16 files changed, 4096 insertions, 1212 deletions

diff --git a/src/data/things/album.js b/src/data/things/album.js
index c012c243..fd8a71d3 100644
--- a/src/data/things/album.js
+++ b/src/data/things/album.js
@@ -1,163 +1,264 @@
-import {empty} from '#sugar';
 import find from '#find';
-
-import Thing from './thing.js';
+import {empty, stitchArrays} from '#sugar';
+import {isDate, isTrackSectionList} from '#validators';
+import {filterMultipleArrays} from '#wiki-data';
+
+import {
+  exitWithoutDependency,
+  exitWithoutUpdateValue,
+  exposeDependency,
+  exposeUpdateValueOrContinue,
+  input,
+  fillMissingListItems,
+  withFlattenedList,
+  withPropertiesFromList,
+  withUnflattenedList,
+} from '#composite';
+
+import Thing, {
+  additionalFiles,
+  commentary,
+  color,
+  commentatorArtists,
+  contribsPresent,
+  contributionList,
+  dimensions,
+  directory,
+  exitWithoutContribs,
+  fileExtension,
+  flag,
+  name,
+  referenceList,
+  simpleDate,
+  simpleString,
+  urls,
+  wikiData,
+  withResolvedReferenceList,
+} 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)) ??
-              [],
-          }));
+    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: [
+      exitWithoutDependency({
+        dependency: 'trackData',
+        value: input.value([]),
+      }),
+
+      exitWithoutUpdateValue({
+        mode: input.value('empty'),
+        value: input.value([]),
+      }),
+
+      withPropertiesFromList({
+        list: input.updateValue(),
+        prefix: input.value('#sections'),
+        properties: input.value([
+          'tracks',
+          'dateOriginallyReleased',
+          'isDefaultTrackSection',
+          'color',
+        ]),
+      }),
+
+      fillMissingListItems({
+        list: '#sections.tracks',
+        fill: input.value([]),
+      }),
+
+      fillMissingListItems({
+        list: '#sections.isDefaultTrackSection',
+        fill: input.value(false),
+      }),
+
+      fillMissingListItems({
+        list: '#sections.color',
+        fill: input.dependency('color'),
+      }),
+
+      withFlattenedList({
+        list: '#sections.tracks',
+      }).outputs({
+        ['#flattenedList']: '#trackRefs',
+        ['#flattenedIndices']: '#sections.startIndex',
+      }),
+
+      withResolvedReferenceList({
+        list: '#trackRefs',
+        data: 'trackData',
+        notFoundMode: input.value('null'),
+        find: input.value(find.track),
+      }).outputs({
+        ['#resolvedReferenceList']: '#tracks',
+      }),
+
+      withUnflattenedList({
+        list: '#tracks',
+        indices: '#sections.startIndex',
+      }).outputs({
+        ['#unflattenedList']: '#sections.tracks',
+      }),
+
+      {
+        flags: {update: true, expose: true},
+
+        update: {validate: isTrackSectionList},
+
+        expose: {
+          dependencies: [
+            '#sections.tracks',
+            '#sections.color',
+            '#sections.dateOriginallyReleased',
+            '#sections.isDefaultTrackSection',
+            '#sections.startIndex',
+          ],
+
+          transform(trackSections, {
+            '#sections.tracks': tracks,
+            '#sections.color': color,
+            '#sections.dateOriginallyReleased': dateOriginallyReleased,
+            '#sections.isDefaultTrackSection': isDefaultTrackSection,
+            '#sections.startIndex': startIndex,
+          }) {
+            filterMultipleArrays(
+              tracks, color, dateOriginallyReleased, isDefaultTrackSection, startIndex,
+              tracks => !empty(tracks));
+
+            return stitchArrays({
+              tracks,
+              color,
+              dateOriginallyReleased,
+              isDefaultTrackSection,
+              startIndex,
+            });
+          }
         },
       },
-    },
-
-    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(),
+    ],
+
+    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)
-            : [],
+    commentatorArtists: commentatorArtists(),
+
+    hasCoverArt: contribsPresent({contribs: 'coverArtistContribs'}),
+    hasWallpaperArt: contribsPresent({contribs: 'wallpaperArtistContribs'}),
+    hasBannerArt: contribsPresent({contribs: 'bannerArtistContribs'}),
+
+    tracks: [
+      exitWithoutDependency({
+        dependency: 'trackData',
+        value: input.value([]),
+      }),
+
+      exitWithoutDependency({
+        dependency: 'trackSections',
+        mode: input.value('empty'),
+        value: input.value([]),
+      }),
+
+      {
+        dependencies: ['trackSections'],
+        compute: (continuation, {trackSections}) =>
+          continuation({
+            '#trackRefs': trackSections
+              .flatMap(section => section.tracks ?? []),
+          }),
       },
-    },
-
-    groups: Thing.common.dynamicThingsFromReferenceList(
-      'groupsByRef',
-      'groupData',
-      find.group
-    ),
-
-    artTags: Thing.common.dynamicThingsFromReferenceList(
-      'artTagsByRef',
-      'artTagData',
-      find.artTag
-    ),
+
+      withResolvedReferenceList({
+        list: '#trackRefs',
+        data: 'trackData',
+        find: input.value(find.track),
+      }),
+
+      exposeDependency({dependency: '#resolvedReferenceList'}),
+    ],
   });
 
   static [Thing.getSerializeDescriptors] = ({
@@ -202,9 +303,9 @@ export class Album extends Thing {
 
 export class TrackSectionHelper extends Thing {
   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 Group'),
+    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..ba3cbd0d 100644
--- a/src/data/things/art-tag.js
+++ b/src/data/things/art-tag.js
@@ -1,35 +1,42 @@
+import {exposeUpdateValueOrContinue, input} from '#composite';
 import {sortAlbumsTracksChronologically} from '#wiki-data';
+import {isName} from '#validators';
 
-import Thing from './thing.js';
+import Thing, {
+  color,
+  directory,
+  flag,
+  name,
+  wikiData,
+} from './thing.js';
 
