Merge branch 'preview' into listing-tweaks

16 files changed, 2327 insertions, 1296 deletions

diff --git a/src/data/things/album.js b/src/data/things/album.js
index c012c243..546fda3b 100644
--- a/src/data/things/album.js
+++ b/src/data/things/album.js
@@ -1,163 +1,143 @@
-import {empty} from '#sugar';
+import {input} from '#composite';
 import find from '#find';
+import {isDate} from '#validators';
+
+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';
 
 import Thing from './thing.js';
 
 export class Album extends Thing {
   static [Thing.referenceType] = 'album';
 
-  static [Thing.getPropertyDescriptors] = ({
-    ArtTag,
-    Artist,
-    Group,
-    Track,
-
-    validators: {
-      isDate,
-      isDimensions,
-      isTrackSectionList,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({ArtTag, Artist, Group, Track}) => ({
     // 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', 'coverArtistContribsByRef'],
-        transform: (coverArtDate, {
-          coverArtistContribsByRef,
-          date,
-        }) =>
-          (!empty(coverArtistContribsByRef)
-            ? coverArtDate ?? date ?? null
-            : 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),
-
-    trackSections: {
-      flags: {update: true, expose: true},
-
-      update: {
-        validate: isTrackSectionList,
-      },
-
-      expose: {
-        dependencies: ['color', 'trackData'],
-        transform(trackSections, {
-          color: albumColor,
-          trackData,
-        }) {
-          let startIndex = 0;
-          return trackSections?.map(section => ({
-            name: section.name ?? null,
-            color: section.color ?? albumColor ?? null,
-            dateOriginallyReleased: section.dateOriginallyReleased ?? null,
-            isDefaultTrackSection: section.isDefaultTrackSection ?? false,
-
-            startIndex: (
-              startIndex += section.tracksByRef.length,
-              startIndex - section.tracksByRef.length
-            ),
-
-            tracksByRef: section.tracksByRef ?? [],
-            tracks:
-              (trackData && section.tracksByRef
-                ?.map(ref => find.track(ref, trackData, {mode: 'quiet'}))
-                .filter(Boolean)) ??
-              [],
-          }));
-        },
-      },
-    },
-
-    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},
-    },
-
-    hasTrackNumbers: Thing.common.flag(true),
-    isListedOnHomepage: Thing.common.flag(true),
-    isListedInGalleries: Thing.common.flag(true),
-
-    commentary: Thing.common.commentary(),
-    additionalFiles: Thing.common.additionalFiles(),
+    name: name('Unnamed Album'),
+    color: color(),
+    directory: directory(),
+    urls: urls(),
+
+    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(),
+    ],
+
+    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: referenceList({
+      class: input.value(ArtTag),
+      find: input.value(find.artTag),
+      data: 'artTagData',
+    }),
 
     // Update only
 
-    artistData: Thing.common.wikiData(Artist),
-    artTagData: Thing.common.wikiData(ArtTag),
-    groupData: Thing.common.wikiData(Group),
-    trackData: Thing.common.wikiData(Track),
+    artistData: wikiData(Artist),
+    artTagData: wikiData(ArtTag),
+    groupData: wikiData(Group),
+    trackData: 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(),
-
-    hasCoverArt: Thing.common.contribsPresent('coverArtistContribsByRef'),
-    hasWallpaperArt: Thing.common.contribsPresent('wallpaperArtistContribsByRef'),
-    hasBannerArt: Thing.common.contribsPresent('bannerArtistContribsByRef'),
-
-    tracks: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['trackSections', 'trackData'],
-        compute: ({trackSections, trackData}) =>
-          trackSections && trackData
-            ? trackSections
-                .flatMap((section) => section.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
-    ),
+    commentatorArtists: commentatorArtists(),
+
+    hasCoverArt: contribsPresent({contribs: 'coverArtistContribs'}),
+    hasWallpaperArt: contribsPresent({contribs: 'wallpaperArtistContribs'}),
+    hasBannerArt: contribsPresent({contribs: 'bannerArtistContribs'}),
+
+    tracks: [
+      withTracks(),
+      exposeDependency({dependency: '#tracks'}),
+    ],
   });
 
   static [Thing.getSerializeDescriptors] = ({
@@ -201,10 +181,12 @@ export class Album extends Thing {
 }
 
 export class TrackSectionHelper extends Thing {
+  static [Thing.friendlyName] = `Track Section`;
+
   static [Thing.getPropertyDescriptors] = () => ({
-    name: Thing.common.name('Unnamed Track Group'),
-    color: Thing.common.color(),
-    dateOriginallyReleased: Thing.common.simpleDate(),
-    isDefaultTrackGroup: Thing.common.flag(false),
+    name: name('Unnamed Track Section'),
+    color: color(),
+    dateOriginallyReleased: simpleDate(),
+    isDefaultTrackGroup: flag(false),
   })
 }
diff --git a/src/data/things/art-tag.js b/src/data/things/art-tag.js
index c103c4d5..6503beec 100644
--- a/src/data/things/art-tag.js
+++ b/src/data/things/art-tag.js
@@ -1,35 +1,47 @@
+import {input} from '#composite';
 import {sortAlbumsTracksChronologically} from '#wiki-data';
+import {isName} from '#validators';
+
+import {exposeUpdateValueOrContinue} from '#composite/control-flow';
+
+import {
+  color,
+  directory,
+  flag,
+  name,
+  wikiData,
+} from '#composite/wiki-properties';
 
 import Thing from './thing.js';
 
 export class ArtTag extends Thing {
   static [Thing.referenceType] = 'tag';
+  static [Thing.friendlyName] = `Art Tag`;
 
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-    Track,
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Album, Track}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Art Tag'),
-    directory: Thing.common.directory(),
-    color: Thing.common.color(),
-    isContentWarning: Thing.common.flag(false),
+    name: name('Unnamed Art Tag'),
+    directory: directory(),
+    color: color(),
+    isContentWarning: flag(false),
 
-    nameShort: {
-      flags: {update: true, expose: true},
+    nameShort: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isName),
+      }),
 
-      expose: {
+      {
         dependencies: ['name'],
-        transform: (value, {name}) =>
-          value ?? name.replace(/ \(.*?\)$/, ''),
+        compute: ({name}) =>
+          name.replace(/ \([^)]*?\)$/, ''),
       },
-    },
+    ],
 
     // Update only
 
-    albumData: Thing.common.wikiData(Album),
-    trackData: Thing.common.wikiData(Track),
+    albumData: wikiData(Album),
+    trackData: wikiData(Track),
 
     // Expose only
 
@@ -37,8 +49,8 @@ export class ArtTag extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['albumData', 'trackData'],
-        compute: ({albumData, trackData, [ArtTag.instance]: artTag}) =>
+        dependencies: ['this', 'albumData', 'trackData'],
+        compute: ({this: artTag, albumData, trackData}) =>
           sortAlbumsTracksChronologically(
             [...albumData, ...trackData]
               .filter(({artTags}) => artTags.includes(artTag)),
diff --git a/src/data/things/artist.js b/src/data/things/artist.js
index 6d4f4a0d..1b313db6 100644
--- a/src/data/things/artist.js
+++ b/src/data/things/artist.js
@@ -1,29 +1,33 @@
+import {input} from '#composite';
 import find from '#find';
+import {isName, validateArrayItems} from '#validators';
+
+import {
+  directory,
+  fileExtension,
+  flag,
+  name,
+  simpleString,
+  singleReference,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
 
 import Thing from './thing.js';
 
 export class Artist extends Thing {
   static [Thing.referenceType] = 'artist';
 
