8 files changed, 168 insertions, 6 deletions
diff --git a/src/data/composite/control-flow/raiseOutputWithoutDependency.js b/src/data/composite/control-flow/raiseOutputWithoutDependency.js
index 03d8036a..3d04f8a9 100644
--- a/src/data/composite/control-flow/raiseOutputWithoutDependency.js
+++ b/src/data/composite/control-flow/raiseOutputWithoutDependency.js
@@ -17,7 +17,7 @@ export default templateCompositeFrom({
 
   outputs: ({
     [input.staticValue('output')]: output,
-  }) => Object.keys(output),
+  }) => Object.keys(output ?? {}),
 
   steps: () => [
     withResultOfAvailabilityCheck({
diff --git a/src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js b/src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js
index 3c39f5ba..ffa83a94 100644
--- a/src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js
+++ b/src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js
@@ -16,7 +16,7 @@ export default templateCompositeFrom({
 
   outputs: ({
     [input.staticValue('output')]: output,
-  }) => Object.keys(output),
+  }) => Object.keys(output ?? {}),
 
   steps: () => [
     withResultOfAvailabilityCheck({
diff --git a/src/data/composite/things/art-tag/index.js b/src/data/composite/things/art-tag/index.js
new file mode 100644
index 00000000..bbd38293
--- /dev/null
+++ b/src/data/composite/things/art-tag/index.js
@@ -0,0 +1,2 @@
+export {default as withAllDescendantArtTags} from './withAllDescendantArtTags.js';
+export {default as withAncestorArtTagBaobabTree} from './withAncestorArtTagBaobabTree.js';
diff --git a/src/data/composite/things/art-tag/withAllDescendantArtTags.js b/src/data/composite/things/art-tag/withAllDescendantArtTags.js
new file mode 100644
index 00000000..c643cf23
--- /dev/null
+++ b/src/data/composite/things/art-tag/withAllDescendantArtTags.js
@@ -0,0 +1,45 @@
+// Gets all the art tags which descend from this one - that means its own direct
+// descendants, but also all the direct and indirect desceands of each of those!
+// The results aren't specially sorted, but they won't contain any duplicates
+// (for example if two descendant tags both route deeper to end up including
+// some of the same tags).
+
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+import {unique} from '#sugar';
+
+import {raiseOutputWithoutDependency} from '#composite/control-flow';
+import {withResolvedReferenceList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `withAllDescendantArtTags`,
+
+  outputs: ['#allDescendantArtTags'],
+
+  steps: () => [
+    raiseOutputWithoutDependency({
+      dependency: 'directDescendantArtTags',
+      mode: input.value('empty'),
+      output: input.value({'#allDescendantArtTags': []})
+    }),
+
+    withResolvedReferenceList({
+      list: 'directDescendantArtTags',
+      data: 'artTagData',
+      find: input.value(find.artTag),
+    }),
+
+    {
+      dependencies: ['#resolvedReferenceList'],
+      compute: (continuation, {
+        ['#resolvedReferenceList']: directDescendantArtTags,
+      }) => continuation({
+        ['#allDescendantArtTags']:
+          unique([
+            ...directDescendantArtTags,
+            ...directDescendantArtTags.flatMap(artTag => artTag.allDescendantArtTags),
+          ]),
+      }),
+    },
+  ],
+})
diff --git a/src/data/composite/things/art-tag/withAncestorArtTagBaobabTree.js b/src/data/composite/things/art-tag/withAncestorArtTagBaobabTree.js
new file mode 100644
index 00000000..d5caa99e
--- /dev/null
+++ b/src/data/composite/things/art-tag/withAncestorArtTagBaobabTree.js
@@ -0,0 +1,46 @@
+// Gets all the art tags which are ancestors of this one as a "baobab tree" -
+// what you'd typically think of as roots are all up in the air! Since this
+// really is backwards from the way that the art tag tree is written in data,
+// chances are pretty good that there will be many of the exact same "leaf"
+// nodes - art tags which don't themselves have any ancestors. In the actual
+// data structure, each node is a Map, with keys for each ancestor and values
+// for each ancestor's own baobab (thus a branching structure, just like normal
+// trees in this regard).
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {raiseOutputWithoutDependency} from '#composite/control-flow';
+import {withReverseReferenceList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `withAncestorArtTagBaobabTree`,
+
+  outputs: ['#ancestorArtTagBaobabTree'],
+
+  steps: () => [
+    withReverseReferenceList({
+      data: 'artTagData',
+      list: input.value('directDescendantArtTags'),
+    }).outputs({
+      ['#reverseReferenceList']: '#directAncestorArtTags',
+    }),
+
+    raiseOutputWithoutDependency({
+      dependency: '#directAncestorArtTags',
+      mode: input.value('empty'),
+      output: input.value({'#ancestorArtTagBaobabTree': {}})
+    }),
+
+    {
+      dependencies: ['#directAncestorArtTags'],
+      compute: (continuation, {
+        ['#directAncestorArtTags']: directAncestorArtTags,
+      }) => continuation({
+        ['#ancestorArtTagBaobabTree']:
+          new Map(
+            directAncestorArtTags
+              .map(artTag => [artTag, artTag.ancestorArtTagBaobabTree])),
+      }),
+    },
+  ],
+});
diff --git a/src/data/things/art-tag.js b/src/data/things/art-tag.js
index 1266a4e0..a530ba8c 100644
--- a/src/data/things/art-tag.js
+++ b/src/data/things/art-tag.js
@@ -1,17 +1,27 @@
 import {input} from '#composite';
-import {sortAlbumsTracksChronologically} from '#wiki-data';
+import find from '#find';
+import {unique} from '#sugar';
 import {isName} from '#validators';
+import {sortAlbumsTracksChronologically} from '#wiki-data';
 
