Merge branch 'preview' into listing-tweaks

82 files changed, 5378 insertions, 1527 deletions

diff --git a/src/data/composite/control-flow/exitWithoutDependency.js b/src/data/composite/control-flow/exitWithoutDependency.js
new file mode 100644
index 00000000..c660a7ef
--- /dev/null
+++ b/src/data/composite/control-flow/exitWithoutDependency.js
@@ -0,0 +1,35 @@
+// Early exits if a dependency isn't available.
+// See withResultOfAvailabilityCheck for {mode} options.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+import withResultOfAvailabilityCheck from './withResultOfAvailabilityCheck.js';
+
+export default templateCompositeFrom({
+  annotation: `exitWithoutDependency`,
+
+  inputs: {
+    dependency: input({acceptsNull: true}),
+    mode: inputAvailabilityCheckMode(),
+    value: input({defaultValue: null}),
+  },
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('dependency'),
+      mode: input('mode'),
+    }),
+
+    {
+      dependencies: ['#availability', input('value')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('value')]: value,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.exit(value)),
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/exitWithoutUpdateValue.js b/src/data/composite/control-flow/exitWithoutUpdateValue.js
new file mode 100644
index 00000000..244b3233
--- /dev/null
+++ b/src/data/composite/control-flow/exitWithoutUpdateValue.js
@@ -0,0 +1,24 @@
+// Early exits if this property's update value isn't available.
+// See withResultOfAvailabilityCheck for {mode} options.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import exitWithoutDependency from './exitWithoutDependency.js';
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+
+export default templateCompositeFrom({
+  annotation: `exitWithoutUpdateValue`,
+
+  inputs: {
+    mode: inputAvailabilityCheckMode(),
+    value: input({defaultValue: null}),
+  },
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input.updateValue(),
+      mode: input('mode'),
+      value: input('value'),
+    }),
+  ],
+});
diff --git a/src/data/composite/control-flow/exposeConstant.js b/src/data/composite/control-flow/exposeConstant.js
new file mode 100644
index 00000000..e0435478
--- /dev/null
+++ b/src/data/composite/control-flow/exposeConstant.js
@@ -0,0 +1,26 @@
+// Exposes a constant value exactly as it is; like exposeDependency, this
+// is typically the base of a composition serving as a particular property
+// descriptor. It generally follows steps which will conditionally early
+// exit with some other value, with the exposeConstant base serving as the
+// fallback default value.
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `exposeConstant`,
+
+  compose: false,
+
+  inputs: {
+    value: input.staticValue(),
+  },
+
+  steps: () => [
+    {
+      dependencies: [input('value')],
+      compute: ({
+        [input('value')]: value,
+      }) => value,
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/exposeDependency.js b/src/data/composite/control-flow/exposeDependency.js
new file mode 100644
index 00000000..3aa3d03a
--- /dev/null
+++ b/src/data/composite/control-flow/exposeDependency.js
@@ -0,0 +1,28 @@
+// Exposes a dependency exactly as it is; this is typically the base of a
+// composition which was created to serve as one property's descriptor.
+//
+// Please note that this *doesn't* verify that the dependency exists, so
+// if you provide the wrong name or it hasn't been set by a previous
+// compositional step, the property will be exposed as undefined instead
+// of null.
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `exposeDependency`,
+
+  compose: false,
+
+  inputs: {
+    dependency: input.staticDependency({acceptsNull: true}),
+  },
+
+  steps: () => [
+    {
+      dependencies: [input('dependency')],
+      compute: ({
+        [input('dependency')]: dependency
+      }) => dependency,
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/exposeDependencyOrContinue.js b/src/data/composite/control-flow/exposeDependencyOrContinue.js
new file mode 100644
index 00000000..0f7f223e
--- /dev/null
+++ b/src/data/composite/control-flow/exposeDependencyOrContinue.js
@@ -0,0 +1,34 @@
+// Exposes a dependency as it is, or continues if it's unavailable.
+// See withResultOfAvailabilityCheck for {mode} options.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+import withResultOfAvailabilityCheck from './withResultOfAvailabilityCheck.js';
+
+export default templateCompositeFrom({
+  annotation: `exposeDependencyOrContinue`,
+
+  inputs: {
+    dependency: input({acceptsNull: true}),
+    mode: inputAvailabilityCheckMode(),
+  },
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('dependency'),
+      mode: input('mode'),
+    }),
+
+    {
+      dependencies: ['#availability', input('dependency')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('dependency')]: dependency,
+      }) =>
+        (availability
+          ? continuation.exit(dependency)
+          : continuation()),
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/exposeUpdateValueOrContinue.js b/src/data/composite/control-flow/exposeUpdateValueOrContinue.js
new file mode 100644
index 00000000..1f94b332
--- /dev/null
+++ b/src/data/composite/control-flow/exposeUpdateValueOrContinue.js
@@ -0,0 +1,40 @@
+// Exposes the update value of an {update: true} property as it is,
+// or continues if it's unavailable.