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-    Flash,
-    Track,
-
-    validators: {
-      isName,
-      validateArrayItems,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Album, Flash, Track}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Artist'),
-    directory: Thing.common.directory(),
-    urls: Thing.common.urls(),
-    contextNotes: Thing.common.simpleString(),
+    name: name('Unnamed Artist'),
+    directory: directory(),
+    urls: urls(),
+    contextNotes: simpleString(),
 
-    hasAvatar: Thing.common.flag(false),
-    avatarFileExtension: Thing.common.fileExtension('jpg'),
+    hasAvatar: flag(false),
+    avatarFileExtension: fileExtension('jpg'),
 
     aliasNames: {
       flags: {update: true, expose: true},
@@ -31,30 +35,23 @@ export class Artist extends Thing {
       expose: {transform: (names) => names ?? []},
     },
 
-    isAlias: Thing.common.flag(),
-    aliasedArtistRef: Thing.common.singleReference(Artist),
+    isAlias: flag(),
+
+    aliasedArtist: singleReference({
+      class: input.value(Artist),
+      find: input.value(find.artist),
+      data: 'artistData',
+    }),
 
     // Update only
 
-    albumData: Thing.common.wikiData(Album),
-    artistData: Thing.common.wikiData(Artist),
-    flashData: Thing.common.wikiData(Flash),
-    trackData: Thing.common.wikiData(Track),
+    albumData: wikiData(Album),
+    artistData: wikiData(Artist),
+    flashData: wikiData(Flash),
+    trackData: 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:
@@ -66,14 +63,14 @@ export class Artist extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['trackData'],
+        dependencies: ['this', 'trackData'],
 
-        compute: ({trackData, [Artist.instance]: artist}) =>
+        compute: ({this: artist, trackData}) =>
           trackData?.filter((track) =>
             [
-              ...track.artistContribs,
-              ...track.contributorContribs,
-              ...track.coverArtistContribs,
+              ...track.artistContribs ?? [],
+              ...track.contributorContribs ?? [],
+              ...track.coverArtistContribs ?? [],
             ].some(({who}) => who === artist)) ?? [],
       },
     },
@@ -82,9 +79,9 @@ export class Artist extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['trackData'],
+        dependencies: ['this', 'trackData'],
 
-        compute: ({trackData, [Artist.instance]: artist}) =>
+        compute: ({this: artist, trackData}) =>
           trackData?.filter(({commentatorArtists}) =>
             commentatorArtists.includes(artist)) ?? [],
       },
@@ -120,18 +117,16 @@ export class Artist extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['albumData'],
+        dependencies: ['this', 'albumData'],
 
-        compute: ({albumData, [Artist.instance]: artist}) =>
+        compute: ({this: artist, albumData}) =>
           albumData?.filter(({commentatorArtists}) =>
             commentatorArtists.includes(artist)) ?? [],
       },
     },
 
-    flashesAsContributor: Artist.filterByContrib(
-      'flashData',
-      'contributorContribs'
-    ),
+    flashesAsContributor:
+      Artist.filterByContrib('flashData', 'contributorContribs'),
   });
 
   static [Thing.getSerializeDescriptors] = ({
@@ -165,15 +160,15 @@ export class Artist extends Thing {
     flags: {expose: true},
 
     expose: {
-      dependencies: [thingDataProperty],
+      dependencies: ['this', thingDataProperty],
 
       compute: ({
+        this: artist,
         [thingDataProperty]: thingData,
-        [Artist.instance]: artist
       }) =>
         thingData?.filter(thing =>
           thing[contribsProperty]
-            .some(contrib => contrib.who === artist)) ?? [],
+            ?.some(contrib => contrib.who === artist)) ?? [],
     },
   });
 }
diff --git a/src/data/things/cacheable-object.js b/src/data/things/cacheable-object.js
index ea705a61..9fda865e 100644
--- a/src/data/things/cacheable-object.js
+++ b/src/data/things/cacheable-object.js
@@ -76,28 +76,24 @@
 
 import {inspect as nodeInspect} from 'node:util';
 
-import {color, ENABLE_COLOR} from '#cli';
+import {colors, ENABLE_COLOR} from '#cli';
 
 function inspect(value) {
   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.
-    */
+  // 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();
@@ -143,7 +139,7 @@ export default class CacheableObject {
 
       const definition = {
         configurable: false,
-        enumerable: true,
+        enumerable: flags.expose,
       };
 
       if (flags.update) {
@@ -183,13 +179,8 @@ export default class CacheableObject {
           } 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
-          ].join(' ');
-          throw error;
+        } catch (caughtError) {
+          throw new CacheableObjectPropertyValueError(property, this[property], newValue, caughtError);
         }
       }
 
@@ -250,20 +241,27 @@ export default class CacheableObject {
 
     let getAllDependencies;
 
-    const dependencyKeys = expose.dependencies;
-    if (dependencyKeys?.length > 0) {
-      const reflectionEntry = [this.constructor.instance, this];
-      const dependencyGetters = dependencyKeys
-        .map(key => () => [key, this.#propertyUpdateValues[key]]);
+    if (expose.dependencies?.length > 0) {
+      const dependencyKeys = expose.dependencies.slice();
+      const shouldReflect = dependencyKeys.includes('this');
+
+      getAllDependencies = () => {
+        const dependencies = Object.create(null);
+
+        for (const key of dependencyKeys) {
+          dependencies[key] = this.#propertyUpdateValues[key];
+        }
+
+        if (shouldReflect) {
+          dependencies.this = this;
+        }
 
-      getAllDependencies = () =>
-        Object.fromEntries(dependencyGetters
-          .map(f => f())
-          .concat([reflectionEntry]));
+        return dependencies;
+      };
     } else {
-      const allDependencies = {[this.constructor.instance]: this};
-      Object.freeze(allDependencies);
-      getAllDependencies = () => allDependencies;
+      const dependencies = Object.create(null);
+      Object.freeze(dependencies);
+      getAllDependencies = () => dependencies;
     }
 
     if (flags.update) {
@@ -347,4 +345,22 @@ export default class CacheableObject {
       console.log(` - ${line}`);
     }
   }
+
+  static getUpdateValue(object, key) {
+    if (!Object.hasOwn(object, key)) {
+      return undefined;
+    }
+
+    return object.#propertyUpdateValues[key] ?? null;
+  }
+}
+
+export class CacheableObjectPropertyValueError extends Error {
+  constructor(property, oldValue, newValue, error) {
+    super(
+      `Error setting ${colors.green(property)} (${inspect(oldValue)} -> ${inspect(newValue)})`,
+      {cause: error});
+
+    this.property = property;
+  }
 }
diff --git a/src/data/things/composite.js b/src/data/things/composite.js
new file mode 100644
index 00000000..51525bc1
--- /dev/null
+++ b/src/data/things/composite.js
@@ -0,0 +1,1301 @@
+import {inspect} from 'node:util';
+
+import {colors} from '#cli';
+import {TupleMap} from '#wiki-data';
+import {a} from '#validators';
+
+import {
+  decorateErrorWithIndex,
+  empty,
+  filterProperties,
+  openAggregate,
+  stitchArrays,
+  typeAppearance,
+  unique,
+  withAggregate,
+} from '#sugar';
+
+const globalCompositeCache = {};
+
+const _valueIntoToken = shape =>
+  (value = null) =>
+    (value === null
+      ? Symbol.for(`hsmusic.composite.${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;
+
+  // 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 anyStepsTransform =
+    stepsTransform.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 && !anyStepsCompute && !anyStepsTransform) {
+    aggregate.push(new TypeError(`Expected at least one step to compute or transform`));
+  }
+
+  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 ? ` ${annotation}` : ''));
+        error.cause = thrownError;
+        throw error;
+      }
+    };
+
+    if (compositionNests) {
+      if (compositionUpdates) {
+        expose.transform = (value, continuation, dependencies) =>
+          _wrapper(value, continuation, dependencies);
+      }
+
+      if (anyStepsCompute && !anyStepsUseUpdateValue && !anyInputsUseUpdateValue) {
+        expose.compute = (continuation, dependencies) =>
+          _wrapper(noTransformSymbol, continuation, dependencies);
+      }
+
+      if (base.cacheComposition) {
+        expose.cache = base.cacheComposition;
+      }
+    } else if (compositionUpdates) {
+      expose.transform = (value, dependencies) =>
+        _wrapper(value, null, dependencies);
+    } else {
+      expose.compute = (dependencies) =>
+        _wrapper(noTransformSymbol, null, dependencies);
+    }
+  }
+
+  return constructedDescriptor;
+}
+
+export function displayCompositeCacheAnalysis() {
+  const showTimes = (cache, key) => {
+    const times = cache.times[key].slice().sort();
+
+    const all = times;
+    const worst10pc = times.slice(-times.length / 10);
+    const best10pc = times.slice(0, times.length / 10);
+    const middle50pc = times.slice(times.length / 4, -times.length / 4);
+    const middle80pc = times.slice(times.length / 10, -times.length / 10);
+
+    const fmt = val => `${(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/things/flash.js b/src/data/things/flash.js
index 6eb5234f..e2afcef4 100644
--- a/src/data/things/flash.js
+++ b/src/data/things/flash.js
@@ -1,25 +1,42 @@
+import {input} from '#composite';
 import find from '#find';
 