-import {exposeUpdateValueOrContinue} from '#composite/control-flow';
+import {exitWithoutDependency, exposeDependency, exposeUpdateValueOrContinue}
+  from '#composite/control-flow';
 
 import {
   color,
   directory,
   flag,
+  referenceList,
+  reverseReferenceList,
+  simpleString,
   name,
+  urls,
   wikiData,
 } from '#composite/wiki-properties';
 
+import {withAllDescendantArtTags, withAncestorArtTagBaobabTree}
+  from '#composite/things/art-tag';
+
 import Thing from './thing.js';
 
 export class ArtTag extends Thing {
@@ -24,6 +34,7 @@ export class ArtTag extends Thing {
     directory: directory(),
     color: color(),
     isContentWarning: flag(false),
+    extraReadingURLs: urls(),
 
     nameShort: [
       exposeUpdateValueOrContinue({
@@ -37,14 +48,36 @@ export class ArtTag extends Thing {
       },
     ],
 
+    description: simpleString(),
+
+    directDescendantArtTags: referenceList({
+      class: input.value(ArtTag),
+      find: input.value(find.artTag),
+      data: 'artTagData',
+    }),
+
     // Update only
 
     albumData: wikiData(Album),
+    artTagData: wikiData(ArtTag),
     trackData: wikiData(Track),
 
     // Expose only
 
-    taggedInThings: {
+    descriptionShort: [
+      exitWithoutDependency({
+        dependency: 'description',
+        mode: input.value('falsy'),
+      }),
+
+      {
+        dependencies: ['description'],
+        compute: ({description}) =>
+          description.split('<hr class="split">')[0],
+      },
+    ],
+
+    directlyTaggedInThings: {
       flags: {expose: true},
 
       expose: {
@@ -56,5 +89,32 @@ export class ArtTag extends Thing {
             {getDate: o => o.coverArtDate}),
       },
     },
+
+    indirectlyTaggedInThings: [
+      withAllDescendantArtTags(),
+
+      {
+        dependencies: ['#allDescendantArtTags'],
+        compute: ({'#allDescendantArtTags': allDescendantArtTags}) =>
+          unique(
+            allDescendantArtTags
+              .flatMap(artTag => artTag.directlyTaggedInThings)),
+      },
+    ],
+
+    allDescendantArtTags: [
+      withAllDescendantArtTags(),
+      exposeDependency({dependency: '#allDescendantArtTags'}),
+    ],
+
+    directAncestorArtTags: reverseReferenceList({
+      data: 'artTagData',
+      list: input.value('directDescendantArtTags'),
+    }),
+
+    ancestorArtTagBaobabTree: [
+      withAncestorArtTagBaobabTree(),
+      exposeDependency({dependency: '#ancestorArtTagBaobabTree'}),
+    ],
   });
 }
diff --git a/src/data/things/language.js b/src/data/things/language.js
index fe74f7bf..cd719d0c 100644
--- a/src/data/things/language.js
+++ b/src/data/things/language.js
@@ -376,6 +376,7 @@ Object.assign(Language.prototype, {
   countAdditionalFiles: countHelper('additionalFiles', 'files'),
   countAlbums: countHelper('albums'),
   countArtworks: countHelper('artworks'),
+  countArtTags: countHelper('artTags', 'tags'),
   countFlashes: countHelper('flashes'),
   countCommentaryEntries: countHelper('commentaryEntries', 'entries'),
   countContributions: countHelper('contributions'),
diff --git a/src/data/yaml.js b/src/data/yaml.js
index c799be5f..09465648 100644
--- a/src/data/yaml.js
+++ b/src/data/yaml.js
@@ -459,9 +459,13 @@ export const processArtTagDocument = makeProcessDocument(T.ArtTag, {
     name: 'Tag',
     nameShort: 'Short Name',
     directory: 'Directory',
+    description: 'Description',
+    extraReadingURLs: 'Extra Reading URLs',
 
     color: 'Color',
     isContentWarning: 'Is CW',
+
+    directDescendantArtTags: 'Direct Descendant Tags',
   },
 });
 
@@ -1359,7 +1363,7 @@ export function linkWikiDataArrays(wikiData, {
   assignWikiData(WD.groupCategoryData, 'groupData');
   assignWikiData(WD.flashData, 'artistData', 'flashActData', 'trackData');
   assignWikiData(WD.flashActData, 'flashData');
-  assignWikiData(WD.artTagData, 'albumData', 'trackData');
+  assignWikiData(WD.artTagData, 'albumData', 'artTagData', 'trackData');
   assignWikiData(WD.homepageLayout?.rows, 'albumData', 'groupData');
 }
 
@@ -1477,6 +1481,10 @@ export function filterReferenceErrors(wikiData) {
       artTags: 'artTag',
     }],
 
+    ['artTagData', processArtTagDocument, {
+      directDescendantArtTags: 'artTag',
+    }],
+
     ['trackData', processTrackDocument, {
       artistContribs: '_contrib',
       contributorContribs: '_contrib',