 export class ArtTag extends Thing {
   static [Thing.referenceType] = '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 +44,8 @@ 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 522ca5f9..085e5663 100644
--- a/src/data/things/artist.js
+++ b/src/data/things/artist.js
@@ -1,29 +1,31 @@
+import {input} from '#composite';
 import find from '#find';
-
-import Thing from './thing.js';
+import {isName, validateArrayItems} from '#validators';
+
+import Thing, {
+  directory,
+  fileExtension,
+  flag,
+  name,
+  simpleString,
+  singleReference,
+  urls,
+  wikiData,
+} 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 +33,23 @@ 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 +61,14 @@ 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 +77,9 @@ 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)) ?? [],
       },
@@ -103,18 +98,16 @@ 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] = ({
@@ -148,15 +141,15 @@ 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..4bc3668d 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) {
@@ -185,7 +181,7 @@ export default class CacheableObject {
           }
         } catch (error) {
           error.message = [
-            `Property ${color.green(property)}`,
+            `Property ${colors.green(property)}`,
             `(${inspect(this[property])} -> ${inspect(newValue)}):`,
             error.message
           ].join(' ');
@@ -250,20 +246,27 @@ 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 +350,12 @@ export default class CacheableObject {
       console.log(` - ${line}`);
     }
   }
+
+  static getUpdateValue(object, key) {
+    if (!Object.hasOwn(object, key)) {
+      return undefined;
+    }
+
+    return object.#propertyUpdateValues[key] ?? null;
+  }
 }
diff --git a/src/data/things/composite.js b/src/data/things/composite.js
new file mode 100644
index 00000000..7a9048c2
--- /dev/null
+++ b/src/data/things/composite.js
@@ -0,0 +1,2188 @@
+import {inspect} from 'node:util';
+
+import {colors} from '#cli';
+import {TupleMap} from '#wiki-data';
+
+import {
+  isArray,
+  isString,
+  isWholeNumber,
+  oneOf,
+  validateArrayItems,
+} from '#validators';
+
+import {
+  decorateErrorWithIndex,
+  empty,
+  filterProperties,
+  openAggregate,
+  stitchArrays,
+  unique,
+} from '#sugar';
+
+// Composes multiple compositional "steps" and a "base" to form a property
+// descriptor out of modular building blocks. This is an extension to the
+// more general-purpose CacheableObject property descriptor syntax, and
+// aims to make modular data processing - which lends to declarativity -
+// much easier, without fundamentally altering much of the typical syntax
+// or terminology, nor building on it to an excessive degree.
+//
+// Think of a composition as being a chain of steps which lead into a final
+// base property, which is usually responsible for returning the value that
+// will actually get exposed when the property being described is accessed.
+//
+// == The compositional base: ==
+//
+// The final item in a compositional list is its base, and it identifies
+// the essential qualities of the property descriptor. The compositional
+// steps preceding it may exit early, in which case the expose function
+// defined on the base won't be called; or they will provide dependencies
+// that the base may use to compute the final value that gets exposed for
+// this property.
+//
+// The base indicates the capabilities of the composition as a whole.
+// It should be {expose: true}, since that's the only area that preceding
+// compositional steps (currently) can actually influence. If it's also
+// {update: true}, then the composition as a whole accepts an update value
+// just like normal update-flag property descriptors - meaning it can be
+// set with `thing.someProperty = value` and that value will be paseed
+// into each (implementing) step's transform() function, as well as the
+// base. Bases usually aren't {compose: true}, but can be - check out the
+// section on "nesting compositions" for details about that.
+//
+// Every composition always has exactly one compositional base, and it's
+// always the last item in the composition list. All items preceding it
+// are compositional steps, described below.
+//
+// == Compositional steps: ==
+//
+// Compositional steps are, in essence, typical property descriptors with
+// the extra flag {compose: true}. They operate on existing dependencies,
+// and are typically dynamically constructed by "utility" functions (but
+// can also be manually declared within the step list of a composition).
+// Compositional steps serve two purposes:
+//
+//  1. exit early, if some condition is matched, returning and exposing
+//     some value directly from that step instead of continuing further
+//     down the step list;
+//
+//  2. and/or provide new, dynamically created "private" dependencies which
+//     can be accessed by further steps down the list, or at the base at
+//     the bottom, modularly supplying information that will contribute to
+//     the final value exposed for this property.
+//
+// Usually it's just one of those two, but it's fine for a step to perform
+// both jobs if the situation benefits.
+//
+// Compositional steps are the real "modular" or "compositional" part of
+// this data processing style - they're designed to be combined together
+// in dynamic, versatile ways, as each property demands it. You usually
+// define a compositional step to be returned by some ordinary static
+// property-descriptor-returning function (customarily namespaced under
+// the relevant Thing class's static `composite` field) - that lets you
+// reuse it in multiple compositions later on.
+//
+// Compositional steps are implemented with "continuation passing style",
+// meaning the connection to the next link on the chain is passed right to
+// each step's compute (or transform) function, and the implementation gets
+// to decide whether to continue on that chain or exit early by returning
+// some other value.
+//
+// Every step along the chain, apart from the base at the bottom, has to
+// have the {compose: true} step. That means its compute() or transform()
+// function will be passed an extra argument at the end, `continuation`.
+// To provide new dependencies to items further down the chain, just pass
+// them directly to this continuation() function, customarily with a hash
+// ('#') prefixing each name - for example:
+//
+//   compute({..some dependencies..}, continuation) {
+//     return continuation({
+//       '#excitingProperty': (..a value made from dependencies..),
+//     });
+//   }
+//
+// Performing an early exit is as simple as returning some other value,
+// instead of the continuation. You may also use `continuation.exit(value)`
+// to perform the exact same kind of early exit - it's just a different
+// syntax that might fit in better in certain longer compositions.
+//
+// It may be fine to simply provide new dependencies under a hard-coded
+// name, such as '#excitingProperty' above, but if you're writing a utility
+// that dynamically returns the compositional step and you suspect you
+// might want to use this step multiple times in a single composition,
+// it's customary to accept a name for the result.
+//
+// Here's a detailed example showing off early exit, dynamically operating
+// on a provided dependency name, and then providing a result in another
+// also-provided dependency name:
+//
+//   withResolvedContribs = ({
+//     from: contribsByRefDependency,
+//     into: outputDependency,
+//   }) => ({
+//     flags: {expose: true, compose: true},
+//     expose: {
+//       dependencies: [contribsByRefDependency, 'artistData'],
+//       compute({
+//         [contribsByRefDependency]: contribsByRef,
+//         artistData,
+//       }, continuation) {
+//         if (!artistData) return null;  /* early exit! */
+//         return continuation({
+//           [outputDependency]:  /* this is the important part */
+//             (..resolve contributions one way or another..),
+//         });
+//       },
+//     },
+//   });
+//
+// And how you might work that into a composition:
+//
+//   Track.coverArtists =
+//     compositeFrom([
+//       doSomethingWhichMightEarlyExit(),
+//
+//       withResolvedContribs({
+//         from: 'coverArtistContribsByRef',
+//         into: '#coverArtistContribs',
+//       }),
+//
+//       {
+//         flags: {expose: true},
+//         expose: {
+//           dependencies: ['#coverArtistContribs'],
+//           compute: ({'#coverArtistContribs': coverArtistContribs}) =>
+//             coverArtistContribs.map(({who}) => who),
+//         },
+//       },
+//     ]);
+//
+// One last note! A super common code pattern when creating more complex
+// compositions is to have several steps which *only* expose and compose.
+// As a syntax shortcut, you can skip the outer section. It's basically
+// like writing out just the {expose: {...}} part. Remember that this
+// indicates that the step you're defining is compositional, so you have
+// to specify the flags manually for the base, even if this property isn't
+// going to get an {update: true} flag.
+//
+// == Cache-safe dependency names: ==
+//
+// [Disclosure: The caching engine hasn't actually been implemented yet.
+//  As such, this section is subject to change, and simply provides sound
+//  forward-facing advice and interfaces.]
+//
+// It's a good idea to write individual compositional steps in such a way
+// that they're "cache-safe" - meaning the same input (dependency) values
+// will always result in the same output (continuation or early exit).
+//
+// In order to facilitate this, compositional step descriptors may specify
+// unique `mapDependencies`, `mapContinuation`, and `options` values.
+//
+// Consider the `withResolvedContribs` example adjusted to make use of
+// two of these options below:
+//
+//   withResolvedContribs = ({
+//     from: contribsByRefDependency,
+//     into: outputDependency,
+//   }) => ({
+//     flags: {expose: true, compose: true},
+//     expose: {
+//       dependencies: ['artistData'],
+//       mapDependencies: {contribsByRef: contribsByRefDependency},
+//       mapContinuation: {outputDependency},
+//       compute({
+//         contribsByRef, /* no longer in square brackets */
+//         artistData,
+//       }, continuation) {
+//         if (!artistData) return null;
+//         return continuation({
+//           outputDependency: /* no longer in square brackets */
+//             (..resolve contributions one way or another..),
+//         });
+//       },
+//     },
+//   });
+//
+// With a little destructuring and restructuring JavaScript sugar, the
+// above can be simplified some more:
+//
+//   withResolvedContribs = ({from, to}) => ({
+//     flags: {expose: true, compose: true},
+//     expose: {
+//       dependencies: ['artistData'],
+//       mapDependencies: {from},
+//       mapContinuation: {into},
+//       compute({artistData, from: contribsByRef}, continuation) {
+//         if (!artistData) return null;
+//         return continuation({
+//           into: (..resolve contributions one way or another..),
+//         });
+//       },
+//     },
+//   });
+//
+// These two properties let you separate the name-mapping behavior (for
+// dependencies and the continuation) from the main body of the compute
+// function. That means the compute function will *always* get inputs in
+// the same form (dependencies 'artistData' and 'from' above), and will
+// *always* provide its output in the same form (early return or 'to').
+//
+// Thanks to that, this `compute` function is cache-safe! Its outputs can
+// be cached corresponding to each set of mapped inputs. So it won't matter
+// whether the `from` dependency is named `coverArtistContribsByRef` or
+// `contributorContribsByRef` or something else - the compute function
+// doesn't care, and only expects that value to be provided via its `from`
+// argument. Likewise, it doesn't matter if the output should be sent to
+// '#coverArtistContribs` or `#contributorContribs` or some other name;
+// the mapping is handled automatically outside, and compute will always
+// output its value to the continuation's `to`.
+//
+// Note that `mapDependencies` and `mapContinuation` should be objects of
+// the same "shape" each run - that is, the values will change depending on
+// outside context, but the keys are always the same. You shouldn't use
+// `mapDependencies` to dynamically select more or fewer dependencies.
+// If you need to dynamically select a range of dependencies, just specify
+// them in the `dependencies` array like usual. The caching engine will
+// understand that differently named `dependencies` indicate separate
+// input-output caches should be used.
+//
+// The 'options' property makes it possible to specify external arguments
+// that fundamentally change the behavior of the `compute` function, while
+// still remaining cache-safe. It indicates that the caching engine should
+// use a completely different input-to-output cache for each permutation
+// of the 'options' values. This way, those functions are still cacheable
+// at all; they'll just be cached separately for each set of option values.
+// Values on the 'options' property will always be provided in compute's
+// dependencies under '#options' (to avoid name conflicts with other
+// dependencies).
+//
+// == To compute or to transform: ==
+//
+// A compositional step can work directly on a property's stored update
+// value, transforming it in place and either early exiting with it or
+// passing it on (via continuation) to the next item(s) in the
+// compositional step list. (If needed, these can provide dependencies
+// the same way as compute functions too - just pass that object after
+// the updated (or same) transform value in your call to continuation().)
+//
+// But in order to make them more versatile, compositional steps have an
+// extra trick up their sleeve. If a compositional step implements compute
+// and *not* transform, it can still be used in a composition targeting a
+// property which updates! These retain their full dependency-providing and
+// early exit functionality - they just won't be provided the update value.
+// If a compute-implementing step returns its continuation, then whichever
+// later step (or the base) next implements transform() will receive the
+// update value that had so far been running - as well as any dependencies
+// the compute() step returned, of course!
+//
+// Please note that a compositional step which transforms *should not*
+// specify, in its flags, {update: true}. Just provide the transform()
+// function in its expose descriptor; it will be automatically detected
+// and used when appropriate.
+//
+// It's actually possible for a step to specify both transform and compute,
+// in which case the transform() implementation will only be selected if
+// the composition's base is {update: true}. It's not exactly known why you
+// would want to specify unique-but-related transform and compute behavior,
+// but the basic possibility was too cool to skip out on.
+//
+// == Nesting compositions: ==
+//
+// Compositional steps are so convenient that you just might want to bundle
+// them together, and form a whole new step-shaped unit of its own!
+//
+// In order to allow for this while helping to ensure internal dependencies
+// remain neatly isolated from the composition which nests your bundle,
+// the compositeFrom() function will accept and adapt to a base that
+// specifies the {compose: true} flag, just like the steps preceding it.
+//
+// The continuation function that gets provided to the base will be mildly
+// special - after all, nothing follows the base within the composition's
+// own list! Instead of appending dependencies alongside any previously
+// provided ones to be available to the next step, the base's continuation
+// function should be used to define "exports" of the composition as a
+// whole. It's similar to the usual behavior of the continuation, just
+// expanded to the scope of the composition instead of following steps.
+//
+// For example, suppose your composition (which you expect to include in
+// other compositions) brings about several private, hash-prefixed
+// dependencies to contribute to its own results. Those dependencies won't
+// end up "bleeding" into the dependency list of whichever composition is
+// nesting this one - they will totally disappear once all the steps in
+// the nested composition have finished up.
+//
+// To "export" the results of processing all those dependencies (provided
+// that's something you want to do and this composition isn't used purely
+// for a conditional early-exit), you'll want to define them in the
+// continuation passed to the base. (Customarily, those should start with
+// a hash just like the exports from any other compositional step; they're
+// still dynamically provided dependencies!)
+//
+// Another way to "export" dependencies is by using calling *any* step's
+// `continuation.raise()` function. This is sort of like early exiting,
+// but instead of quitting out the whole entire property, it will just
+// break out of the current, nested composition's list of steps, acting
+// as though the composition had finished naturally. The dependencies
+// passed to `raise` will be the ones which get exported.
+//
+// Since `raise` is another way to export dependencies, if you're using
+// dynamic export names, you should specify `mapContinuation` on the step