+import {
+  isColor,
+  isDirectory,
+  isNumber,
+  isString,
+  oneOf,
+} from '#validators';
+
+import {exposeDependency, exposeUpdateValueOrContinue}
+  from '#composite/control-flow';
+import {withPropertyFromObject} from '#composite/data';
+
+import {
+  color,
+  contributionList,
+  directory,
+  fileExtension,
+  name,
+  referenceList,
+  simpleDate,
+  simpleString,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
+import {withFlashAct} from '#composite/things/flash';
+
 import Thing from './thing.js';
 
 export class Flash extends Thing {
   static [Thing.referenceType] = 'flash';
 
-  static [Thing.getPropertyDescriptors] = ({
-    Artist,
-    Track,
-    FlashAct,
-
-    validators: {
-      isDirectory,
-      isNumber,
-      isString,
-      oneOf,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Artist, Track, FlashAct}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Flash'),
+    name: name('Unnamed Flash'),
 
     directory: {
       flags: {update: true, expose: true},
@@ -47,51 +64,51 @@ export class Flash extends Thing {
       },
     },
 
-    date: Thing.common.simpleDate(),
-
-    coverArtFileExtension: Thing.common.fileExtension('jpg'),
+    color: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isColor),
+      }),
 
-    contributorContribsByRef: Thing.common.contribsByRef(),
+      withFlashAct(),
 
-    featuredTracksByRef: Thing.common.referenceList(Track),
+      withPropertyFromObject({
+        object: '#flashAct',
+        property: input.value('color'),
+      }),
 
-    urls: Thing.common.urls(),
+      exposeDependency({dependency: '#flashAct.color'}),
+    ],
 
-    // Update only
+    date: simpleDate(),
 
-    artistData: Thing.common.wikiData(Artist),
-    trackData: Thing.common.wikiData(Track),
-    flashActData: Thing.common.wikiData(FlashAct),
+    coverArtFileExtension: fileExtension('jpg'),
 
-    // Expose only
+    contributorContribs: contributionList(),
 
-    contributorContribs: Thing.common.dynamicContribs('contributorContribsByRef'),
+    featuredTracks: referenceList({
+      class: input.value(Track),
+      find: input.value(find.track),
+      data: 'trackData',
+    }),
 
-    featuredTracks: Thing.common.dynamicThingsFromReferenceList(
-      'featuredTracksByRef',
-      'trackData',
-      find.track
-    ),
+    urls: urls(),
 
-    act: {
-      flags: {expose: true},
+    // Update only
 
-      expose: {
-        dependencies: ['flashActData'],
+    artistData: wikiData(Artist),
+    trackData: wikiData(Track),
+    flashActData: wikiData(FlashAct),
 
-        compute: ({flashActData, [Flash.instance]: flash}) =>
-          flashActData.find((act) => act.flashes.includes(flash)) ?? null,
-      },
-    },
+    // Expose only
 
-    color: {
+    act: {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['flashActData'],
+        dependencies: ['this', 'flashActData'],
 
-        compute: ({flashActData, [Flash.instance]: flash}) =>
-          flashActData.find((act) => act.flashes.includes(flash))?.color ?? null,
+        compute: ({this: flash, flashActData}) =>
+          flashActData.find((act) => act.flashes.includes(flash)) ?? null,
       },
     },
   });
@@ -111,17 +128,18 @@ export class Flash extends Thing {
 }
 
 export class FlashAct extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    validators: {
-      isColor,
-    },
-  }) => ({
+  static [Thing.referenceType] = 'flash-act';
+  static [Thing.friendlyName] = `Flash Act`;
+
+  static [Thing.getPropertyDescriptors] = () => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Flash Act'),
-    color: Thing.common.color(),
-    anchor: Thing.common.simpleString(),
-    jump: Thing.common.simpleString(),
+    name: name('Unnamed Flash Act'),
+    directory: directory(),
+    color: color(),
+    listTerminology: simpleString(),
+
+    jump: simpleString(),
 
     jumpColor: {
       flags: {update: true, expose: true},
@@ -133,18 +151,14 @@ export class FlashAct extends Thing {
       }
     },
 
-    flashesByRef: Thing.common.referenceList(Flash),
+    flashes: referenceList({
+      class: input.value(Flash),
+      find: input.value(find.flash),
+      data: 'flashData',
+    }),
 
     // Update only
 
-    flashData: Thing.common.wikiData(Flash),
-
-    // Expose only
-
-    flashes: Thing.common.dynamicThingsFromReferenceList(
-      'flashesByRef',
-      'flashData',
-      find.flash
-    ),
+    flashData: wikiData(Flash),
   })
 }
diff --git a/src/data/things/group.js b/src/data/things/group.js
index ba339b3e..8764a9db 100644
--- a/src/data/things/group.js
+++ b/src/data/things/group.js
@@ -1,33 +1,44 @@
+import {input} from '#composite';
 import find from '#find';
 
+import {
+  color,
+  directory,
+  name,
+  referenceList,
+  simpleString,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
 import Thing from './thing.js';
 
 export class Group extends Thing {
   static [Thing.referenceType] = 'group';
 
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Album}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Group'),
-    directory: Thing.common.directory(),
+    name: name('Unnamed Group'),
+    directory: directory(),
 
-    description: Thing.common.simpleString(),
+    description: simpleString(),
 
-    urls: Thing.common.urls(),
+    urls: urls(),
 
-    featuredAlbumsByRef: Thing.common.referenceList(Album),
+    featuredAlbums: referenceList({
+      class: input.value(Album),
+      find: input.value(find.album),
+      data: 'albumData',
+    }),
 
     // Update only
 
-    albumData: Thing.common.wikiData(Album),
-    groupCategoryData: Thing.common.wikiData(GroupCategory),
+    albumData: wikiData(Album),
+    groupCategoryData: wikiData(GroupCategory),
 
     // Expose only
 
-    featuredAlbums: Thing.common.dynamicThingsFromReferenceList('featuredAlbumsByRef', 'albumData', find.album),
-
     descriptionShort: {
       flags: {expose: true},
 