+//
+// See withResultOfAvailabilityCheck for {mode} options.
+//
+// Provide {validate} here to conveniently set a custom validation check
+// for this property's update value.
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+import exposeDependencyOrContinue from './exposeDependencyOrContinue.js';
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+
+export default templateCompositeFrom({
+  annotation: `exposeUpdateValueOrContinue`,
+
+  inputs: {
+    mode: inputAvailabilityCheckMode(),
+
+    validate: input({
+      type: 'function',
+      defaultValue: null,
+    }),
+  },
+
+  update: ({
+    [input.staticValue('validate')]: validate,
+  }) =>
+    (validate
+      ? {validate}
+      : {}),
+
+  steps: () => [
+    exposeDependencyOrContinue({
+      dependency: input.updateValue(),
+      mode: input('mode'),
+    }),
+  ],
+});
diff --git a/src/data/composite/control-flow/index.js b/src/data/composite/control-flow/index.js
new file mode 100644
index 00000000..dfc53db7
--- /dev/null
+++ b/src/data/composite/control-flow/index.js
@@ -0,0 +1,9 @@
+export {default as exitWithoutDependency} from './exitWithoutDependency.js';
+export {default as exitWithoutUpdateValue} from './exitWithoutUpdateValue.js';
+export {default as exposeConstant} from './exposeConstant.js';
+export {default as exposeDependency} from './exposeDependency.js';
+export {default as exposeDependencyOrContinue} from './exposeDependencyOrContinue.js';
+export {default as exposeUpdateValueOrContinue} from './exposeUpdateValueOrContinue.js';
+export {default as raiseOutputWithoutDependency} from './raiseOutputWithoutDependency.js';
+export {default as raiseOutputWithoutUpdateValue} from './raiseOutputWithoutUpdateValue.js';
+export {default as withResultOfAvailabilityCheck} from './withResultOfAvailabilityCheck.js';
diff --git a/src/data/composite/control-flow/inputAvailabilityCheckMode.js b/src/data/composite/control-flow/inputAvailabilityCheckMode.js
new file mode 100644
index 00000000..8008fdeb
--- /dev/null
+++ b/src/data/composite/control-flow/inputAvailabilityCheckMode.js
@@ -0,0 +1,9 @@
+import {input} from '#composite';
+import {is} from '#validators';
+
+export default function inputAvailabilityCheckMode() {
+  return input({
+    validate: is('null', 'empty', 'falsy', 'index'),
+    defaultValue: 'null',
+  });
+}
diff --git a/src/data/composite/control-flow/raiseOutputWithoutDependency.js b/src/data/composite/control-flow/raiseOutputWithoutDependency.js
new file mode 100644
index 00000000..03d8036a
--- /dev/null
+++ b/src/data/composite/control-flow/raiseOutputWithoutDependency.js
@@ -0,0 +1,39 @@
+// Raises if a dependency isn't available.
+// See withResultOfAvailabilityCheck for {mode} options.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+import withResultOfAvailabilityCheck from './withResultOfAvailabilityCheck.js';
+
+export default templateCompositeFrom({
+  annotation: `raiseOutputWithoutDependency`,
+
+  inputs: {
+    dependency: input({acceptsNull: true}),
+    mode: inputAvailabilityCheckMode(),
+    output: input.staticValue({defaultValue: {}}),
+  },
+
+  outputs: ({
+    [input.staticValue('output')]: output,
+  }) => Object.keys(output),
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('dependency'),
+      mode: input('mode'),
+    }),
+
+    {
+      dependencies: ['#availability', input('output')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('output')]: output,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.raiseOutputAbove(output)),
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js b/src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js
new file mode 100644
index 00000000..3c39f5ba
--- /dev/null
+++ b/src/data/composite/control-flow/raiseOutputWithoutUpdateValue.js
@@ -0,0 +1,47 @@
+// Raises if this property's update value isn't available.
+// See withResultOfAvailabilityCheck for {mode} options!
+
+import {input, templateCompositeFrom} from '#composite';
+
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+import withResultOfAvailabilityCheck from './withResultOfAvailabilityCheck.js';
+
+export default templateCompositeFrom({
+  annotation: `raiseOutputWithoutUpdateValue`,
+
+  inputs: {
+    mode: inputAvailabilityCheckMode(),
+    output: input.staticValue({defaultValue: {}}),
+  },
+
+  outputs: ({
+    [input.staticValue('output')]: output,
+  }) => Object.keys(output),
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input.updateValue(),
+      mode: input('mode'),
+    }),
+
+    // TODO: A bit of a kludge, below. Other "do something with the update
+    // value" type functions can get by pretty much just passing that value
+    // as an input (input.updateValue()) into the corresponding "do something
+    // with a dependency/arbitrary value" function. But we can't do that here,
+    // because the special behavior, raiseOutputAbove(), only works to raise
+    // output above the composition it's *directly* nested in. Other languages
+    // have a throw/catch system that might serve as inspiration for something
+    // better here.