+// calling `continuation.raise` as well.
+//
+// An important note on `mapDependencies` here: A nested composition gets
+// free access to all the ordinary properties defined on the thing it's
+// working on, but if you want it to depend on *private* dependencies -
+// ones prefixed with '#' - which were provided by some other compositional
+// step preceding wherever this one gets nested, then you *have* to use
+// `mapDependencies` to gain access. Check out the section on "cache-safe
+// dependency names" for information on this syntax!
+//
+// Also - on rare occasion - you might want to make a reusable composition
+// that itself causes the composition *it's* nested in to raise. If that's
+// the case, give `composition.raiseAbove()` a go! This effectively means
+// kicking out of *two* layers of nested composition - the one including
+// the step with the `raiseAbove` call, and the composition which that one
+// is nested within. You don't need to use `raiseAbove` if the reusable
+// utility function just returns a single compositional step, but if you
+// want to make use of other compositional steps, it gives you access to
+// the same conditional-raise capabilities.
+//
+// Have some syntax sugar! Since nested compositions are defined by having
+// the base be {compose: true}, the composition will infer as much if you
+// don't specifying the base's flags at all. Simply use the same shorthand
+// syntax as for other compositional steps, and it'll work out cleanly!
+//
+
+const globalCompositeCache = {};
+
+export function input(nameOrDescription) {
+  if (typeof nameOrDescription === 'string') {
+    return Symbol.for(`hsmusic.composite.input:${nameOrDescription}`);
+  } else {
+    return {
+      symbol: Symbol.for('hsmusic.composite.input'),
+      shape: 'input',
+      value: nameOrDescription,
+    };
+  }
+}
+
+input.symbol = Symbol.for('hsmusic.composite.input');
+
+input.updateValue = (description = null) =>
+  (description
+    ? {
+        symbol: input.symbol,
+        shape: 'input.updateValue',
+        value: description,
+      }
+    : Symbol.for('hsmusic.composite.input.updateValue'));
+
+input.myself = () => Symbol.for(`hsmusic.composite.input.myself`);
+
+input.value = value => ({symbol: input.symbol, shape: 'input.value', value});
+input.dependency = name => Symbol.for(`hsmusic.composite.input.dependency:${name}`);
+input.staticDependency = name => Symbol.for(`hsmusic.composite.input.staticDependency:${name}`);
+input.staticValue = name => Symbol.for(`hsmusic.composite.input.staticValue:${name}`);
+
+function isInputToken(token) {
+  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 ${token}`);
+  }
+
+  if (typeof token === 'object') {
+    return token.shape;
+  } else {
+    return token.description.match(/hsmusic\.composite\.(input.*?)(:|$)/)[1];
+  }
+}
+
+function getInputTokenValue(token) {
+  if (!isInputToken(token)) {
+    throw new TypeError(`Expected an input token, got ${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;
+}
+
+export function templateCompositeFrom(description) {
+  const compositeName =
+    (description.annotation
+      ? description.annotation
+      : `unnamed composite`);
+
+  const descriptionAggregate = openAggregate({message: `Errors in description for ${compositeName}`});
+
+  if ('steps' in description) {
+    if (Array.isArray(description.steps)) {
+      descriptionAggregate.push(new TypeError(`Wrap steps array in a function`));
+    } else if (typeof description.steps !== 'function') {
+      descriptionAggregate.push(new TypeError(`Expected steps to be a function (returning an array)`));
+    }
+  }
+
+  validateInputs:
+  if ('inputs' in description) {
+    if (Array.isArray(description.inputs)) {
+      descriptionAggregate.push(new Error(`Expected inputs to be object, got array`));
+      break validateInputs;
+    } else if (typeof description.inputs !== 'object') {
+      descriptionAggregate.push(new Error(`Expected inputs to be object, got ${typeof description.inputs}`));
+      break validateInputs;
+    }
+
+    descriptionAggregate.nest({message: `Errors in input descriptions for ${compositeName}`}, ({push}) => {
+      const missingCallsToInput = [];
+      const wrongCallsToInput = [];
+
+      for (const [name, value] of Object.entries(description.inputs)) {
+        if (!isInputToken(value)) {
+          missingCallsToInput.push(name);
+          continue;
+        }
+
+        if (!['input', 'input.staticDependency', 'input.staticValue'].includes(getInputTokenShape(value))) {
+          wrongCallsToInput.push(name);
+        }
+      }
+
+      for (const name of missingCallsToInput) {
+        push(new Error(`${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'
+    ) {
+      descriptionAggregate.push(new Error(`Expected outputs to be array or function, got ${typeof description.outputs}`));
+      break validateOutputs;
+    }
+
+    if (Array.isArray(description.outputs)) {
+      descriptionAggregate.map(
+        description.outputs,
+        decorateErrorWithIndex(value => {
+          if (typeof value !== 'string') {
+            throw new Error(`${value}: Expected string, got ${typeof value}`)
+          } else if (!value.startsWith('#')) {
+            throw new Error(`${value}: Expected "#" at start`);
+          }
+        }),
+        {message: `Errors in output descriptions for ${compositeName}`});
+    }
+  }
+
+  descriptionAggregate.close();
+
+  const expectedInputNames =
+    (description.inputs
+      ? Object.keys(description.inputs)
+      : []);
+
+  const instantiate = (inputOptions = {}) => {
+    const inputOptionsAggregate = openAggregate({message: `Errors in input options passed to ${compositeName}`});
+
+    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 = description.inputs[name].value;
+          if (!inputDescription) return true;
+          if ('defaultValue' in inputDescription) return false;
+          if ('defaultDependency' in inputDescription) return false;
+          if (inputDescription.null === true) return false;
+          return true;
+        });
+
+    const wrongTypeInputNames = [];
+    const wrongInputCallInputNames = [];
+
+    for (const [name, value] of Object.entries(inputOptions)) {
+      if (misplacedInputNames.includes(name)) {
+        continue;
+      }
+
+      if (typeof value !== 'string' && !isInputToken(value)) {
+        wrongTypeInputNames.push(name);
+        continue;
+      }
+    }
+
+    if (!empty(misplacedInputNames)) {
+      inputOptionsAggregate.push(new Error(`Unexpected input names: ${misplacedInputNames.join(', ')}`));
+    }
+
+    if (!empty(missingInputNames)) {
+      inputOptionsAggregate.push(new Error(`Required these inputs: ${missingInputNames.join(', ')}`));
+    }
+
+    for (const name of wrongTypeInputNames) {
+      const type = typeof inputOptions[name];
+      inputOptionsAggregate.push(new Error(`${name}: Expected string or input() call, got ${type}`));
+    }
+
+    inputOptionsAggregate.close();
+
+    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) {
+        const outputOptionsAggregate = openAggregate({message: `Errors in output options passed to ${compositeName}`});
+
+        const misplacedOutputNames = [];
+        const wrongTypeOutputNames = [];
+        // const notPrivateOutputNames = [];
+
+        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 (!value.startsWith('#')) {
+            notPrivateOutputNames.push(name);
+            continue;
+          }
+          */
+        }
+
+        if (!empty(misplacedOutputNames)) {
+          outputOptionsAggregate.push(new Error(`Unexpected output names: ${misplacedOutputNames.join(', ')}`));
+        }
+
+        for (const name of wrongTypeOutputNames) {
+          const type = typeof providedOptions[name];
+          outputOptionsAggregate.push(new Error(`${name}: Expected string, got ${type}`));
+        }
+
+        /*
+        for (const name of notPrivateOutputNames) {
+          const into = providedOptions[name];
+          outputOptionsAggregate.push(new Error(`${name}: Expected "#" at start, got ${into}`));
+        }
+        */
+
+        outputOptionsAggregate.close();
+
+        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 finalInputs = {};
+
+          for (const [name, description_] of Object.entries(description.inputs)) {
+            // TODO: Validate inputOptions[name] against staticValue, staticDependency shapes
+            const description = getInputTokenValue(description_);
+            const tokenShape = getInputTokenShape(description_);
+
+            if (name in inputOptions) {
+              if (typeof inputOptions[name] === 'string') {
+                finalInputs[name] = input.dependency(inputOptions[name]);
+              } else {
+                finalInputs[name] = inputOptions[name];
+              }
+            } else if (description.defaultValue) {
+              finalInputs[name] = input.value(description.defaultValue);
+            } else if (description.defaultDependency) {
+              finalInputs[name] = input.dependency(description.defaultDependency);
+            } else {
+              finalInputs[name] = input.value(null);
+            }
+          }
+
+          finalDescription.inputs = finalInputs;
+        }
+
+        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 ${compositeName}`});
+
+        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 function compositeFrom(description) {
+  const {annotation} = 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 ${typeof description.steps}` +
+      (annotation ? ` (${annotation})` : ''));
+  }
+
+  const composition =
+    description.steps.map(step =>
+      ('toResolvedComposition' in step
+        ? compositeFrom(step.toResolvedComposition())
+        : step));
+
+  const inputMetadata = getStaticInputMetadata(description.inputs ?? {});
+
+  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.inputs ?? {})
+      .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 base = composition.at(-1);
+  const steps = composition.slice();
+
+  const aggregate = openAggregate({
+    message:
+      `Errors preparing composition` +
+      (annotation ? ` (${annotation})` : ''),
+  });
+
+  // TODO: Check description.compose ?? true instead.
+  const compositionNests = description.compose ?? true;
+
+  const exposeDependencies = new Set();
+
+  // 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 don't update unless the corresponding flag is explicitly set.
+  const stepsUpdate =
+    steps
+      .map(step =>
+        (step.flags
+          ? step.flags.update ?? false
+          : false));
+
+  // 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 =
+    anyInputsUseUpdateValue ||
+    anyStepsUseUpdateValue ||
+    anyStepsUpdate;
+
+  const stepEntries = stitchArrays({
+    step: steps,
+    expose: stepExposeDescriptions,
+    stepComposes: stepsCompose,
+    stepComputes: stepsCompute,
+    stepTransforms: stepsTransform,
+  });
+
+  for (let i = 0; i < stepEntries.length; i++) {
+    const {
+      step,
+      expose,
+      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 && !anyStepsUpdate && !anyStepsCompute) {
+    aggregate.push(new TypeError(`Expected at least one step to compute or update`));
+  }
+
+  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};
+  }
+
+  const continuationSymbol = Symbol.for('compositeFrom: continuation symbol');
+  const noTransformSymbol = Symbol.for('compositeFrom: no-transform symbol');
+
+  function _computeOrTransform(initialValue, continuationIfApplicable, initialDependencies) {
+    const expectingTransform = initialValue !== noTransformSymbol;
+
+    let valueSoFar =
+      (expectingTransform
+        ? initialValue
+        : undefined);
+
+    const availableDependencies = {...initialDependencies};
+
+    const inputValues =
+      ('inputs' in description
+        ? Object.fromEntries(Object.entries(description.inputs)
+            .map(([name, token]) => {
+              const tokenShape = getInputTokenShape(token);
+              const tokenValue = getInputTokenValue(token);
+              switch (tokenShape) {
+                case 'input.dependency':
+                  return [input(name), initialDependencies[tokenValue]];
+                case 'input.value':
+                  return [input(name), tokenValue];
+                case 'input.updateValue':
+                  if (!expectingTransform) {
+                    throw new Error(`Unexpected input.updateValue() accessed on non-transform call`);
+                  }
+                  return [input(name), valueSoFar];
+                case 'input.myself':
+                  return [input(name), initialDependencies['this']];
+                case 'input':
+                  return [input(name), initialDependencies[token]];
+                default:
+                  throw new TypeError(`Unexpected input shape ${tokenShape}`);
+              }
+            }))
+        : {});
+
+    if (expectingTransform) {
+      debug(() => [colors.bright(`begin composition - transforming from:`), initialValue]);
+    } else {
+      debug(() => colors.bright(`begin composition - not transforming`));
+    }
+
+    for (let i = 0; i < steps.length; i++) {
+      const step = steps[i];
+      const isBase = i === steps.length - 1;
+
+      debug(() => [
+        `step #${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 filteredDependencies =
+        filterProperties({
+          ...availableDependencies,
+          ...inputMetadata,
+          ...inputValues,
+          ...
+            (callingTransformForThisStep
+              ? {[input.updateValue()]: valueSoFar}
+              : {}),
+          [input.myself()]: initialDependencies['this'],
+        }, expose.dependencies ?? []);
+
+      debug(() => [
+        `step #${i+1} - ${callingTransformForThisStep ? 'transform' : 'compute'}`,
+        `with dependencies:`, filteredDependencies,
+        ...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);
+            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 ?? {}},
+        ...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 ? ` ${annotation}` : ''));
+        error.cause = thrownError;
+        throw error;
+      }
+    };
+
+    if (compositionNests) {
+      if (compositionUpdates) {
+        expose.transform = (value, continuation, dependencies) =>
+          _wrapper(value, continuation, dependencies);
+      }
+
+      if (anyStepsCompute) {
+        expose.compute = (continuation, dependencies) =>
+          _wrapper(noTransformSymbol, continuation, dependencies);
+      }
+
+      if (base.cacheComposition) {
+        expose.cache = base.cacheComposition;
+      }
+    } else if (compositionUpdates) {
+      expose.transform = (value, dependencies) =>
+        _wrapper(value, null, dependencies);
+    } else {
+      expose.compute = (dependencies) =>
+        _wrapper(noTransformSymbol, null, dependencies);
+    }
+  }
+
+  return constructedDescriptor;
+}
+
+export function displayCompositeCacheAnalysis() {
+  const showTimes = (cache, key) => {
+    const times = cache.times[key].slice().sort();
+
+    const all = times;
+    const worst10pc = times.slice(-times.length / 10);
+    const best10pc = times.slice(0, times.length / 10);
+    const middle50pc = times.slice(times.length / 4, -times.length / 4);
+    const middle80pc = times.slice(times.length / 10, -times.length / 10);
+
+    const fmt = val => `${(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;
+}
+
+// Exposes a dependency exactly as it is; this is typically the base of a
+// composition which was created to serve as one property's descriptor.
+//
+// Please note that this *doesn't* verify that the dependency exists, so
+// if you provide the wrong name or it hasn't been set by a previous
+// compositional step, the property will be exposed as undefined instead
+// of null.
+//
+export const exposeDependency = templateCompositeFrom({
+  annotation: `exposeDependency`,
+
+  compose: false,
+
+  inputs: {
+    dependency: input.staticDependency(),
+  },
+
+  steps: () => [
+    {
+      dependencies: [input('dependency')],
+      compute: ({
+        [input('dependency')]: dependency
+      }) => dependency,
+    },
+  ],
+});
+
+// Exposes a constant value exactly as it is; like exposeDependency, this
+// is typically the base of a composition serving as a particular property
+// descriptor. It generally follows steps which will conditionally early
+// exit with some other value, with the exposeConstant base serving as the
+// fallback default value.
+export const exposeConstant = templateCompositeFrom({
+  annotation: `exposeConstant`,
+
+  compose: false,
+
+  inputs: {
+    value: input.staticValue(),
+  },
+
+  steps: () => [
+    {
+      dependencies: [input('value')],
+      compute: ({
+        [input('value')]: value,
+      }) => value,
+    },
+  ],
+});
+
+// Checks the availability of a dependency and provides the result to later
+// steps under '#availability' (by default). This is mainly intended for use
+// by the more specific utilities, which you should consider using instead.
+// Customize {mode} to select one of these modes, or default to 'null':
+//
+// * 'null':  Check that the value isn't null (and not undefined either).
+// * 'empty': Check that the value is neither null, undefined, nor an empty
+//            array.
+// * 'falsy': Check that the value isn't false when treated as a boolean
+//            (nor an empty array). Keep in mind this will also be false