@@ -41,8 +52,8 @@ export class Group extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['albumData'],
-        compute: ({albumData, [Group.instance]: group}) =>
+        dependencies: ['this', 'albumData'],
+        compute: ({this: group, albumData}) =>
           albumData?.filter((album) => album.groups.includes(group)) ?? [],
       },
     },
@@ -51,9 +62,8 @@ export class Group extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['groupCategoryData'],
-
-        compute: ({groupCategoryData, [Group.instance]: group}) =>
+        dependencies: ['this', 'groupCategoryData'],
+        compute: ({this: group, groupCategoryData}) =>
           groupCategoryData.find((category) => category.groups.includes(group))
             ?.color,
       },
@@ -63,8 +73,8 @@ export class Group extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['groupCategoryData'],
-        compute: ({groupCategoryData, [Group.instance]: group}) =>
+        dependencies: ['this', 'groupCategoryData'],
+        compute: ({this: group, groupCategoryData}) =>
           groupCategoryData.find((category) => category.groups.includes(group)) ??
           null,
       },
@@ -73,26 +83,22 @@ export class Group extends Thing {
 }
 
 export class GroupCategory extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    Group,
-  }) => ({
+  static [Thing.friendlyName] = `Group Category`;
+
+  static [Thing.getPropertyDescriptors] = ({Group}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Group Category'),
-    color: Thing.common.color(),
+    name: name('Unnamed Group Category'),
+    color: color(),
 
-    groupsByRef: Thing.common.referenceList(Group),
+    groups: referenceList({
+      class: input.value(Group),
+      find: input.value(find.group),
+      data: 'groupData',
+    }),
 
     // Update only
 
-    groupData: Thing.common.wikiData(Group),
-
-    // Expose only
-
-    groups: Thing.common.dynamicThingsFromReferenceList(
-      'groupsByRef',
-      'groupData',
-      find.group
-    ),
+    groupData: wikiData(Group),
   });
 }
diff --git a/src/data/things/homepage-layout.js b/src/data/things/homepage-layout.js
index ec9e9556..bfa971ca 100644
--- a/src/data/things/homepage-layout.js
+++ b/src/data/things/homepage-layout.js
@@ -1,20 +1,37 @@
+import {input} from '#composite';
 import find from '#find';
 
+import {
+  is,
+  isCountingNumber,
+  isString,
+  isStringNonEmpty,
+  oneOf,
+  validateArrayItems,
+  validateInstanceOf,
+  validateReference,
+} from '#validators';
+
+import {exposeDependency} from '#composite/control-flow';
+import {withResolvedReference} from '#composite/wiki-data';
+
+import {
+  color,
+  name,
+  referenceList,
+  simpleString,
+  wikiData,
+} from '#composite/wiki-properties';
+
 import Thing from './thing.js';
 
 export class HomepageLayout extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    HomepageLayoutRow,
+  static [Thing.friendlyName] = `Homepage Layout`;
 
-    validators: {
-      isStringNonEmpty,
-      validateArrayItems,
-      validateInstanceOf,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({HomepageLayoutRow}) => ({
     // Update & expose
 
-    sidebarContent: Thing.common.simpleString(),
+    sidebarContent: simpleString(),
 
     navbarLinks: {
       flags: {update: true, expose: true},
@@ -32,13 +49,12 @@ export class HomepageLayout extends Thing {
 }
 
 export class HomepageLayoutRow extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-    Group,
-  }) => ({
+  static [Thing.friendlyName] = `Homepage Row`;
+
+  static [Thing.getPropertyDescriptors] = ({Album, Group}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Homepage Row'),
+    name: name('Unnamed Homepage Row'),
 
     type: {
       flags: {update: true, expose: true},
@@ -50,30 +66,22 @@ export class HomepageLayoutRow extends Thing {
       },
     },
 
-    color: Thing.common.color(),
+    color: 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),
+    albumData: wikiData(Album),
+    groupData: wikiData(Group),
   });
 }
 
 export class HomepageLayoutAlbumsRow extends HomepageLayoutRow {
-  static [Thing.getPropertyDescriptors] = (opts, {
-    Album,
-    Group,
-
-    validators: {
-      is,
-      isCountingNumber,
-      isString,
-      validateArrayItems,
-    },
-  } = opts) => ({
+  static [Thing.friendlyName] = `Homepage Albums Row`;
+
+  static [Thing.getPropertyDescriptors] = (opts, {Album, Group} = opts) => ({
     ...HomepageLayoutRow[Thing.getPropertyDescriptors](opts),
 
     // Update & expose
@@ -104,8 +112,39 @@ export class HomepageLayoutAlbumsRow extends HomepageLayoutRow {
       },
     },
 
-    sourceGroupByRef: Thing.common.singleReference(Group),
-    sourceAlbumsByRef: Thing.common.referenceList(Album),
+    sourceGroup: [
+      {
+        flags: {expose: true, update: true, compose: true},
+
+        update: {
+          validate:
+            oneOf(
+              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},
@@ -116,19 +155,5 @@ export class HomepageLayoutAlbumsRow extends HomepageLayoutRow {
       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
-    ),
   });
 }
diff --git a/src/data/things/index.js b/src/data/things/index.js
index 591cdc3b..4ea1f007 100644
--- a/src/data/things/index.js
+++ b/src/data/things/index.js
@@ -2,9 +2,9 @@ import * as path from 'node:path';
 import {fileURLToPath} from 'node:url';
 
 import {logError} from '#cli';
+import {compositeFrom} from '#composite';
 import * as serialize from '#serialize';
 import {openAggregate, showAggregate} from '#sugar';
-import * as validators from '#validators';
 
 import Thing from './thing.js';
 
@@ -21,7 +21,11 @@ import * as trackClasses from './track.js';
 import * as wikiInfoClasses from './wiki-info.js';
 
 export {default as Thing} from './thing.js';
-export {default as CacheableObject} from './cacheable-object.js';
+
+export {
+  default as CacheableObject,
+  CacheableObjectPropertyValueError,
+} from './cacheable-object.js';
 
 const allClassLists = {
   'album.js': albumClasses,
@@ -82,6 +86,8 @@ function errorDuplicateClassNames() {
 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;
     }
   }
@@ -119,7 +125,7 @@ function descriptorAggregateHelper({
 }
 
 function evaluatePropertyDescriptors() {
-  const opts = {...allClasses, validators};
+  const opts = {...allClasses};
 
   return descriptorAggregateHelper({
     message: `Errors evaluating Thing class property descriptors`,
@@ -129,8 +135,21 @@ function evaluatePropertyDescriptors() {
         throw new Error(`Missing [Thing.getPropertyDescriptors] function`);
       }
 
-      constructor.propertyDescriptors =
-        constructor[Thing.getPropertyDescriptors](opts);
+      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) {
diff --git a/src/data/things/language.js b/src/data/things/language.js
index 7755c505..646eb6d1 100644
--- a/src/data/things/language.js
+++ b/src/data/things/language.js
@@ -1,11 +1,17 @@
+import {Tag} from '#html';
+import {isLanguageCode} from '#validators';
+
+import {
+  externalFunction,
+  flag,
+  simpleString,
+} from '#composite/wiki-properties';
+
+import CacheableObject from './cacheable-object.js';
 import Thing from './thing.js';
 
 export class Language extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    validators: {
-      isLanguageCode,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = () => ({
     // Update & expose
 
     // General language code. This is used to identify the language distinctly
@@ -18,7 +24,7 @@ export class Language extends Thing {
 
     // Human-readable name. This should be the language's own native name, not
     // localized to any other language.
-    name: Thing.common.simpleString(),
+    name: simpleString(),
 
