is so that the object
-    // caching system understands that it's working with a new ordering.
-    // We still need to actually provide those updated arrays over again!
-    linkWikiDataArrays(wikiData);
-}
-
-// Warn about directories which are reused across more than one of the same type
-// of Thing. Directories are the unique identifier for most data objects across
-// the wiki, so we have to make sure they aren't duplicated!  This also
-// altogether filters out instances of things with duplicate directories (so if
-// two tracks share the directory "megalovania", they'll both be skipped for the
-// build, for example).
-export function filterDuplicateDirectories(wikiData) {
-    const deduplicateSpec = [
-        'albumData',
-        'artTagData',
-        'flashData',
-        'groupData',
-        'newsData',
-        'trackData',
-    ];
-
-    const aggregate = openAggregate({message: `Duplicate directories found`});
-    for (const thingDataProp of deduplicateSpec) {
-        const thingData = wikiData[thingDataProp];
-        aggregate.nest({message: `Duplicate directories found in ${color.green('wikiData.' + thingDataProp)}`}, ({ call }) => {
-            const directoryPlaces = Object.create(null);
-            const duplicateDirectories = [];
-            for (const thing of thingData) {
-                const { directory } = thing;
-                if (directory in directoryPlaces) {
-                    directoryPlaces[directory].push(thing);
-                    duplicateDirectories.push(directory);
-                } else {
-                    directoryPlaces[directory] = [thing];
-                }
-            }
-            if (!duplicateDirectories.length) return;
-            duplicateDirectories.sort((a, b) => {
-                const aL = a.toLowerCase();
-                const bL = b.toLowerCase();
-                return aL < bL ? -1 : aL > bL ? 1 : 0;
-            });
-            for (const directory of duplicateDirectories) {
-                const places = directoryPlaces[directory];
-                call(() => {
-                    throw new Error(`Duplicate directory ${color.green(directory)}:\n` +
-                        places.map(thing => ` - ` + inspect(thing)).join('\n'));
-                });
-            }
-            const allDuplicatedThings = Object.values(directoryPlaces).filter(arr => arr.length > 1).flat();
-            const filteredThings = thingData.filter(thing => !allDuplicatedThings.includes(thing));
-            wikiData[thingDataProp] = filteredThings;
-        });
-    }
-
-    // TODO: This code closes the aggregate but it generally gets closed again
-    // by the caller. This works but it might be weird to assume closing an
-    // aggregate twice is okay, maybe there's a better solution? Expose a new
-    // function on aggregates for checking if it *would* error?
-    // (i.e: errors.length > 0)
-    try {
-        aggregate.close();
-    } catch (error) {
-        // Duplicate entries were found and filtered out, resulting in altered
-        // wikiData arrays. These must be re-linked so objects receive the new
-        // data.
-        linkWikiDataArrays(wikiData);
-    }
-    return aggregate;
-}
-
-// Warn about references across data which don't match anything.  This involves
-// using the find() functions on all references, setting it to 'error' mode, and
-// collecting everything in a structured logged (which gets logged if there are
-// any errors). At the same time, we remove errored references from the thing's
-// data array.
-export function filterReferenceErrors(wikiData) {
-    const referenceSpec = [
-        ['wikiInfo', {
-            divideTrackListsByGroupsByRef: 'group',
-        }],
-
-        ['albumData', {
-            artistContribsByRef: '_contrib',
-            coverArtistContribsByRef: '_contrib',
-            trackCoverArtistContribsByRef: '_contrib',
-            wallpaperArtistContribsByRef: '_contrib',
-            bannerArtistContribsByRef: '_contrib',
-            groupsByRef: 'group',
-            artTagsByRef: 'artTag',
-        }],
-
-        ['trackData', {
-            artistContribsByRef: '_contrib',
-            contributorContribsByRef: '_contrib',
-            coverArtistContribsByRef: '_contrib',
-            referencedTracksByRef: 'track',
-            artTagsByRef: 'artTag',
-            originalReleaseTrackByRef: 'track',
-        }],
-
-        ['groupCategoryData', {
-            groupsByRef: 'group',
-        }],
-
-        ['homepageLayout.rows', {
-            sourceGroupsByRef: 'group',
-            sourceAlbumsByRef: 'album',
-        }],
-
-        ['flashData', {
-            contributorContribsByRef: '_contrib',
-            featuredTracksByRef: 'track',
-        }],
-
-        ['flashActData', {
-            flashesByRef: 'flash',
-        }],
-    ];
-
-    function getNestedProp(obj, key) {
-        const recursive = (o, k) => (k.length === 1
-            ? o[k[0]]
-            : recursive(o[k[0]], k.slice(1)));
-        const keys = key.split(/(?<=(?<!\\)(?:\\\\)*)\./);
-        return recursive(obj, keys);
-    }
-
-    const aggregate = openAggregate({message: `Errors validating between-thing references in data`});
-    const boundFind = bindFind(wikiData, {mode: 'error'});
-    for (const [ thingDataProp, propSpec ] of referenceSpec) {
-        const thingData = getNestedProp(wikiData, thingDataProp);
-        aggregate.nest({message: `Reference errors in ${color.green('wikiData.' + thingDataProp)}`}, ({ nest }) => {
-            const things = Array.isArray(thingData) ? thingData : [thingData];
-            for (const thing of things) {
-                nest({message: `Reference errors in ${inspect(thing)}`}, ({ filter }) => {
-                    for (const [ property, findFnKey ] of Object.entries(propSpec)) {
-                        if (!thing[property]) continue;
-                        if (findFnKey === '_contrib') {
-                            thing[property] = filter(thing[property],
-                                decorateErrorWithIndex(({ who }) => {
-                                    const alias = find.artist(who, wikiData.artistAliasData, {mode: 'quiet'});
-                                    if (alias) {
-                                        const original = find.artist(alias.aliasedArtistRef, wikiData.artistData, {mode: 'quiet'});
-                                        throw new Error(`Reference ${color.red(who)} is to an alias, should be ${color.green(original.name)}`);
-                                    }
-                                    return boundFind.artist(who);
-                                }),
-                                {message: `Reference errors in contributions ${color.green(property)} (${color.green('find.artist')})`});
-                            continue;
-                        }
-                        const findFn = boundFind[findFnKey];
-                        const value = thing[property];
-                        if (Array.isArray(value)) {
-                            thing[property] = filter(value, decorateErrorWithIndex(findFn),
-                                {message: `Reference errors in property ${color.green(property)} (${color.green('find.' + findFnKey)})`});
-                        } else {
-                            nest({message: `Reference error in property ${color.green(property)} (${color.green('find.' + findFnKey)})`}, ({ call }) => {
-                                try {
-                                    call(findFn, value);
-                                } catch (error) {
-                                    thing[property] = null;
-                                    throw error;
-                                }
-                            });
-                        }
-                    }
-                });
-            }
-        });
-    }
-
-    return aggregate;
+  for (const [key, value] of Object.entries(wikiData)) {
+    if (!Array.isArray(value)) continue;
+    wikiData[key] = value.slice();
+  }
+
+  const steps = getDataSteps();
+
+  for (const step of steps) {
+    if (!step.sort) continue;
+    step.sort(wikiData);
+  }
+
+  // Re-link data arrays, so that every object has the new, sorted versions.
+  // Note that the sorting step deliberately creates new arrays (mutating
+  // slices instead of the original arrays) - this is so that the object
+  // caching system understands that it's working with a new ordering.
+  // We still need to actually provide those updated arrays over again!
+  linkWikiDataArrays(wikiData);
 }
 