+//            for values like zero and the empty string!
+//
+
+const availabilityCheckModeInput = {
+  validate: oneOf('null', 'empty', 'falsy'),
+  defaultValue: 'null',
+};
+
+export const withResultOfAvailabilityCheck = templateCompositeFrom({
+  annotation: `withResultOfAvailabilityCheck`,
+
+  inputs: {
+    from: input(),
+    mode: input(availabilityCheckModeInput),
+  },
+
+  outputs: ['#availability'],
+
+  steps: () => [
+    {
+      dependencies: [input('from'), input('mode')],
+
+      compute: (continuation, {
+        [input('from')]: value,
+        [input('mode')]: mode,
+      }) => {
+        let availability;
+
+        switch (mode) {
+          case 'null':
+            availability = value !== undefined && value !== null;
+            break;
+
+          case 'empty':
+            availability = value !== undefined && !empty(value);
+            break;
+
+          case 'falsy':
+            availability = !!value && (!Array.isArray(value) || !empty(value));
+            break;
+        }
+
+        return continuation({'#availability': availability});
+      },
+    },
+  ],
+});
+
+// Exposes a dependency as it is, or continues if it's unavailable.
+// See withResultOfAvailabilityCheck for {mode} options!
+export const exposeDependencyOrContinue = templateCompositeFrom({
+  annotation: `exposeDependencyOrContinue`,
+
+  inputs: {
+    dependency: input(),
+    mode: input(availabilityCheckModeInput),
+  },
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('dependency'),
+      mode: input('mode'),
+    }),
+
+    {
+      dependencies: ['#availability', input('dependency')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('dependency')]: dependency,
+      }) =>
+        (availability
+          ? continuation.exit(dependency)
+          : continuation()),
+    },
+  ],
+});
+
+// Exposes the update value of an {update: true} property as it is,
+// or continues if it's unavailable. See withResultOfAvailabilityCheck
+// for {mode} options! Also provide {validate} here to conveniently
+// set a custom validation check for this property's update value.
+export const exposeUpdateValueOrContinue = templateCompositeFrom({
+  annotation: `exposeUpdateValueOrContinue`,
+
+  inputs: {
+    mode: input(availabilityCheckModeInput),
+    validate: input({type: 'function', null: true}),
+  },
+
+  update: ({
+    [input.staticValue('validate')]: validate,
+  }) =>
+    (validate
+      ? {validate}
+      : {}),
+
+  steps: () => [
+    exposeDependencyOrContinue({
+      dependency: input.updateValue(),
+      mode: input('mode'),
+    }),
+  ],
+});
+
+// Early exits if a dependency isn't available.
+// See withResultOfAvailabilityCheck for {mode} options!
+export const exitWithoutDependency = templateCompositeFrom({
+  annotation: `exitWithoutDependency`,
+
+  inputs: {
+    dependency: input(),
+    mode: input(availabilityCheckModeInput),
+    value: input({null: true}),
+  },
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('dependency'),
+      mode: input('mode'),
+    }),
+
+    {
+      dependencies: ['#availability', input('value')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('value')]: value,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.exit(value)),
+    },
+  ],
+});
+
+// Early exits if this property's update value isn't available.
+// See withResultOfAvailabilityCheck for {mode} options!
+export const exitWithoutUpdateValue = templateCompositeFrom({
+  annotation: `exitWithoutUpdateValue`,
+
+  inputs: {
+    mode: input(availabilityCheckModeInput),
+    value: input({defaultValue: null}),
+  },
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input.updateValue(),
+      mode: input('mode'),
+    }),
+  ],
+});
+
+// Raises if a dependency isn't available.
+// See withResultOfAvailabilityCheck for {mode} options!
+export const raiseOutputWithoutDependency = templateCompositeFrom({
+  annotation: `raiseOutputWithoutDependency`,
+
+  inputs: {
+    dependency: input(),
+    mode: input(availabilityCheckModeInput),
+    output: input.staticValue({defaultValue: {}}),
+  },
+
+  outputs: ({
+    [input.staticValue('output')]: output,
+  }) => Object.keys(output),
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('dependency'),
+      mode: input('mode'),
+    }),
+
+    {
+      dependencies: ['#availability', input('output')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('output')]: output,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.raiseOutputAbove(output)),
+    },
+  ],
+});
+
+// Raises if this property's update value isn't available.
+// See withResultOfAvailabilityCheck for {mode} options!
+export const raiseOutputWithoutUpdateValue = templateCompositeFrom({
+  annotation: `raiseOutputWithoutUpdateValue`,
+
+  inputs: {
+    mode: input(availabilityCheckModeInput),
+    output: input.staticValue({defaultValue: {}}),
+  },
+
+  outputs: ({
+    [input.staticValue('output')]: output,
+  }) => Object.keys(output),
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input.updateValue(),
+      mode: input('mode'),
+    }),
+
+    {
+      dependencies: ['#availability', input('output')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('output')]: output,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.raiseOutputAbove(output)),
+    },
+  ],
+});
+
+// Gets a property of some object (in a dependency) and provides that value.
+// If the object itself is null, or the object doesn't have the listed property,
+// the provided dependency will also be null.
+export const withPropertyFromObject = templateCompositeFrom({
+  annotation: `withPropertyFromObject`,
+
+  inputs: {
+    object: input({type: 'object', null: true}),
+    property: input({type: 'string'}),
+  },
+
+  outputs: ({
+    [input.staticDependency('object')]: object,
+    [input.staticValue('property')]: property,
+  }) => {
+    return [
+      (object && property
+        ? (object.startsWith('#')
+            ? `${object}.${property}`
+            : `#${object}.${property}`)
+        : '#value'),
+    ];
+  },
+
+  steps: () => [
+    {
+      dependencies: [
+        input.staticDependency('object'),
+        input.staticValue('property'),
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('object')]: object,
+        [input.staticValue('property')]: property,
+      }) => continuation({
+        '#output':
+          (object && property
+            ? (object.startsWith('#')
+                ? `${object}.${property}`
+                : `#${object}.${property}`)
+            : '#value'),
+      }),
+    },
+
+    {
+      dependencies: [
+        '#output',
+        input('object'),
+        input('property'),
+      ],
+
+      compute: (continuation, {
+        ['#output']: output,
+        [input('object')]: object,
+        [input('property')]: property,
+      }) => continuation({
+        [output]:
+          (object === null
+            ? null
+            : object[property] ?? null),
+      }),
+    },
+  ],
+});
+
+// Gets the listed properties from some object, providing each property's value
+// as a dependency prefixed with the same name as the object (by default).
+// If the object itself is null, all provided dependencies will be null;
+// if it's missing only select properties, those will be provided as null.
+export const withPropertiesFromObject = templateCompositeFrom({
+  annotation: `withPropertiesFromObject`,
+
+  inputs: {
+    object: input({
+      type: 'object',
+      null: true,
+    }),
+
+    properties: input({
+      validate: validateArrayItems(isString),
+    }),
+
+    prefix: input.staticValue({
+      type: 'string',
+      null: true,
+    }),
+  },
+
+  outputs: ({
+    [input.staticDependency('object')]: object,
+    [input.staticValue('properties')]: properties,
+    [input.staticValue('prefix')]: prefix,
+  }) =>
+    (properties
+      ? properties.map(property =>
+          (prefix
+            ? `${prefix}.${property}`
+         : object
+            ? `${object}.${property}`
+            : `#object.${property}`))
+      : '#object'),
+
+  steps: () => [
+    {
+      dependencies: [input('object'), input('properties')],
+      compute: (continuation, {
+        [input('object')]: object,
+        [input('properties')]: properties,
+      }) => continuation({
+        ['#entries']:
+          (object === null
+            ? properties.map(property => [property, null])
+            : properties.map(property => [property, object[property]])),
+      }),
+    },
+
+    {
+      dependencies: [
+        input.staticDependency('object'),
+        input.staticValue('properties'),
+        input.staticValue('prefix'),
+        '#entries',
+      ],
+
+      compute: ({
+        [input.staticDependency('object')]: object,
+        [input.staticValue('properties')]: properties,
+        [input.staticValue('prefix')]: prefix,
+        ['#entries']: entries,
+      }) =>
+        (properties
+          ? continuation(
+              Object.fromEntries(
+                entries.map(([property, value]) => [
+                  (prefix
+                    ? `${prefix}.${property}`
+                 : object
+                    ? `${object}.${property}`
+                    : `#object.${property}`),
+                  value ?? null,
+                ])))
+          : continuation({
+              ['#object']:
+                Object.fromEntries(entries),
+            })),
+    },
+  ],
+});
+
+// Gets a property from each of a list of objects (in a dependency) and
+// provides the results. This doesn't alter any list indices, so positions
+// which were null in the original list are kept null here. Objects which don't
+// have the specified property are retained in-place as null.
+export function withPropertyFromList({
+  list,
+  property,
+  into = null,
+}) {
+  into ??=
+    (list.startsWith('#')
+      ? `${list}.${property}`
+      : `#${list}.${property}`);
+
+  return {
+    annotation: `withPropertyFromList`,
+    flags: {expose: true, compose: true},
+
+    expose: {
+      mapDependencies: {list},
+      mapContinuation: {into},
+      options: {property},
+
+      compute(continuation, {list, '#options': {property}}) {
+        if (list === undefined || empty(list)) {
+          return continuation({into: []});
+        }
+
+        return continuation({
+          into:
+            list.map(item =>
+              (item === null || item === undefined
+                ? null
+                : item[property] ?? null)),
+        });
+      },
+    },
+  };
+}
+
+// Gets the listed properties from each of a list of objects, providing lists
+// of property values each into a dependency prefixed with the same name as the
+// list (by default). Like withPropertyFromList, this doesn't alter indices.
+export const withPropertiesFromList = templateCompositeFrom({
+  annotation: `withPropertiesFromList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+
+    properties: input({
+      validate: validateArrayItems(isString),
+    }),
+
+    prefix: input.staticValue({
+      type: 'string',
+      null: true,
+    }),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+    [input.staticValue('properties')]: properties,
+    [input.staticValue('prefix')]: prefix,
+  }) =>
+    (properties
+      ? properties.map(property =>
+          (prefix
+            ? `${prefix}.${property}`
+         : list
+            ? `${list}.${property}`
+            : `#list.${property}`))
+      : '#lists'),
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('properties')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('properties')]: properties,
+      }) => continuation({
+        ['#lists']:
+          Object.fromEntries(
+            properties.map(property => [
+              property,
+              list.map(item => item[property] ?? null),
+            ])),
+      }),
+    },
+
+    {
+      dependencies: [
+        input.staticDependency('list'),
+        input.staticValue('properties'),
+        input.staticValue('prefix'),
+        '#lists',
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('list')]: list,
+        [input.staticValue('properties')]: properties,
+        [input.staticValue('prefix')]: prefix,
+        ['#lists']: lists,
+      }) =>
+        (properties
+          ? continuation(
+              Object.fromEntries(
+                properties.map(property => [
+                  (prefix
+                    ? `${prefix}.${property}`
+                 : list
+                    ? `${list}.${property}`
+                    : `#list.${property}`),
+                  lists[property],
+                ])))
+          : continuation({'#lists': lists})),
+    },
+  ],
+});
+
+// Replaces items of a list, which are null or undefined, with some fallback
+// value. By default, this replaces the passed dependency.
+export const fillMissingListItems = templateCompositeFrom({
+  annotation: `fillMissingListItems`,
+
+  inputs: {
+    list: input({type: 'array'}),
+    fill: input(),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+  }) => [list ?? '#list'],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('fill')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('fill')]: fill,
+      }) => continuation({
+        ['#filled']:
+          list.map(item => item ?? fill),
+      }),
+    },
+
+    {
+      dependencies: [input.staticDependency('list'), '#filled'],
+      compute: (continuation, {
+        [input.staticDependency('list')]: list,
+        ['#filled']: filled,
+      }) => continuation({
+        [list ?? '#list']:
+          filled,
+      }),
+    },
+  ],
+});
+
+// Filters particular values out of a list. Note that this will always
+// completely skip over null, but can be used to filter out any other
+// primitive or object value.
+export const excludeFromList = templateCompositeFrom({
+  annotation: `excludeFromList`,
+
+  inputs: {
+    list: input(),
+
+    item: input({null: true}),
+    items: input({validate: isArray, null: true}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+  }) => [list ?? '#list'],
+
+  steps: () => [
+    {
+      dependencies: [
+        input.staticDependency('list'),
+        input('list'),
+        input('item'),
+        input('items'),
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('list')]: listName,
+        [input('list')]: listContents,
+        [input('item')]: excludeItem,
+        [input('items')]: excludeItems,
+      }) => continuation({
+        [listName ?? '#list']:
+          listContents.filter(item => {
+            if (excludeItem !== null && item === excludeItem) return false;
+            if (!empty(excludeItems) && excludeItems.includes(item)) return false;
+            return true;
+          }),
+      }),
+    },
+  ],
+});
+
+// Flattens an array with one level of nested arrays, providing as dependencies
+// both the flattened array as well as the original starting indices of each
+// successive source array.
+export const withFlattenedList = templateCompositeFrom({
+  annotation: `withFlattenedList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+  },
+
+  outputs: ['#flattenedList', '#flattenedIndices'],
+
+  steps: () => [
+    {
+      dependencies: [input('list')],
+      compute(continuation, {
+        [input('list')]: sourceList,
+      }) {
+        const flattenedList = sourceList.flat();
+        const indices = [];
+        let lastEndIndex = 0;
+        for (const {length} of sourceList) {
+          indices.push(lastEndIndex);
+          lastEndIndex += length;
+        }
+
+        return continuation({
+          ['#flattenedList']: flattenedList,
+          ['#flattenedIndices']: indices,
+        });
+      },
+    },
+  ],
+});
+
+// After mapping the contents of a flattened array in-place (being careful to
+// retain the original indices by replacing unmatched results with null instead
+// of filtering them out), this function allows for recombining them. It will
+// filter out null and undefined items by default (pass {filter: false} to
+// disable this).
+export const withUnflattenedList = templateCompositeFrom({
+  annotation: `withUnflattenedList`,
+
+  inputs: {
+    list: input({
+      type: 'array',
+      defaultDependency: '#flattenedList',
+    }),
+
+    indices: input({
+      validate: validateArrayItems(isWholeNumber),
+      defaultDependency: '#flattenedIndices',
+    }),
+
+    filter: input({
+      type: 'boolean',
+      defaultValue: true,
+    }),
+  },
+
+  outputs: ['#unflattenedList'],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('indices'), input('filter')],
+      compute(continuation, {
+        [input('list')]: list,
+        [input('indices')]: indices,
+        [input('filter')]: filter,
+      }) {
+        const unflattenedList = [];
+
+        for (let i = 0; i < indices.length; i++) {
+          const startIndex = indices[i];
+          const endIndex =
+            (i === indices.length - 1
+              ? list.length
+              : indices[i + 1]);
+
+          const values = list.slice(startIndex, endIndex);
+          unflattenedList.push(
+            (filter
+              ? values.filter(value => value !== null && value !== undefined)
+              : values));
+        }
+
+        return continuation({
+          ['#unflattenedList']: unflattenedList,
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/things/flash.js b/src/data/things/flash.js
index 6eb5234f..c3f90260 100644
--- a/src/data/things/flash.js
+++ b/src/data/things/flash.js
@@ -1,25 +1,33 @@
+import {input} from '#composite';
 import find from '#find';
 