     // Language code specific to JavaScript's Internationalization (Intl) API.
     // Usually this will be the same as the language's general code, but it
@@ -40,7 +46,7 @@ export class Language extends Thing {
     // 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),
+    hidden: flag(false),
 
     // Mapping of translation keys to values (strings). Generally, don't
     // access this object directly - use methods instead.
@@ -68,7 +74,7 @@ export class Language extends Thing {
 
     // Update only
 
-    escapeHTML: Thing.common.externalFunction(),
+    escapeHTML: externalFunction(),
 
     // Expose only
 
@@ -95,6 +101,7 @@ export class Language extends Thing {
       },
     },
 
+    // TODO: This currently isn't used. Is it still needed?
     strings_htmlEscaped: {
       flags: {expose: true},
       expose: {
@@ -124,8 +131,8 @@ export class Language extends Thing {
     };
   }
 
-  $(key, args = {}) {
-    return this.formatString(key, args);
+  $(...args) {
+    return this.formatString(...args);
   }
 
   assertIntlAvailable(property) {
@@ -139,20 +146,22 @@ export class Language extends Thing {
     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`);
-    }
+  formatString(...args) {
+    const hasOptions =
+      typeof args.at(-1) === 'object' &&
+      args.at(-1) !== null;
 
-    return this.formatStringHelper(this.strings_htmlEscaped, key, args);
-  }
+    const key =
+      (hasOptions ? args.slice(0, -1) : args)
+        .filter(Boolean)
+        .join('.');
 
-  formatStringNoHTMLEscape(key, args = {}) {
-    return this.formatStringHelper(this.strings, key, args);
-  }
+    const options =
+      (hasOptions
+        ? args.at(-1)
+        : null);
 
-  formatStringHelper(strings, key, args = {}) {
-    if (!strings) {
+    if (!this.strings) {
       throw new Error(`Strings unavailable`);
     }
 
@@ -160,30 +169,94 @@ export class Language extends Thing {
       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
-    );
+    const template = this.strings[key];
+
+    let output;
+
+    if (hasOptions) {
+      // Convert the keys on the options 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. Also strip HTML from arguments
+      // that are literal strings - real HTML content should always be proper
+      // HTML objects (see html.js).
+      const processedOptions =
+        Object.entries(options).map(([k, v]) => [
+          k.replace(/[A-Z]/g, '_$&').toUpperCase(),
+          this.#sanitizeStringArg(v),
+        ]);
+
+      // Replacement time! Woot. Reduce comes in handy here!
+      output =
+        processedOptions.reduce(
+          (x, [k, v]) => x.replaceAll(`{${k}}`, v),
+          template);
+    } else {
+      // Without any options provided, just use the template as-is. This will
+      // still error if the template expected arguments, and otherwise will be
+      // the right value.
+      output = template;
+    }
 
     // Post-processing: if any expected arguments *weren't* replaced, that
     // is almost definitely an error.
-    if (output.match(/\{[A-Z_]+\}/)) {
+    if (output.match(/\{[A-Z][A-Z0-9_]*\}/)) {
       throw new Error(`Args in ${key} were missing - output: ${output}`);
     }
 
-    return output;
+    // Last caveat: Wrap the output in an HTML tag so that it doesn't get
+    // treated as unsanitized HTML if *it* gets passed as an argument to
+    // *another* formatString call.
+    return this.#wrapSanitized(output);
+  }
+
+  // Escapes HTML special characters so they're displayed as-are instead of
+  // treated by the browser as a tag. This does *not* have an effect on actual
+  // html.Tag objects, which are treated as sanitized by default (so that they
+  // can be nested inside strings at all).
+  #sanitizeStringArg(arg) {
+    const escapeHTML = CacheableObject.getUpdateValue(this, 'escapeHTML');
+
+    if (!escapeHTML) {
+      throw new Error(`escapeHTML unavailable`);
+    }
+
+    if (typeof arg !== 'string') {
+      return arg.toString();
+    }
+
+    return escapeHTML(arg);
+  }
+
+  // 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(output) {
+    return new Tag(null, null, output);
+  }
+
+  // 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 as an argument to formatString,
+  // but if you used a custom validation function ({validate: v => v.isHTML}
+  // instead of {type: 'string'} / {type: 'html'}) and are embedding the
+  // contents of a slot directly, it should be manually sanitized with this
+  // function first.
+  sanitize(arg) {
+    const escapeHTML = CacheableObject.getUpdateValue(this, 'escapeHTML');
+
+    if (!escapeHTML) {
+      throw new Error(`escapeHTML unavailable`);
+    }
+
+    return (
+      (typeof arg === 'string'
+        ? new Tag(null, null, escapeHTML(arg))
+        : arg));
   }
 
   formatDate(date) {
@@ -252,19 +325,32 @@ export class Language extends Thing {
   // Conjunction list: A, B, and C
   formatConjunctionList(array) {
     this.assertIntlAvailable('intl_listConjunction');
-    return this.intl_listConjunction.format(array.map(arr => arr.toString()));
+    return this.#wrapSanitized(
+      this.intl_listConjunction.format(
+        array.map(item => this.#sanitizeStringArg(item))));
   }
 
   // Disjunction lists: A, B, or C
   formatDisjunctionList(array) {
     this.assertIntlAvailable('intl_listDisjunction');
-    return this.intl_listDisjunction.format(array.map(arr => arr.toString()));
+    return this.#wrapSanitized(
+      this.intl_listDisjunction.format(
+        array.map(item => this.#sanitizeStringArg(item))));
   }
 
   // Unit lists: A, B, C
   formatUnitList(array) {
     this.assertIntlAvailable('intl_listUnit');
-    return this.intl_listUnit.format(array.map(arr => arr.toString()));
+    return this.#wrapSanitized(
+      this.intl_listUnit.format(
+        array.map(item => this.#sanitizeStringArg(item))));
+  }
+
+  // Lists without separator: A B C
+  formatListWithoutSeparator(array) {
+    return this.#wrapSanitized(
+      array.map(item => this.#sanitizeStringArg(item))
+        .join(' '));
   }
 
   // File sizes: 42.5 kB, 127.2 MB, 4.13 GB, 998.82 TB
diff --git a/src/data/things/news-entry.js b/src/data/things/news-entry.js
index 43911410..36da0299 100644
--- a/src/data/things/news-entry.js
+++ b/src/data/things/news-entry.js
@@ -1,16 +1,24 @@
+import {
+  directory,
+  name,
+  simpleDate,
+  simpleString,
+} from '#composite/wiki-properties';
+
 import Thing from './thing.js';
 
 export class NewsEntry extends Thing {
   static [Thing.referenceType] = 'news-entry';
+  static [Thing.friendlyName] = `News Entry`;
 
   static [Thing.getPropertyDescriptors] = () => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed News Entry'),
-    directory: Thing.common.directory(),
-    date: Thing.common.simpleDate(),
+    name: name('Unnamed News Entry'),
+    directory: directory(),
+    date: simpleDate(),
 
-    content: Thing.common.simpleString(),
+    content: simpleString(),
 
     // Expose only
 
diff --git a/src/data/things/static-page.js b/src/data/things/static-page.js
index 3d8d474c..ab9c5f98 100644
--- a/src/data/things/static-page.js
+++ b/src/data/things/static-page.js
@@ -1,16 +1,21 @@
+import {isName} from '#validators';
+
+import {
+  directory,
+  name,
+  simpleString,
+} from '#composite/wiki-properties';
+
 import Thing from './thing.js';
 
 export class StaticPage extends Thing {
   static [Thing.referenceType] = 'static';
+  static [Thing.friendlyName] = `Static Page`;
 