 // Utility function for loading all wiki data from the provided YAML data
@@ -1297,47 +1016,56 @@ export function filterReferenceErrors(wikiData) {
 // where reporting info about data loading isn't as relevant as during the
 // main wiki build process.
 export async function quickLoadAllFromYAML(dataPath, {
-    showAggregate: customShowAggregate = showAggregate,
-} = {}) {
-    const showAggregate = customShowAggregate;
+  bindFind,
+  getAllFindSpecs,
 
-    let wikiData;
-
-    {
-        const { aggregate, result } = await loadAndProcessDataDocuments({
-            dataPath,
-        });
+  showAggregate: customShowAggregate = showAggregate,
+}) {
+  const showAggregate = customShowAggregate;
 
-        wikiData = result;
+  let wikiData;
 
-        try {
-            aggregate.close();
-            logInfo`Loaded data without errors. (complete data)`;
-        } catch (error) {
-            showAggregate(error);
-            logWarn`Loaded data with errors. (partial data)`;
-        }
-    }
+  {
+    const {aggregate, result} = await loadAndProcessDataDocuments({dataPath});
 
-    linkWikiDataArrays(wikiData);
+    wikiData = result;
 
     try {
-        filterDuplicateDirectories(wikiData).close();
-        logInfo`No duplicate directories found. (complete data)`;
+      aggregate.close();
+      logInfo`Loaded data without errors. (complete data)`;
     } catch (error) {
-        showAggregate(error);
-        logWarn`Duplicate directories found. (partial data)`;
+      showAggregate(error);
+      logWarn`Loaded data with errors. (partial data)`;
     }
-
-    try {
-        filterReferenceErrors(wikiData).close();
-        logInfo`No reference errors found. (complete data)`;
-    } catch (error) {
-        showAggregate(error);
-        logWarn`Duplicate directories found. (partial data)`;
-    }
-
-    sortWikiDataArrays(wikiData);
-
-    return wikiData;
+  }
+
+  linkWikiDataArrays(wikiData);
+
+  try {
+    reportDuplicateDirectories(wikiData, {getAllFindSpecs});
+    logInfo`No duplicate directories found. (complete data)`;
+  } catch (error) {
+    showAggregate(error);
+    logWarn`Duplicate directories found. (partial data)`;
+  }
+
+  try {
+    filterReferenceErrors(wikiData, {bindFind}).close();
+    logInfo`No reference errors found. (complete data)`;
+  } catch (error) {
+    showAggregate(error);
+    logWarn`Reference errors found. (partial data)`;
+  }
+
+  try {
+    reportContentTextErrors(wikiData, {bindFind});
+    logInfo`No content text errors found.`;
+  } catch (error) {
+    showAggregate(error);
+    logWarn`Content text errors found.`;
+  }
+
+  sortWikiDataArrays(wikiData);
+
+  return wikiData;
 }