-import Thing from './thing.js';
+import {
+  isColor,
+  isDirectory,
+  isNumber,
+  isString,
+  oneOf,
+} from '#validators';
+
+import Thing, {
+  color,
+  contributionList,
+  fileExtension,
+  name,
+  referenceList,
+  simpleDate,
+  simpleString,
+  urls,
+  wikiData,
+} 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},
@@ -47,39 +55,35 @@ export class Flash extends Thing {
       },
     },
 
-    date: Thing.common.simpleDate(),
+    date: simpleDate(),
 
-    coverArtFileExtension: Thing.common.fileExtension('jpg'),
+    coverArtFileExtension: fileExtension('jpg'),
 
-    contributorContribsByRef: Thing.common.contribsByRef(),
+    contributorContribs: contributionList(),
 
-    featuredTracksByRef: Thing.common.referenceList(Track),
+    featuredTracks: referenceList({
+      class: input.value(Track),
+      find: input.value(find.track),
+      data: 'trackData',
+    }),
 
-    urls: Thing.common.urls(),
+    urls: urls(),
 
     // Update only
 
-    artistData: Thing.common.wikiData(Artist),
-    trackData: Thing.common.wikiData(Track),
-    flashActData: Thing.common.wikiData(FlashAct),
+    artistData: wikiData(Artist),
+    trackData: wikiData(Track),
+    flashActData: wikiData(FlashAct),
 
     // Expose only
 
-    contributorContribs: Thing.common.dynamicContribs('contributorContribsByRef'),
-
-    featuredTracks: Thing.common.dynamicThingsFromReferenceList(
-      'featuredTracksByRef',
-      'trackData',
-      find.track
-    ),
-
     act: {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['flashActData'],
+        dependencies: ['this', 'flashActData'],
 
-        compute: ({flashActData, [Flash.instance]: flash}) =>
+        compute: ({this: flash, flashActData}) =>
           flashActData.find((act) => act.flashes.includes(flash)) ?? null,
       },
     },
@@ -88,9 +92,9 @@ export class Flash extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['flashActData'],
+        dependencies: ['this', 'flashActData'],
 
-        compute: ({flashActData, [Flash.instance]: flash}) =>
+        compute: ({this: flash, flashActData}) =>
           flashActData.find((act) => act.flashes.includes(flash))?.color ?? null,
       },
     },
@@ -111,17 +115,13 @@ export class Flash extends Thing {
 }
 
 export class FlashAct extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    validators: {
-      isColor,
-    },
-  }) => ({
+  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'),
+    color: color(),
+    anchor: simpleString(),
+    jump: simpleString(),
 
     jumpColor: {
       flags: {update: true, expose: true},
@@ -133,18 +133,14 @@ 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..0b117801 100644
--- a/src/data/things/group.js
+++ b/src/data/things/group.js
@@ -1,33 +1,42 @@
+import {input} from '#composite';
 import find from '#find';
 
-import Thing from './thing.js';
+import Thing, {
+  color,
+  directory,
+  name,
+  referenceList,
+  simpleString,
+  urls,
+  wikiData,
+} 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 +50,8 @@ 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 +60,8 @@ 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 +71,8 @@ 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 +81,20 @@ export class Group extends Thing {
 }
 
 export class GroupCategory extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    Group,
-  }) => ({
+  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..bade280c 100644
--- a/src/data/things/homepage-layout.js
+++ b/src/data/things/homepage-layout.js
@@ -1,20 +1,36 @@
 import find from '#find';
 
-import Thing from './thing.js';
+import {
+  compositeFrom,
+  exposeDependency,
+  input,
+} from '#composite';
+
+import {
+  is,
+  isCountingNumber,
+  isString,
+  isStringNonEmpty,
+  oneOf,
+  validateArrayItems,
+  validateInstanceOf,
+  validateReference,
+} from '#validators';
+
+import Thing, {
+  color,
+  name,
+  referenceList,
+  simpleString,
+  wikiData,
+  withResolvedReference,
+} from './thing.js';
 
 export class HomepageLayout extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    HomepageLayoutRow,
-
-    validators: {
-      isStringNonEmpty,
-      validateArrayItems,
-      validateInstanceOf,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({HomepageLayoutRow}) => ({
     // Update & expose
 
-    sidebarContent: Thing.common.simpleString(),
+    sidebarContent: simpleString(),
 
     navbarLinks: {
       flags: {update: true, expose: true},
@@ -32,13 +48,10 @@ export class HomepageLayout extends Thing {
 }
 
 export class HomepageLayoutRow extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-    Group,
-  }) => ({
+  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 +63,20 @@ 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.getPropertyDescriptors] = (opts, {Album, Group} = opts) => ({
     ...HomepageLayoutRow[Thing.getPropertyDescriptors](opts),
 
     // Update & expose
@@ -104,8 +107,39 @@ 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 +150,5 @@ 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..77e5fa76 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';
 
@@ -82,6 +82,8 @@ 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 +121,7 @@ function descriptorAggregateHelper({
 }
 
 function evaluatePropertyDescriptors() {
-  const opts = {...allClasses, validators};
+  const opts = {...allClasses};
 
   return descriptorAggregateHelper({
     message: `Errors evaluating Thing class property descriptors`,
@@ -129,8 +131,21 @@ 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 afa9f1ee..a325d6a6 100644
--- a/src/data/things/language.js
+++ b/src/data/things/language.js
@@ -1,16 +1,16 @@
-import Thing from './thing.js';
-
 import {Tag} from '#html';
 import {isLanguageCode} from '#validators';
 
 import CacheableObject from './cacheable-object.js';
 
+import Thing, {
+  externalFunction,
+  flag,
+  simpleString,
+} 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
@@ -23,7 +23,7 @@ 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
@@ -45,7 +45,7 @@ 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.
@@ -73,7 +73,7 @@ export class Language extends Thing {
 
     // Update only
 
-    escapeHTML: Thing.common.externalFunction({expose: true}),
+    escapeHTML: externalFunction(),
 
     // Expose only
 
@@ -192,7 +192,7 @@ export class Language extends Thing {
   // html.Tag objects, which are treated as sanitized by default (so that they
   // can be nested inside strings at all).
   #sanitizeStringArg(arg) {
-    const escapeHTML = this.escapeHTML;
+    const escapeHTML = CacheableObject.getUpdateValue(this, 'escapeHTML');
 
     if (!escapeHTML) {
       throw new Error(`escapeHTML unavailable`);
@@ -224,7 +224,7 @@ export class Language extends Thing {
   // contents of a slot directly, it should be manually sanitized with this
   // function first.
   sanitize(arg) {
-    const escapeHTML = this.escapeHTML;
+    const escapeHTML = CacheableObject.getUpdateValue(this, 'escapeHTML');
 
     if (!escapeHTML) {
       throw new Error(`escapeHTML unavailable`);
diff --git a/src/data/things/news-entry.js b/src/data/things/news-entry.js
index 43911410..6984874e 100644
--- a/src/data/things/news-entry.js
+++ b/src/data/things/news-entry.js
@@ -1,4 +1,9 @@
-import Thing from './thing.js';
+import Thing, {
+  directory,
+  name,
+  simpleDate,
+  simpleString,
+} from './thing.js';
 
 export class NewsEntry extends Thing {
   static [Thing.referenceType] = 'news-entry';
@@ -6,11 +11,11 @@ export class NewsEntry extends Thing {
   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..0133e0b6 100644
--- a/src/data/things/static-page.js
+++ b/src/data/things/static-page.js
@@ -1,16 +1,18 @@
-import Thing from './thing.js';
+import {isName} from '#validators';
+
+import Thing, {
+  directory,
+  name,
+  simpleString,
+} from './thing.js';
 
 export class StaticPage extends Thing {
   static [Thing.referenceType] = 'static';
 
-  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 +24,8 @@ 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 5705ee7e..77f549fe 100644
--- a/src/data/things/thing.js
+++ b/src/data/things/thing.js
@@ -3,10 +3,24 @@
 
 import {inspect} from 'node:util';
 
-import {color} from '#cli';
+import {colors} from '#cli';
 import find from '#find';
-import {empty} from '#sugar';
-import {getKebabCase} from '#wiki-data';
+import {stitchArrays, unique} from '#sugar';
+import {filterMultipleArrays, getKebabCase} from '#wiki-data';
+import {oneOf} from '#validators';
+
+import {
+  compositeFrom,
+  exitWithoutDependency,
+  exposeConstant,
+  exposeDependency,
+  exposeDependencyOrContinue,
+  input,
+  raiseOutputWithoutDependency,
+  templateCompositeFrom,
+  withResultOfAvailabilityCheck,
+  withPropertiesFromList,
+} from '#composite';
 
 import {
   isAdditionalFileList,
@@ -15,10 +29,13 @@ import {
   isColor,
   isContributionList,
   isDate,
+  isDimensions,
   isDirectory,
+  isDuration,
   isFileExtension,
   isName,
   isString,
+  isType,
   isURL,
   validateArrayItems,
   validateInstanceOf,
@@ -34,388 +51,687 @@ export default class Thing extends CacheableObject {
   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},
-    }),
+  // Default custom inspect function, which may be overridden by Thing
+  // subclasses. This will be used when displaying aggregate errors and other
+  // command-line logging - it's the place to provide information useful in
+  // identifying the Thing being presented.
+  [inspect.custom]() {
+    const cname = this.constructor.name;
 
-    color: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: isColor},
-    }),
+    return (
+      (this.name ? `${cname} ${colors.green(`"${this.name}"`)}` : `${cname}`) +
+      (this.directory ? ` (${colors.blue(Thing.getReference(this))})` : '')
+    );
+  }
 
-    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;
-        },
+  static getReference(thing) {
+    if (!thing.constructor[Thing.referenceType]) {
+      throw TypeError(`Passed Thing is ${thing.constructor.name}, which provides no [Thing.referenceType]`);
+    }
+
+    if (!thing.directory) {
+      throw TypeError(`Passed ${thing.constructor.name} is missing its directory`);
+    }
+
+    return `${thing.constructor[Thing.referenceType]}:${thing.directory}`;
+  }
+}
+
+// Property descriptor templates
+//
+// 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!
+
+export function name(defaultName) {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isName, default: defaultName},
+  };
+}
+
+export function color() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isColor},
+  };
+}
+
+export function directory() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDirectory},
+    expose: {
+      dependencies: ['name'],
+      transform(directory, {name}) {
+        if (directory === null && name === null) return null;
+        else if (directory === null) return getKebabCase(name);
+        else return directory;
       },
-    }),
+    },
+  };
+}
 
-    urls: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: validateArrayItems(isURL)},
-      expose: {transform: (value) => value ?? []},
-    }),
+export function urls() {
+  return {
+    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},
-    }),
+// A file extension! Or the default, if provided when calling this.
+export function fileExtension(defaultFileExtension = null) {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isFileExtension},
+    expose: {transform: (value) => value ?? defaultFileExtension},
+  };
+}
+
+// Plain ol' image dimensions. This is a two-item array of positive integers,
+// corresponding to width and height respectively.
+export function dimensions() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDimensions},
+  };
+}
+
+// Duration! This is a number of seconds, possibly floating point, always
+// at minimum zero.
+export function duration() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDuration},
+  };
+}
+
+// Straightforward flag descriptor for a variety of property purposes.
+// Provide a default value, true or false!
+export function flag(defaultValue = false) {
+  // TODO:                        ^ Are you actually kidding me
+  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.
+export function simpleDate() {
+  return {
+    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.
+export function simpleString() {
+  return {
+    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.
+export function externalFunction() {
+  return {
+    flags: {update: true},
+    update: {validate: (t) => typeof t === 'function'},
+  };
+}
 