-  static [Thing.getPropertyDescriptors] = ({
-    validators: {
-      isName,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = () => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Static Page'),
+    name: name('Unnamed Static Page'),
 
     nameShort: {
       flags: {update: true, expose: true},
@@ -22,8 +27,8 @@ export class StaticPage extends Thing {
       },
     },
 
-    directory: Thing.common.directory(),
-    content: Thing.common.simpleString(),
-    stylesheet: Thing.common.simpleString(),
+    directory: directory(),
+    content: simpleString(),
+    stylesheet: simpleString(),
   });
 }
diff --git a/src/data/things/thing.js b/src/data/things/thing.js
index c2876f56..def7e914 100644
--- a/src/data/things/thing.js
+++ b/src/data/things/thing.js
@@ -1,399 +1,19 @@
-// Thing: base class for wiki data types, providing wiki-specific utility
-// functions on top of essential CacheableObject behavior.
+// 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 {color} from '#cli';
-import find from '#find';
-import {empty} from '#sugar';
-import {getKebabCase} from '#wiki-data';
-
-import {
-  isAdditionalFileList,
-  isBoolean,
-  isCommentary,
-  isColor,
-  isContributionList,
-  isDate,
-  isDirectory,
-  isFileExtension,
-  isName,
-  isString,
-  isURL,
-  validateArrayItems,
-  validateInstanceOf,
-  validateReference,
-  validateReferenceList,
-} from '#validators';
+import {colors} from '#cli';
 
 import CacheableObject from './cacheable-object.js';
 
 export default class Thing extends CacheableObject {
-  static referenceType = Symbol('Thing.referenceType');
+  static referenceType = Symbol.for('Thing.referenceType');
+  static friendlyName = Symbol.for(`Thing.friendlyName`);
 
   static getPropertyDescriptors = Symbol('Thing.getPropertyDescriptors');
   static getSerializeDescriptors = Symbol('Thing.getSerializeDescriptors');
 
-  // 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!
-  static 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)},
-      expose: {transform: (value) => value ?? []},
-    }),
-
-    // 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},
-      expose: {
-        transform: (additionalFiles) =>
-          additionalFiles ?? [],
-      },
-    }),
-
-    // 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.
-    dynamicInheritContribs: (
-      // If this property is explicitly false, the contribution list returned
-      // will always be empty.
-      nullerProperty,
-
-      // Property holding contributions on the current object.
-      contribsByRefProperty,
-
-      // Property holding corresponding "default" contributions on the parent
-      // object, which will fallen back to if the object doesn't have its own
-      // contribs.
-      parentContribsByRefProperty,
-
-      // Data array to search in and "find" function to locate parent object
-      // (which will be passed the child object and the wiki data array).
-      thingDataProperty,
-      findFn
-    ) => ({
-      flags: {expose: true},
-      expose: {
-        dependencies: [
-          contribsByRefProperty,
-          thingDataProperty,
-          nullerProperty,
-          'artistData',
-        ].filter(Boolean),
-
-        compute({
-          [Thing.instance]: thing,
-          [nullerProperty]: nuller,
-          [contribsByRefProperty]: contribsByRef,
-          [thingDataProperty]: thingData,
-          artistData,
-        }) {
-          if (!artistData) return [];
-          if (nuller === false) 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);
-        },
-      },
-    }),
-
-    // Nice 'n simple shorthand for an exposed-only flag which is true when any
-    // contributions are present in the specified property.
-    contribsPresent: (contribsByRefProperty) => ({
-      flags: {expose: true},
-      expose: {
-        dependencies: [contribsByRefProperty],
-        compute({
-          [contribsByRefProperty]: contribsByRef,
-        }) {
-          return !empty(contribsByRef);
-        },
-      }
-    }),
-
-    // 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: (thingDataProperty, referencerRefListProperty) => ({
-      flags: {expose: true},
-
-      expose: {
-        dependencies: [thingDataProperty],
-
-        compute: ({[thingDataProperty]: thingData, [Thing.instance]: thing}) =>
-          thingData?.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: (thingDataProperty, referencerRefListProperty) => ({
-      flags: {expose: true},
-
-      expose: {
-        dependencies: [thingDataProperty],
-
-        compute: ({[thingDataProperty]: thingData, [Thing.instance]: thing}) =>
-          thingData?.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'})
-                  )
-                )
-              )
-            : [],
-      },
-    }),
-  };
-
   // 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
@@ -402,8 +22,8 @@ export default class Thing extends CacheableObject {
     const cname = this.constructor.name;
 
     return (
-      (this.name ? `${cname} ${color.green(`"${this.name}"`)}` : `${cname}`) +
-      (this.directory ? ` (${color.blue(Thing.getReference(this))})` : '')
+      (this.name ? `${cname} ${colors.green(`"${this.name}"`)}` : `${cname}`) +
+      (this.directory ? ` (${colors.blue(Thing.getReference(this))})` : '')
     );
   }
 
diff --git a/src/data/things/track.js b/src/data/things/track.js
index e176acb4..db325a17 100644
--- a/src/data/things/track.js
+++ b/src/data/things/track.js
@@ -1,460 +1,330 @@
 import {inspect} from 'node:util';
 
-import {color} from '#cli';
+import {colors} from '#cli';
+import {input} from '#composite';
 import find from '#find';
-import {empty} from '#sugar';
 
+import {
+  isColor,
+  isContributionList,
+  isDate,
+  isFileExtension,
+} from '#validators';
+
+import {withPropertyFromObject} from '#composite/data';
+import {withResolvedContribs} from '#composite/wiki-data';
+
+import {
+  exitWithoutDependency,
+  exposeConstant,
+  exposeDependency,
+  exposeDependencyOrContinue,
+  exposeUpdateValueOrContinue,
+} from '#composite/control-flow';
+
+import {
+  additionalFiles,
+  commentary,
+  commentatorArtists,
+  contributionList,
+  directory,
+  duration,
+  flag,
+  name,
+  referenceList,
+  reverseReferenceList,
+  simpleDate,
+  singleReference,
+  simpleString,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
+import {
+  exitWithoutUniqueCoverArt,
+  inheritFromOriginalRelease,
+  trackReverseReferenceList,
+  withAlbum,
+  withAlwaysReferenceByDirectory,
+  withContainingTrackSection,
+  withHasUniqueCoverArt,
+  withOtherReleases,
+  withPropertyFromAlbum,
+} from '#composite/things/track';
+
+import CacheableObject from './cacheable-object.js';
 import Thing from './thing.js';
 
 export class Track extends Thing {
   static [Thing.referenceType] = 'track';
 