-    // 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},
-      };
+// Strong 'n sturdy contribution list, rolling a list of references (provided
+// as this property's update value) and the resolved results (as get exposed)
+// into one property. Update value will look something like this:
+//
+//   [
+//     {who: 'Artist Name', what: 'Viola'},
+//     {who: 'artist:john-cena', what: null},
+//     ...
+//   ]
+//
+// ...typically as processed from YAML, spreadsheet, or elsewhere.
+// Exposes as the same, but with the "who" replaced with matches found in
+// artistData - which means this always depends on an `artistData` property
+// also existing on this object!
+//
+export function contributionList() {
+  return compositeFrom({
+    annotation: `contributionList`,
+
+    compose: false,
+
+    update: {validate: isContributionList},
+
+    steps: [
+      withResolvedContribs({from: input.updateValue()}),
+      exposeDependencyOrContinue({dependency: '#resolvedContribs'}),
+      exposeConstant({value: input.value([])}),
+    ],
+  });
+}
+
+// Artist commentary! Generally present on tracks and albums.
+export function commentary() {
+  return {
+    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: [...]},
+//         ...
+//     ]
+//
+export function additionalFiles() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isAdditionalFileList},
+    expose: {
+      transform: (additionalFiles) =>
+        additionalFiles ?? [],
     },
+  };
+}
 
-    // 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},
-    }),
+const thingClassInput = {
+  validate(thingClass) {
+    isType(thingClass, 'function');
 
-    // 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},
-    }),
+    if (!Object.hasOwn(thingClass, Thing.referenceType)) {
+      throw new TypeError(`Expected a Thing constructor, missing Thing.referenceType`);
+    }
 
-    // External function. These should only be used as dependencies for other
-    // properties, so they're left unexposed.
-    externalFunction: ({expose = false} = {}) => ({
-      flags: {update: true, expose},
-      update: {validate: (t) => typeof t === 'function'},
+    return true;
+  },
+};
+
+// 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.
+export const referenceList = templateCompositeFrom({
+  annotation: `referenceList`,
+
+  compose: false,
+
+  inputs: {
+    class: input.staticValue(thingClassInput),
+
+    find: input({type: 'function'}),
+
+    // todo: validate
+    data: input(),
+  },
+
+  update: ({
+    [input.staticValue('class')]: thingClass,
+  }) => {
+    const {[Thing.referenceType]: referenceType} = thingClass;
+    return {validate: validateReferenceList(referenceType)};
+  },
+
+  steps: () => [
+    withResolvedReferenceList({
+      list: input.updateValue(),
+      data: input('data'),
+      find: input('find'),
     }),
 
-    // 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},
+    exposeDependency({dependency: '#resolvedReferenceList'}),
+  ],
+});
+
+// Corresponding function for a single reference.
+export const singleReference = templateCompositeFrom({
+  annotation: `singleReference`,
+
+  compose: false,
+
+  inputs: {
+    class: input(thingClassInput),
+    find: input({type: 'function'}),
+
+    // todo: validate
+    data: input(),
+  },
+
+  update: ({
+    [input.staticValue('class')]: thingClass,
+  }) => {
+    const {[Thing.referenceType]: referenceType} = thingClass;
+    return {validate: validateReference(referenceType)};
+  },
+
+  steps: () => [
+    withResolvedReference({
+      ref: input.updateValue(),
+      data: input('data'),
+      find: input('find'),
     }),
 
-    // Artist commentary! Generally present on tracks and albums.
-    commentary: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: isCommentary},
+    exposeDependency({dependency: '#resolvedReference'}),
+  ],
+});
+
+// Nice 'n simple shorthand for an exposed-only flag which is true when any
+// contributions are present in the specified property.
+export const contribsPresent = templateCompositeFrom({
+  annotation: `contribsPresent`,
+
+  compose: false,
+
+  inputs: {
+    contribs: input({type: 'string'}),
+  },
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('contribs'),
+      mode: input.value('empty'),
     }),
 
-    // 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 ?? [],
-      },
+    exposeDependency({dependency: '#availability'}),
+  ],
+});
+
+// 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.
+export const reverseReferenceList = templateCompositeFrom({
+  annotation: `reverseReferenceList`,
+
+  compose: false,
+
+  inputs: {
+    // todo: validate
+    data: input(),
+
+    list: input({type: 'string'}),
+  },
+
+  steps: () => [
+    withReverseReferenceList({
+      data: input('data'),
+      list: input('list'),
     }),
 
-    // 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)},
-      };
+    exposeDependency({dependency: '#reverseReferenceList'}),
+  ],
+});
+
+// General purpose wiki data constructor, for properties like artistData,
+// trackData, etc.
+export function wikiData(thingClass) {
+  return {
+    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.
+export const commentatorArtists = templateCompositeFrom({
+  annotation: `commentatorArtists`,
+
+  compose: false,
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: 'commentary',
+      mode: input.value('falsy'),
+      value: input.value([]),
+    }),
 
-    // 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)},
-      };
+    {
+      dependencies: ['commentary'],
+      compute: (continuation, {commentary}) =>
+        continuation({
+          '#artistRefs':
+            Array.from(
+              commentary
+                .replace(/<\/?b>/g, '')
+                .matchAll(/<i>(?<who>.*?):<\/i>/g))
+              .map(({groups: {who}}) => who),
+        }),
     },
 
-    // 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
-    ) => ({
+    withResolvedReferenceList({
+      list: '#artistRefs',
+      data: 'artistData',
+      find: input.value(find.artist),
+    }).outputs({
+      '#resolvedReferenceList': '#artists',
+    }),
+
+    {
       flags: {expose: true},
 
       expose: {
-        dependencies: [referenceListProperty, thingDataProperty],
-        compute: ({
-          [referenceListProperty]: refs,
-          [thingDataProperty]: thingData,
-        }) =>
-          refs && thingData
-            ? refs
-                .map((ref) => findFn(ref, thingData, {mode: 'quiet'}))
-                .filter(Boolean)
-            : [],
+        dependencies: ['#artists'],
+        compute: ({'#artists': artists}) =>
+          unique(artists),
       },
+    },
+  ],
+});
+
+// Compositional utilities
+
+// Resolves the contribsByRef contained in the provided dependency,
+// providing (named by the second argument) the result. "Resolving"
+// means mapping the "who" reference of each contribution to an artist
+// object, and filtering out those whose "who" doesn't match any artist.
+export const withResolvedContribs = templateCompositeFrom({
+  annotation: `withResolvedContribs`,
+
+  inputs: {
+    // todo: validate
+    from: input(),
+
+    notFoundMode: input({
+      validate: oneOf('exit', 'filter', 'null'),
+      defaultValue: 'null',
     }),
+  },
 
-    // Corresponding function for a single reference.
-    dynamicThingFromSingleReference: (
-      singleReferenceProperty,
-      thingDataProperty,
-      findFn
-    ) => ({
-      flags: {expose: true},
+  outputs: ['#resolvedContribs'],
 
-      expose: {
-        dependencies: [singleReferenceProperty, thingDataProperty],
-        compute: ({
-          [singleReferenceProperty]: ref,
-          [thingDataProperty]: thingData,
-        }) => (ref && thingData ? findFn(ref, thingData, {mode: 'quiet'}) : null),
-      },
+  steps: () => [
+    raiseOutputWithoutDependency({
+      dependency: input('from'),
+      mode: input.value('empty'),
+      output: input.value({
+        ['#resolvedContribs']: [],
+      }),
     }),
 
-    // 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)
-            : [],
-      },
+    withPropertiesFromList({
+      list: input('from'),
+      properties: input.value(['who', 'what']),
+      prefix: input.value('#contribs'),
     }),
 
-    // 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);
-        },
+    withResolvedReferenceList({
+      list: '#contribs.who',
+      data: 'artistData',
+      find: input.value(find.artist),
+      notFoundMode: input('notFoundMode'),
+    }).outputs({
+      ['#resolvedReferenceList']: '#contribs.who',
+    }),
+
+    {
+      dependencies: ['#contribs.who', '#contribs.what'],
+
+      compute(continuation, {
+        ['#contribs.who']: who,
+        ['#contribs.what']: what,
+      }) {
+        filterMultipleArrays(who, what, (who, _what) => who);
+        return continuation({
+          ['#resolvedContribs']: stitchArrays({who, what}),
+        });
       },
+    },
+  ],
+});
+
+// Shorthand for exiting if the contribution list (usually a property's update
+// value) resolves to empty - ensuring that the later computed results are only
+// returned if these contributions are present.
+export const exitWithoutContribs = templateCompositeFrom({
+  annotation: `exitWithoutContribs`,
+
+  inputs: {
+    // todo: validate
+    contribs: input(),
+
+    value: input({null: true}),
+  },
+
+  steps: () => [
+    withResolvedContribs({
+      from: input('contribs'),
     }),
 
-    // 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);
-        },
-      }
+    withResultOfAvailabilityCheck({
+      from: '#resolvedContribs',
+      mode: input.value('empty'),
     }),
 
-    // 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},
+    {
+      dependencies: ['#availability', input('value')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('value')]: value,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.exit(value)),
+    },
+  ],
+});
+
+// Resolves a reference by using the provided find function to match it
+// within the provided thingData dependency. This will early exit if the
+// data dependency is null, or, if notFoundMode is set to 'exit', if the find
+// function doesn't match anything for the reference. Otherwise, the data
+// object is provided on the output dependency; or null, if the reference
+// doesn't match anything or itself was null to begin with.
+export const withResolvedReference = templateCompositeFrom({
+  annotation: `withResolvedReference`,
+
+  inputs: {
+    // todo: validate
+    ref: input(),
+
+    // todo: validate
+    data: input(),
+
+    find: input({type: 'function'}),
+
+    notFoundMode: input({
+      validate: oneOf('null', 'exit'),
+      defaultValue: 'null',
+    }),
+  },
 
-      expose: {
-        dependencies: [thingDataProperty],
+  outputs: ['#resolvedReference'],
 
-        compute: ({[thingDataProperty]: thingData, [Thing.instance]: thing}) =>
-          thingData?.filter(t => t[referencerRefListProperty].includes(thing)) ?? [],
-      },
+  steps: () => [
+    raiseOutputWithoutDependency({
+      dependency: input('ref'),
+      output: input.value({
+        ['#resolvedReference']: null,
+      }),
     }),
 
-    // 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],
+    exitWithoutDependency({
+      dependency: input('data'),
+    }),
 
-        compute: ({[thingDataProperty]: thingData, [Thing.instance]: thing}) =>
-          thingData?.filter((t) => t[referencerRefListProperty] === thing) ?? [],
+    {
+      dependencies: [
+        input('ref'),
+        input('data'),
+        input('find'),
+        input('notFoundMode'),
+      ],
+
+      compute(continuation, {
+        [input('ref')]: ref,
+        [input('data')]: data,
+        [input('find')]: findFunction,
+        [input('notFoundMode')]: notFoundMode,
+      }) {
+        const match = findFunction(ref, data, {mode: 'quiet'});
+
+        if (match === null && notFoundMode === 'exit') {
+          return continuation.exit(null);
+        }
+
+        return continuation.raiseOutput({
+          ['#resolvedReference']: match ?? null,
+        });
       },
+    },
+  ],
+});
+
+// Resolves a list of references, with each reference matched with provided
+// data in the same way as withResolvedReference. This will early exit if the
+// data dependency is null (even if the reference list is empty). By default
+// it will filter out references which don't match, but this can be changed
+// to early exit ({notFoundMode: 'exit'}) or leave null in place ('null').
+export const withResolvedReferenceList = templateCompositeFrom({
+  annotation: `withResolvedReferenceList`,
+
+  inputs: {
+    // todo: validate
+    list: input(),
+
+    // todo: validate
+    data: input(),
+
+    find: input({type: 'function'}),
+
+    notFoundMode: input({
+      validate: oneOf('exit', 'filter', 'null'),
+      defaultValue: 'filter',
     }),
+  },
 
-    // General purpose wiki data constructor, for properties like artistData,
-    // trackData, etc.
-    wikiData: (thingClass) => ({
-      flags: {update: true},
-      update: {
-        validate: validateArrayItems(validateInstanceOf(thingClass)),
-      },
+  outputs: ['#resolvedReferenceList'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input('data'),
+      value: input.value([]),
     }),
 
-    // 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},
+    raiseOutputWithoutDependency({
+      dependency: input('list'),
+      mode: input.value('empty'),
+      output: input.value({
+        ['#resolvedReferenceList']: [],
+      }),
+    }),
 
-      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'})
-                  )
-                )
-              )
-            : [],
+    {
+      dependencies: [input('list'), input('data'), input('find')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('data')]: data,
+        [input('find')]: findFunction,
+      }) =>
+        continuation({
+          '#matches': list.map(ref => findFunction(ref, data, {mode: 'quiet'})),
+        }),
+    },
+
+    {
+      dependencies: ['#matches'],
+      compute: (continuation, {'#matches': matches}) =>
+        (matches.every(match => match)
+          ? continuation.raiseOutput({
+              ['#resolvedReferenceList']: matches,
+            })
+          : continuation()),
+    },
+
+    {
+      dependencies: ['#matches', input('notFoundMode')],
+      compute(continuation, {
+        ['#matches']: matches,
+        [input('notFoundMode')]: notFoundMode,
+      }) {
+        switch (notFoundMode) {
+          case 'exit':
+            return continuation.exit([]);
+
+          case 'filter':
+            return continuation.raiseOutput({
+              ['#resolvedReferenceList']:
+                matches.filter(match => match),
+            });
+
+          case 'null':
+            return continuation.raiseOutput({
+              ['#resolvedReferenceList']:
+                matches.map(match => match ?? null),
+            });
+
+          default:
+            throw new TypeError(`Expected notFoundMode to be exit, filter, or null`);
+        }
       },
-    }),
-  };
+    },
+  ],
+});
 
-  // Default custom inspect function, which may be overridden by Thing
-  // subclasses. This will be used when displaying aggregate errors and other
-  // command-line logging - it's the place to provide information useful in
-  // identifying the Thing being presented.
-  [inspect.custom]() {
-    const cname = this.constructor.name;
+// Check out the info on reverseReferenceList!
+// This is its composable form.
+export const withReverseReferenceList = templateCompositeFrom({
+  annotation: `withReverseReferenceList`,
 
-    return (
-      (this.name ? `${cname} ${color.green(`"${this.name}"`)}` : `${cname}`) +
-      (this.directory ? ` (${color.blue(Thing.getReference(this))})` : '')
-    );
-  }
+  inputs: {
+    // todo: validate
+    data: input(),
 
-  static getReference(thing) {
-    if (!thing.constructor[Thing.referenceType]) {
-      throw TypeError(`Passed Thing is ${thing.constructor.name}, which provides no [Thing.referenceType]`);
-    }
+    list: input({type: 'string'}),
+  },
 
-    if (!thing.directory) {
-      throw TypeError(`Passed ${thing.constructor.name} is missing its directory`);
-    }
+  outputs: ['#reverseReferenceList'],
 
-    return `${thing.constructor[Thing.referenceType]}:${thing.directory}`;
-  }
-}
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input('data'),
+      value: input.value([]),
+    }),
+
+    {
+      dependencies: [input.myself(), input('data'), input('list')],
+
+      compute: (continuation, {
+        [input.myself()]: thisThing,
+        [input('data')]: data,
+        [input('list')]: refListProperty,
+      }) =>
+        continuation({
+          ['#reverseReferenceList']:
+            data.filter(thing => thing[refListProperty].includes(thisThing)),
+        }),
+    },
+  ],
+});
diff --git a/src/data/things/track.js b/src/data/things/track.js
index 14510d96..3e0d95bf 100644
--- a/src/data/things/track.js
+++ b/src/data/things/track.js
@@ -1,330 +1,343 @@
 import {inspect} from 'node:util';
 
-import {color} from '#cli';
+import {colors} from '#cli';
 import find from '#find';
 import {empty} from '#sugar';
 
-import Thing from './thing.js';
+import {
+  exitWithoutDependency,
+  excludeFromList,
+  exposeConstant,
+  exposeDependency,
+  exposeDependencyOrContinue,
+  exposeUpdateValueOrContinue,
+  input,
+  raiseOutputWithoutDependency,
+  templateCompositeFrom,
+  withPropertyFromObject,
+} from '#composite';
+
+import {
+  isBoolean,
+  isColor,
+  isContributionList,
+  isDate,
+  isFileExtension,
+  oneOf,
+} from '#validators';
+
+import CacheableObject from './cacheable-object.js';
+
+import Thing, {
+  additionalFiles,
+  commentary,
+  commentatorArtists,
+  contributionList,
+  directory,
+  duration,
+  flag,
+  name,
+  referenceList,
+  reverseReferenceList,
+  simpleDate,
+  singleReference,
+  simpleString,
+  urls,
+  wikiData,
+  withResolvedContribs,
+  withResolvedReference,
+  withReverseReferenceList,
+} 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(),
-
-    // Controls how find.track works - it'll never be matched by a reference
-    // just to the track's name, which means you don't have to always reference
-    // some *other* (much more commonly referenced) track by directory instead
-    // of more naturally by name.
-    alwaysReferenceByDirectory: {
-      flags: {update: true, expose: true},
-
-      // Deliberately defaults to null - this will fall back to false in most
-      // cases.
-      update: {validate: isBoolean, default: null},
+    name: name('Unnamed Track'),
+    directory: directory(),
 
-      expose: {
-        dependencies: ['name', 'originalReleaseTrackByRef', 'trackData'],
+    duration: duration(),
+    urls: urls(),
+    dateFirstReleased: simpleDate(),
 
-        transform(value, {
-          name,
-          originalReleaseTrackByRef,
-          trackData,
-          [Track.instance]: thisTrack,
-        }) {
-          if (value !== null) return value;
+    color: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isColor),
+      }),
 
-          const original =
-            find.track(
-              originalReleaseTrackByRef,
-              trackData.filter(track => track !== thisTrack),
-              {quiet: true});
+      withContainingTrackSection(),
 
-          if (!original) return false;
+      withPropertyFromObject({
+        object: '#trackSection',
+        property: input.value('color'),
+      }),
 
-          return name === original.name;
-        }
-      },
-    },
+      exposeDependencyOrContinue({dependency: '#trackSection.color'}),
 
-    artistContribsByRef: Thing.common.contribsByRef(),
-    contributorContribsByRef: Thing.common.contribsByRef(),
-    coverArtistContribsByRef: Thing.common.contribsByRef(),
+      withPropertyFromAlbum({
+        property: input.value('color'),
+      }),
 
-    referencedTracksByRef: Thing.common.referenceList(Track),
-    sampledTracksByRef: Thing.common.referenceList(Track),
-    artTagsByRef: Thing.common.referenceList(ArtTag),
+      exposeDependency({dependency: '#album.color'}),
+    ],
 
-    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',
+    // Controls how find.track works - it'll never be matched by a reference
+    // just to the track's name, which means you don't have to always reference
+    // some *other* (much more commonly referenced) track by directory instead
+    // of more naturally by name.
+    alwaysReferenceByDirectory: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isBoolean),
+      }),
+
+      excludeFromList({
+        list: 'trackData',
+        item: input.myself(),
+      }),
+
+      withOriginalRelease({
+        data: '#trackData',
+      }),
+
+      exitWithoutDependency({
+        dependency: '#originalRelease',
+        value: input.value(false),
+      }),
+
+      withPropertyFromObject({
+        object: '#originalRelease',
+        property: input.value('name'),
+      }),
+
+      {
+        dependencies: ['name', '#originalRelease.name'],
+        compute: ({name, '#originalRelease.name': originalName}) =>
+          name === originalName,
       },
-    },
-
-    originalReleaseTrackByRef: Thing.common.singleReference(Track),
-
-    dataSourceAlbumByRef: Thing.common.singleReference(Album),
+    ],
+
+    // 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'}),
+
+      withPropertyFromAlbum({
+        property: input.value('artistContribs'),
+      }),
+
+      exposeDependency({dependency: '#album.artistContribs'}),
+    ],
+
+    contributorContribs: [
+      inheritFromOriginalRelease({
+        property: input.value('contributorContribs'),
+      }),
+
+      contributionList(),
+    ],
 
-    commentary: Thing.common.commentary(),
-    lyrics: Thing.common.simpleString(),
-    additionalFiles: Thing.common.additionalFiles(),
-    sheetMusicFiles: Thing.common.additionalFiles(),
-    midiProjectFiles: Thing.common.additionalFiles(),
+    // 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(),
+
+      withResolvedContribs({
+        from: input.updateValue({validate: isContributionList}),
+      }).outputs({
+        '#resolvedContribs': '#coverArtistContribs',
+      }),
+
+      exposeDependencyOrContinue({dependency: '#coverArtistContribs'}),
+
+      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);
+    commentatorArtists: commentatorArtists(),
+
+    album: [
+      withAlbum(),
+      exposeDependency({dependency: '#album'}),
+    ],
+
+    date: [
+      exposeDependencyOrContinue({dependency: 'dateFirstReleased'}),
+
+      withPropertyFromAlbum({
+        property: input.value('date'),
+      }),
+
+      exposeDependency({dependency: '#album.date'}),
+    ],
+
+    // Whether or not the track has "unique" cover artwork - a cover which is
+    // specifically associated with this track in particular, rather than with
+    // the track's album as a whole. This is typically used to select between
+    // displaying the track artwork and a fallback, such as the album artwork
+    // or a placeholder. (This property is named hasUniqueCoverArt instead of
+    // the usual hasCoverArt to emphasize that it does not inherit from the
+    // album.)
+    hasUniqueCoverArt: [
+      withHasUniqueCoverArt(),
+      exposeDependency({dependency: '#hasUniqueCoverArt'}),
+    ],
+
+    otherReleases: [
+      exitWithoutDependency({
+        dependency: 'trackData',
+        mode: input.value('empty'),
+      }),
+
+      withOriginalRelease({
+        selfIfOriginal: input.value(true),
+      }),
+
+      {
+        flags: {expose: true},
+        expose: {
+          dependencies: [input.myself(), '#originalRelease', 'trackData'],
+          compute: ({
+            [input.myself()]: thisTrack,
+            ['#originalRelease']: originalRelease,
+            trackData,
+          }) =>
+            (originalRelease === thisTrack
+              ? []
+              : [originalRelease])
+              .concat(trackData.filter(track =>
+                track !== originalRelease &&
+                track !== thisTrack &&
+                track.originalReleaseTrack === originalRelease)),
         },
       },
-    },
-
-    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
@@ -334,162 +347,380 @@ export class Track extends Thing {
     // 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))
-            : [],
-      },
-    },
+    referencedByTracks: trackReverseReferenceList({
+      list: input.value('referencedTracks'),
+    }),
 
     // 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))
-            : [],
-      },
-    },
+    sampledByTracks: trackReverseReferenceList({
+      list: input.value('sampledTracks'),
+    }),
+
+    featuredInFlashes: reverseReferenceList({
+      data: 'flashData',
+      list: input.value('featuredTracks'),
+    }),
+  });
 
-    featuredInFlashes: Thing.common.reverseReferenceList(
-      'flashData',
-      'featuredTracks'
-    ),
+  [inspect.custom](depth) {
+    const parts = [];
 
-    artTags: Thing.common.dynamicThingsFromReferenceList(
-      'artTagsByRef',
-      'artTagData',
-      find.artTag
-    ),
-  });
+    parts.push(Thing.prototype[inspect.custom].apply(this));
 
-  // 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;
+    if (CacheableObject.getUpdateValue(this, 'originalReleaseTrack')) {
+      parts.unshift(`${colors.yellow('[rerelease]')} `);
     }
 
-    const album = Track.findAlbum(track, albumData);
-    if (album && !empty(album.trackCoverArtistContribsByRef)) {
-      return true;
+    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)})`);
     }
 