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-    ArtTag,
-    Artist,
-    Flash,
-
-    validators: {
-      isBoolean,
-      isColor,
-      isDate,
-      isDuration,
-      isFileExtension,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Album, ArtTag, Artist, Flash}) => ({
     // 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(),
-
-    artistContribsByRef: Thing.common.contribsByRef(),
-    contributorContribsByRef: Thing.common.contribsByRef(),
-    coverArtistContribsByRef: Thing.common.contribsByRef(),
-
-    referencedTracksByRef: Thing.common.referenceList(Track),
-    sampledTracksByRef: Thing.common.referenceList(Track),
-    artTagsByRef: Thing.common.referenceList(ArtTag),
-
-    hasCoverArt: {
-      flags: {update: true, expose: true},
-
-      update: {
-        validate(value) {
-          if (value !== false) {
-            throw new TypeError(`Expected false or null`);
-          }
-
-          return true;
-        },
-      },
-
-      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',
-      },
-    },
-
-    originalReleaseTrackByRef: Thing.common.singleReference(Track),
-
-    dataSourceAlbumByRef: Thing.common.singleReference(Album),
-
-    commentary: Thing.common.commentary(),
-    lyrics: Thing.common.simpleString(),
-    additionalFiles: Thing.common.additionalFiles(),
-    sheetMusicFiles: Thing.common.additionalFiles(),
-    midiProjectFiles: Thing.common.additionalFiles(),
+    name: name('Unnamed Track'),
+    directory: directory(),
+
+    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'}),
+    ],
+
+    commentary: commentary(),
+    lyrics: simpleString(),
+
+    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'),
+      }),
+
+      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'),
+      }),
+
+      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'),
+      }),
+
+      referenceList({
+        class: input.value(Track),
+        find: input.value(find.track),
+        data: 'trackData',
+      }),
+    ],
+
+    sampledTracks: [
+      inheritFromOriginalRelease({
+        property: input.value('sampledTracks'),
+      }),
+
+      referenceList({
+        class: input.value(Track),
+        find: input.value(find.track),
+        data: 'trackData',
+      }),
+    ],
+
+    artTags: referenceList({
+      class: input.value(ArtTag),
+      find: input.value(find.artTag),
+      data: 'artTagData',
+    }),
 
     // 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),
+    albumData: wikiData(Album),
+    artistData: wikiData(Artist),
+    artTagData: wikiData(ArtTag),
+    flashData: wikiData(Flash),
+    trackData: 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: {update: true, expose: true},
-
-      update: {validate: isColor},
-
-      expose: {
-        dependencies: ['albumData'],
-
-        transform: (color, {albumData, [Track.instance]: track}) =>
-          color ??
-            Track.findAlbum(track, albumData)
-              ?.trackSections.find(({tracks}) => tracks.includes(track))
-                ?.color ?? null,
-      },
-    },
-
-    coverArtDate: {
-      flags: {update: true, expose: true},
-
-      update: {validate: isDate},
-
-      expose: {
-        dependencies: [
-          'albumData',
-          'coverArtistContribsByRef',
-          'dateFirstReleased',
-          'hasCoverArt',
-        ],
-        transform: (coverArtDate, {
-          albumData,
-          coverArtistContribsByRef,
-          dateFirstReleased,
-          hasCoverArt,
-          [Track.instance]: track,
-        }) =>
-          (Track.hasCoverArt(track, albumData, coverArtistContribsByRef, hasCoverArt)
-            ? coverArtDate ??
-              dateFirstReleased ??
-              Track.findAlbum(track, albumData)?.trackArtDate ??
-              Track.findAlbum(track, albumData)?.date ??
-              null
-            : null),
-      },
-    },
-
-    hasUniqueCoverArt: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['albumData', 'coverArtistContribsByRef', 'hasCoverArt'],
-        compute: ({
-          albumData,
-          coverArtistContribsByRef,
-          hasCoverArt,
-          [Track.instance]: track,
-        }) =>
-          Track.hasUniqueCoverArt(
-            track,
-            albumData,
-            coverArtistContribsByRef,
-            hasCoverArt
-          ),
-      },
-    },
-
-    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);
-        },
-      },
-    },
-
-    artistContribs:
-      Track.inheritFromOriginalRelease('artistContribs', [],
-        Thing.common.dynamicInheritContribs(
-          null,
-          'artistContribsByRef',
-          'artistContribsByRef',
-          'albumData',
-          Track.findAlbum)),
-
-    contributorContribs:
-      Track.inheritFromOriginalRelease('contributorContribs', [],
-        Thing.common.dynamicContribs('contributorContribsByRef')),
-
-    // 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:
-      Thing.common.dynamicInheritContribs(
-        'hasCoverArt',
-        'coverArtistContribsByRef',
-        'trackCoverArtistContribsByRef',
-        'albumData',
-        Track.findAlbum),
-
-    referencedTracks:
-      Track.inheritFromOriginalRelease('referencedTracks', [],
-        Thing.common.dynamicThingsFromReferenceList(
-          'referencedTracksByRef',
-          'trackData',
-          find.track)),
-
-    sampledTracks:
-      Track.inheritFromOriginalRelease('sampledTracks', [],
-        Thing.common.dynamicThingsFromReferenceList(
-          'sampledTracksByRef',
-          '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))
-            : [],
-      },
-    },
-
-    // For the same reasoning, exclude re-releases from sampled tracks too.
-    sampledByTracks: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['trackData'],
-
-        compute: ({trackData, [Track.instance]: track}) =>
-          trackData
-            ? trackData
-                .filter((t) => !t.originalReleaseTrack)
-                .filter((t) => t.sampledTracks?.includes(track))
-            : [],
-      },
-    },
-
-    featuredInFlashes: Thing.common.reverseReferenceList(
-      'flashData',
-      'featuredTracks'
-    ),
-
-    artTags: Thing.common.dynamicThingsFromReferenceList(
-      'artTagsByRef',
-      'artTagData',
-      find.artTag
-    ),
-  });
-
-  // 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
-  static findAlbum = (track, albumData) =>
-    albumData?.find((album) => album.tracks.includes(track));
-
-  // Another reused utility function. This one's logic is a bit more complicated.
-  static hasCoverArt(
-    track,
-    albumData,
-    coverArtistContribsByRef,
-    hasCoverArt
-  ) {
-    if (!empty(coverArtistContribsByRef)) {
-      return true;
-    }
+    commentatorArtists: commentatorArtists(),
 
-    const album = Track.findAlbum(track, albumData);
-    if (album && !empty(album.trackCoverArtistContribsByRef)) {
-      return true;
-    }
+    album: [
+      withAlbum(),
+      exposeDependency({dependency: '#album'}),
+    ],
 
-    return false;
-  }
+    date: [
+      exposeDependencyOrContinue({dependency: 'dateFirstReleased'}),
 
-  static hasUniqueCoverArt(
-    track,
-    albumData,
-    coverArtistContribsByRef,
-    hasCoverArt
-  ) {
-    if (!empty(coverArtistContribsByRef)) {
-      return true;
-    }
+      withPropertyFromAlbum({
+        property: input.value('date'),
+      }),
 
-    if (hasCoverArt === false) {
-      return false;
-    }
+      exposeDependency({dependency: '#album.date'}),
+    ],
 
-    const album = Track.findAlbum(track, albumData);
-    if (album && !empty(album.trackCoverArtistContribsByRef)) {
-      return true;
-    }
+    hasUniqueCoverArt: [
+      withHasUniqueCoverArt(),
+      exposeDependency({dependency: '#hasUniqueCoverArt'}),
+    ],
 
-    return false;
-  }
+    otherReleases: [
+      withOtherReleases(),
+      exposeDependency({dependency: '#otherReleases'}),
+    ],
 
-  static inheritFromOriginalRelease(
-    originalProperty,
-    originalMissingValue,
-    ownPropertyDescriptor
-  ) {
-    return {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: [
-          ...ownPropertyDescriptor.expose.dependencies,
-          'originalReleaseTrackByRef',
-          'trackData',
-        ],
-
-        compute(dependencies) {
-          const {
-            originalReleaseTrackByRef,
-            trackData,
-          } = dependencies;
-
-          if (originalReleaseTrackByRef) {
-            if (!trackData) return originalMissingValue;
-            const original = find.track(originalReleaseTrackByRef, trackData, {mode: 'quiet'});
-            if (!original) return originalMissingValue;
-            return original[originalProperty];
-          }
-
-          return ownPropertyDescriptor.expose.compute(dependencies);
-        },
-      },
-    };
-  }
+    referencedByTracks: trackReverseReferenceList({
+      list: input.value('referencedTracks'),
+    }),
 