-    return false;
+    return parts.join('');
   }
+}
 
-  static hasUniqueCoverArt(
-    track,
-    albumData,
-    coverArtistContribsByRef,
-    hasCoverArt
-  ) {
-    if (!empty(coverArtistContribsByRef)) {
-      return true;
-    }
-
-    if (hasCoverArt === false) {
-      return false;
-    }
-
-    const album = Track.findAlbum(track, albumData);
-    if (album && !empty(album.trackCoverArtistContribsByRef)) {
-      return true;
-    }
+// Early exits with a value inherited from the original release, if
+// this track is a rerelease, and otherwise continues with no further
+// dependencies provided. If allowOverride is true, then the continuation
+// will also be called if the original release exposed the requested
+// property as null.
+export const inheritFromOriginalRelease = templateCompositeFrom({
+  annotation: `Track.inheritFromOriginalRelease`,
+
+  inputs: {
+    property: input({type: 'string'}),
+    allowOverride: input({type: 'boolean', defaultValue: false}),
+  },
+
+  steps: () => [
+    withOriginalRelease(),
+
+    {
+      dependencies: [
+        '#originalRelease',
+        input('property'),
+        input('allowOverride'),
+      ],
+
+      compute: (continuation, {
+        ['#originalRelease']: originalRelease,
+        [input('property')]: originalProperty,
+        [input('allowOverride')]: allowOverride,
+      }) => {
+        if (!originalRelease) return continuation();
+
+        const value = originalRelease[originalProperty];
+        if (allowOverride && value === null) return continuation();
+
+        return continuation.exit(value);
+      },
+    },
+  ],
+});
+
+// Gets the track's album. This will early exit if albumData is missing.
+// By default, if there's no album whose list of tracks includes this track,
+// the output dependency will be null; set {notFoundMode: 'exit'} to early
+// exit instead.
+export const withAlbum = templateCompositeFrom({
+  annotation: `Track.withAlbum`,
+
+  inputs: {
+    notFoundMode: input({
+      validate: oneOf('exit', 'null'),
+      defaultValue: 'null',
+    }),
+  },
+
+  outputs: ['#album'],
+
+  steps: () => [
+    raiseOutputWithoutDependency({
+      dependency: 'albumData',
+      mode: input.value('empty'),
+      output: input.value({
+        ['#album']: null,
+      }),
+    }),
+
+    {
+      dependencies: [input.myself(), 'albumData'],
+      compute: (continuation, {
+        [input.myself()]: track,
+        ['albumData']: albumData,
+      }) =>
+        continuation({
+          ['#album']:
+            albumData.find(album => album.tracks.includes(track)),
+        }),
+    },
 
-    return false;
-  }
+    raiseOutputWithoutDependency({
+      dependency: '#album',
+      output: input.value({
+        ['#album']: null,
+      }),
+    }),
+
+    {
+      dependencies: ['#album'],
+      compute: (continuation, {'#album': album}) =>
+        continuation.raiseOutput({'#album': album}),
+    },
+  ],
+});
+
+// Gets a single property from this track's album, providing it as the same
+// property name prefixed with '#album.' (by default). If the track's album
+// isn't available, then by default, the property will be provided as null;
+// set {notFoundMode: 'exit'} to early exit instead.
+export const withPropertyFromAlbum = templateCompositeFrom({
+  annotation: `withPropertyFromAlbum`,
+
+  inputs: {
+    property: input.staticValue({type: 'string'}),
+
+    notFoundMode: input({
+      validate: oneOf('exit', 'null'),
+      defaultValue: 'null',
+    }),
+  },
+
+  outputs: ({
+    [input.staticValue('property')]: property,
+  }) => ['#album.' + property],
+
+  steps: () => [
+    withAlbum({
+      notFoundMode: input('notFoundMode'),
+    }),
+
+    withPropertyFromObject({
+      object: '#album',
+      property: input('property'),
+    }),
+
+    {
+      dependencies: ['#value', input.staticValue('property')],
+      compute: (continuation, {
+        ['#value']: value,
+        [input.staticValue('property')]: property,
+      }) => continuation({
+        ['#album.' + property]: value,
+      }),
+    },
+  ],
+});
+
+// Gets the track section containing this track from its album's track list.
+// If notFoundMode is set to 'exit', this will early exit if the album can't be
+// found or if none of its trackSections includes the track for some reason.
+export const withContainingTrackSection = templateCompositeFrom({
+  annotation: `withContainingTrackSection`,
+
+  inputs: {
+    notFoundMode: input({
+      validate: oneOf('exit', 'null'),
+      defaultValue: 'null',
+    }),
+  },
+
+  outputs: ['#trackSection'],
+
+  steps: () => [
+    withPropertyFromAlbum({
+      property: input.value('trackSections'),
+      notFoundMode: input('notFoundMode'),
+    }),
+
+    {
+      dependencies: [
+        input.myself(),
+        input('notFoundMode'),
+        '#album.trackSections',
+      ],
+
+      compute(continuation, {
+        [input.myself()]: track,
+        [input('notFoundMode')]: notFoundMode,
+        ['#album.trackSections']: trackSections,
+      }) {
+        if (!trackSections) {
+          return continuation.raiseOutput({
+            ['#trackSection']: null,
+          });
+        }
 
-  static inheritFromOriginalRelease(
-    originalProperty,
-    originalMissingValue,
-    ownPropertyDescriptor
-  ) {
-    return {
-      flags: {expose: true},
+        const trackSection =
+          trackSections.find(({tracks}) => tracks.includes(track));
+
+        if (trackSection) {
+          return continuation.raiseOutput({
+            ['#trackSection']: trackSection,
+          });
+        } else if (notFoundMode === 'exit') {
+          return continuation.exit(null);
+        } else {
+          return continuation.raiseOutput({
+            ['#trackSection']: null,
+          });
+        }
+      },
+    },
+  ],
+});
+
+// Just includes the original release of this track as a dependency.
+// If this track isn't a rerelease, then it'll provide null, unless the
+// {selfIfOriginal} option is set, in which case it'll provide this track
+// itself. Note that this will early exit if the original release is
+// specified by reference and that reference doesn't resolve to anything.
+// Outputs to '#originalRelease' by default.
+export const withOriginalRelease = templateCompositeFrom({
+  annotation: `withOriginalRelease`,
+
+  inputs: {
+    selfIfOriginal: input({type: 'boolean', defaultValue: false}),
+
+    // todo: validate
+    data: input({defaultDependency: 'trackData'}),
+  },
+
+  outputs: ['#originalRelease'],
+
+  steps: () => [
+    withResolvedReference({
+      ref: 'originalReleaseTrack',
+      data: input('data'),
+      find: input.value(find.track),
+      notFoundMode: input.value('exit'),
+    }).outputs({
+      ['#resolvedReference']: '#originalRelease',
+    }),
+
+    {
+      dependencies: [
+        input.myself(),
+        input('selfIfOriginal'),
+        '#originalRelease',
+      ],
+
+      compute: (continuation, {
+        [input.myself()]: track,
+        [input('selfIfOriginal')]: selfIfOriginal,
+        ['#originalRelease']: originalRelease,
+      }) =>
+        continuation({
+          ['#originalRelease']:
+            (originalRelease ??
+              (selfIfOriginal
+                ? track
+                : null)),
+        }),
+    },
+  ],
+});
+
+// The algorithm for checking if a track has unique cover art is used in a
+// couple places, so it's defined in full as a compositional step.
+export const withHasUniqueCoverArt = templateCompositeFrom({
+  annotation: 'withHasUniqueCoverArt',
+
+  outputs: ['#hasUniqueCoverArt'],
+
+  steps: () => [
+    {
+      dependencies: ['disableUniqueCoverArt'],
+      compute: (continuation, {disableUniqueCoverArt}) =>
+        (disableUniqueCoverArt
+          ? continuation.raiseOutput({
+              ['#hasUniqueCoverArt']: false,
+            })
+          : continuation()),
+    },
 
-      expose: {
-        dependencies: [
-          ...ownPropertyDescriptor.expose.dependencies,
-          'originalReleaseTrackByRef',
-          'trackData',
-        ],
-
-        compute(dependencies) {
-          const {
-            originalReleaseTrackByRef,
-            trackData,
-          } = dependencies;
+    withResolvedContribs({from: 'coverArtistContribs'}),
+
+    {
+      dependencies: ['#resolvedContribs'],
+      compute: (continuation, {
+        ['#resolvedContribs']: contribsFromTrack,
+      }) =>
+        (empty(contribsFromTrack)
+          ? continuation()
+          : continuation.raiseOutput({
+              ['#hasUniqueCoverArt']: true,
+            })),
+    },
 
-          if (originalReleaseTrackByRef) {
-            if (!trackData) return originalMissingValue;
-            const original = find.track(originalReleaseTrackByRef, trackData, {mode: 'quiet'});
-            if (!original) return originalMissingValue;
-            return original[originalProperty];
-          }
+    withPropertyFromAlbum({
+      property: input.value('trackCoverArtistContribs'),
+    }),
+
+    {
+      dependencies: ['#album.trackCoverArtistContribs'],
+      compute: (continuation, {
+        ['#album.trackCoverArtistContribs']: contribsFromAlbum,
+      }) =>
+        continuation.raiseOutput({
+          ['#hasUniqueCoverArt']:
+            !empty(contribsFromAlbum),
+        }),
+    },
+  ],
+});
 
-          return ownPropertyDescriptor.expose.compute(dependencies);
-        },
-      },
-    };
-  }
+// Shorthand for checking if the track has unique cover art and exposing a
+// fallback value if it isn't.
+export const exitWithoutUniqueCoverArt = templateCompositeFrom({
+  annotation: `exitWithoutUniqueCoverArt`,
 
-  [inspect.custom]() {
-    const base = Thing.prototype[inspect.custom].apply(this);
+  inputs: {
+    value: input({null: true}),
+  },
 
-    const rereleasePart =
-      (this.originalReleaseTrackByRef
-        ? `${color.yellow('[rerelease]')} `
-        : ``);
+  steps: () => [
+    withHasUniqueCoverArt(),
 
-    const {album, dataSourceAlbum} = this;
+    exitWithoutDependency({
+      dependency: '#hasUniqueCoverArt',
+      mode: input.value('falsy'),
+      value: input('value'),
+    }),
+  ],
+});
 
-    const albumName =
-      (album
-        ? album.name
-        : dataSourceAlbum?.name);
+export const trackReverseReferenceList = templateCompositeFrom({
+  annotation: `trackReverseReferenceList`,
 
-    const albumIndex =
-      albumName &&
-        (album
-          ? album.tracks.indexOf(this)
-          : dataSourceAlbum.tracks.indexOf(this));
+  compose: false,
 
-    const trackNum =
-      albumName &&
-        (albumIndex === -1
-          ? '#?'
-          : `#${albumIndex + 1}`);
+  inputs: {
+    list: input({type: 'string'}),
+  },
 
-    const albumPart =
-      albumName
-        ? ` (${color.yellow(trackNum)} in ${color.green(albumName)})`
-        : ``;
+  steps: () => [
+    withReverseReferenceList({
+      data: 'trackData',
+      list: input('list'),
+    }),
 
-    return rereleasePart + base + albumPart;
-  }
-}
+    {
+      flags: {expose: true},
+      expose: {
+        dependencies: ['#reverseReferenceList'],
+        compute: ({
+          ['#reverseReferenceList']: reverseReferenceList,
+        }) =>
+          reverseReferenceList.filter(track => !track.originalReleaseTrack),
+      },
+    },
+  ],
+});
diff --git a/src/data/things/validators.js b/src/data/things/validators.js
index 5748eacf..cd4c2b46 100644
--- a/src/data/things/validators.js
+++ b/src/data/things/validators.js
@@ -1,6 +1,6 @@
 import {inspect as nodeInspect} from 'node:util';
 
-import {color, ENABLE_COLOR} from '#cli';
+import {colors, ENABLE_COLOR} from '#cli';
 import {withAggregate} from '#sugar';
 
 function inspect(value) {
@@ -9,11 +9,11 @@ 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}`);
 
@@ -174,7 +174,7 @@ function validateArrayItemsHelper(itemValidator) {
         throw new Error(`Expected validator to return true`);
       }
     } catch (error) {
-      error.message = `(index: ${color.yellow(`#${index}`)}, item: ${inspect(item)}) ${error.message}`;
+      error.message = `(index: ${colors.yellow(`#${index}`)}, item: ${inspect(item)}) ${error.message}`;
       throw error;
     }
   };
@@ -264,7 +264,7 @@ 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 +308,7 @@ 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);
diff --git a/src/data/things/wiki-info.js b/src/data/things/wiki-info.js
index e906cab1..c764b528 100644
--- a/src/data/things/wiki-info.js
+++ b/src/data/things/wiki-info.js
@@ -1,20 +1,21 @@
+import {input} from '#composite';
 import find from '#find';
+import {isLanguageCode, isName, isURL} from '#validators';
 
-import Thing from './thing.js';
+import Thing, {
+  color,
+  flag,
+  name,
+  referenceList,
+  simpleString,
+  wikiData,
+} from './thing.js';
 
 export class WikiInfo extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    Group,
-
-    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 +28,12 @@ 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 +45,21 @@ 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),
   });
 }