-  [inspect.custom]() {
-    const base = Thing.prototype[inspect.custom].apply(this);
+    sampledByTracks: trackReverseReferenceList({
+      list: input.value('sampledTracks'),
+    }),
 
-    const rereleasePart =
-      (this.originalReleaseTrackByRef
-        ? `${color.yellow('[rerelease]')} `
-        : ``);
+    featuredInFlashes: reverseReferenceList({
+      data: 'flashData',
+      list: input.value('featuredTracks'),
+    }),
+  });
 
-    const {album, dataSourceAlbum} = this;
+  [inspect.custom](depth) {
+    const parts = [];
 
-    const albumName =
-      (album
-        ? album.name
-        : dataSourceAlbum?.name);
+    parts.push(Thing.prototype[inspect.custom].apply(this));
 
-    const albumIndex =
-      albumName &&
-        (album
-          ? album.tracks.indexOf(this)
-          : dataSourceAlbum.tracks.indexOf(this));
+    if (CacheableObject.getUpdateValue(this, 'originalReleaseTrack')) {
+      parts.unshift(`${colors.yellow('[rerelease]')} `);
+    }
 
-    const trackNum =
-      albumName &&
+    let album;
+    if (depth >= 0 && (album = this.album ?? this.dataSourceAlbum)) {
+      const albumName = album.name;
+      const albumIndex = album.tracks.indexOf(this);
+      const trackNum =
         (albumIndex === -1
           ? '#?'
           : `#${albumIndex + 1}`);
+      parts.push(` (${colors.yellow(trackNum)} in ${colors.green(albumName)})`);
+    }
 
-    const albumPart =
-      albumName
-        ? ` (${color.yellow(trackNum)} in ${color.green(albumName)})`
-        : ``;
-
-    return rereleasePart + base + albumPart;
+    return parts.join('');
   }
 }
diff --git a/src/data/things/validators.js b/src/data/things/validators.js
index fc953c2a..ee301f15 100644
--- a/src/data/things/validators.js
+++ b/src/data/things/validators.js
@@ -1,7 +1,7 @@
 import {inspect as nodeInspect} from 'node:util';
 
-import {color, ENABLE_COLOR} from '#cli';
-import {withAggregate} from '#sugar';
+import {colors, ENABLE_COLOR} from '#cli';
+import {empty, typeAppearance, withAggregate} from '#sugar';
 
 function inspect(value) {
   return nodeInspect(value, {colors: ENABLE_COLOR});
@@ -9,13 +9,13 @@ function inspect(value) {
 
 // Basic types (primitives)
 
-function a(noun) {
+export function a(noun) {
   return /[aeiou]/.test(noun[0]) ? `an ${noun}` : `a ${noun}`;
 }
 
-function isType(value, type) {
+export function isType(value, type) {
   if (typeof value !== type)
-    throw new TypeError(`Expected ${a(type)}, got ${typeof value}`);
+    throw new TypeError(`Expected ${a(type)}, got ${typeAppearance(value)}`);
 
   return true;
 }
@@ -132,7 +132,7 @@ export function isObject(value) {
 
 export function isArray(value) {
   if (typeof value !== 'object' || value === null || !Array.isArray(value))
-    throw new TypeError(`Expected an array, got ${value}`);
+    throw new TypeError(`Expected an array, got ${typeAppearance(value)}`);
 
   return true;
 }
@@ -174,7 +174,8 @@ function validateArrayItemsHelper(itemValidator) {
         throw new Error(`Expected validator to return true`);
       }
     } catch (error) {
-      error.message = `(index: ${color.green(index)}, item: ${inspect(item)}) ${error.message}`;
+      error.message = `(index: ${colors.yellow(`${index}`)}, item: ${inspect(item)}) ${error.message}`;
+      error[Symbol.for('hsmusic.decorate.indexInSourceArray')] = index;
       throw error;
     }
   };
@@ -264,7 +265,7 @@ export function validateProperties(spec) {
           try {
             specValidator(value);
           } catch (error) {
-            error.message = `(key: ${color.green(specKey)}, value: ${inspect(value)}) ${error.message}`;
+            error.message = `(key: ${colors.green(specKey)}, value: ${inspect(value)}) ${error.message}`;
             throw error;
           }
         });
@@ -308,7 +309,7 @@ export const isTrackSection = validateProperties({
   color: optional(isColor),
   dateOriginallyReleased: optional(isDate),
   isDefaultTrackSection: optional(isBoolean),
-  tracksByRef: optional(validateReferenceList('track')),
+  tracks: optional(validateReferenceList('track')),
 });
 
 export const isTrackSectionList = validateArrayItems(isTrackSection);
@@ -404,6 +405,76 @@ export function validateReferenceList(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(array.map(object =>
+          object.constructor[Symbol.for('Thing.referenceType')]));
+
+      if (allRefTypes.has(undefined)) {
+        if (allRefTypes.size === 1) {
+          throw new TypeError(`Expected array of wiki data objects, got array of other objects`);
+        } else {
+          throw new TypeError(`Expected array of wiki data objects, got mixed items`);
+        }
+      }
+
+      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()}`);
+      }
+
+      if (referenceType && !allRefTypes.has(referenceType)) {
+        throw new TypeError(`Expected array of ${referenceType}, got array of ${allRefTypes[0]}`)
+      }
+
+      OK = true; return true;
+    } finally {
+      subcache.set(array, OK);
+    }
+  };
+}
+
 // Compositional utilities
 
 export function oneOf(...checks) {
diff --git a/src/data/things/wiki-info.js b/src/data/things/wiki-info.js
index e906cab1..6286a267 100644
--- a/src/data/things/wiki-info.js
+++ b/src/data/things/wiki-info.js
@@ -1,20 +1,25 @@
+import {input} from '#composite';
 import find from '#find';
+import {isLanguageCode, isName, isURL} from '#validators';
+
+import {
+  color,
+  flag,
+  name,
+  referenceList,
+  simpleString,
+  wikiData,
+} from '#composite/wiki-properties';
 
 import Thing from './thing.js';
 
 export class WikiInfo extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    Group,
+  static [Thing.friendlyName] = `Wiki Info`;
 
-    validators: {
-      isLanguageCode,
-      isName,
-      isURL,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Group}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Wiki'),
+    name: name('Unnamed Wiki'),
 
     // Displayed in nav bar.
     nameShort: {
@@ -27,12 +32,12 @@ export class WikiInfo extends Thing {
       },
     },
 
-    color: Thing.common.color(),
+    color: color(),
 
     // One-line description used for <meta rel="description"> tag.
-    description: Thing.common.simpleString(),
+    description: simpleString(),
 
-    footerContent: Thing.common.simpleString(),
+    footerContent: simpleString(),
 
     defaultLanguage: {
       flags: {update: true, expose: true},
@@ -44,25 +49,21 @@ export class WikiInfo extends Thing {
       update: {validate: isURL},
     },
 
-    divideTrackListsByGroupsByRef: Thing.common.referenceList(Group),
+    divideTrackListsByGroups: referenceList({
+      class: input.value(Group),
+      find: input.value(find.group),
+      data: 'groupData',
+    }),
 
     // 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),
+    enableFlashesAndGames: flag(false),
+    enableListings: flag(false),
+    enableNews: flag(false),
+    enableArtTagUI: flag(false),
+    enableGroupUI: flag(false),
 
     // Update only
 
-    groupData: Thing.common.wikiData(Group),
-
-    // Expose only
-
-    divideTrackListsByGroups: Thing.common.dynamicThingsFromReferenceList(
-      'divideTrackListsByGroupsByRef',
-      'groupData',
-      find.group
-    ),
+    groupData: wikiData(Group),
   });
 }