+
+    {
+      dependencies: ['#availability', input('output')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('output')]: output,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.raiseOutputAbove(output)),
+    },
+  ],
+});
diff --git a/src/data/composite/control-flow/withResultOfAvailabilityCheck.js b/src/data/composite/control-flow/withResultOfAvailabilityCheck.js
new file mode 100644
index 00000000..a6942014
--- /dev/null
+++ b/src/data/composite/control-flow/withResultOfAvailabilityCheck.js
@@ -0,0 +1,71 @@
+// Checks the availability of a dependency and provides the result to later
+// steps under '#availability' (by default). This is mainly intended for use
+// by the more specific utilities, which you should consider using instead.
+//
+// Customize {mode} to select one of these modes, or default to 'null':
+//
+// * 'null':  Check that the value isn't null (and not undefined either).
+// * 'empty': Check that the value is neither null, undefined, nor an empty
+//            array.
+// * 'falsy': Check that the value isn't false when treated as a boolean
+//            (nor an empty array). Keep in mind this will also be false
+//            for values like zero and the empty string!
+// * 'index': Check that the value is a number, and is at least zero.
+//
+// See also:
+//  - exitWithoutDependency
+//  - exitWithoutUpdateValue
+//  - exposeDependencyOrContinue
+//  - exposeUpdateValueOrContinue
+//  - raiseOutputWithoutDependency
+//  - raiseOutputWithoutUpdateValue
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {empty} from '#sugar';
+
+import inputAvailabilityCheckMode from './inputAvailabilityCheckMode.js';
+
+export default templateCompositeFrom({
+  annotation: `withResultOfAvailabilityCheck`,
+
+  inputs: {
+    from: input({acceptsNull: true}),
+    mode: inputAvailabilityCheckMode(),
+  },
+
+  outputs: ['#availability'],
+
+  steps: () => [
+    {
+      dependencies: [input('from'), input('mode')],
+
+      compute: (continuation, {
+        [input('from')]: value,
+        [input('mode')]: mode,
+      }) => {
+        let availability;
+
+        switch (mode) {
+          case 'null':
+            availability = value !== undefined && value !== null;
+            break;
+
+          case 'empty':
+            availability = value !== undefined && !empty(value);
+            break;
+
+          case 'falsy':
+            availability = !!value && (!Array.isArray(value) || !empty(value));
+            break;
+
+          case 'index':
+            availability = typeof value === 'number' && value >= 0;
+            break;
+        }
+
+        return continuation({'#availability': availability});
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/data/excludeFromList.js b/src/data/composite/data/excludeFromList.js
new file mode 100644
index 00000000..718f2294
--- /dev/null
+++ b/src/data/composite/data/excludeFromList.js
@@ -0,0 +1,56 @@
+// Filters particular values out of a list. Note that this will always
+// completely skip over null, but can be used to filter out any other
+// primitive or object value.
+//
+// See also:
+//  - fillMissingListItems
+//
+// More list utilities:
+//  - withFlattenedList
+//  - withPropertyFromList
+//  - withPropertiesFromList
+//  - withUnflattenedList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {empty} from '#sugar';
+
+export default templateCompositeFrom({
+  annotation: `excludeFromList`,
+
+  inputs: {
+    list: input(),
+
+    item: input({defaultValue: null}),
+    items: input({type: 'array', defaultValue: null}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+  }) => [list ?? '#list'],
+
+  steps: () => [
+    {
+      dependencies: [
+        input.staticDependency('list'),
+        input('list'),
+        input('item'),
+        input('items'),
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('list')]: listName,
+        [input('list')]: listContents,
+        [input('item')]: excludeItem,
+        [input('items')]: excludeItems,
+      }) => continuation({
+        [listName ?? '#list']:
+          listContents.filter(item => {
+            if (excludeItem !== null && item === excludeItem) return false;
+            if (!empty(excludeItems) && excludeItems.includes(item)) return false;
+            return true;
+          }),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/data/fillMissingListItems.js b/src/data/composite/data/fillMissingListItems.js
new file mode 100644
index 00000000..c06eceda
--- /dev/null
+++ b/src/data/composite/data/fillMissingListItems.js
@@ -0,0 +1,51 @@
+// Replaces items of a list, which are null or undefined, with some fallback
+// value. By default, this replaces the passed dependency.
+//
+// See also:
+//  - excludeFromList
+//
+// More list utilities:
+//  - withFlattenedList
+//  - withPropertyFromList
+//  - withPropertiesFromList
+//  - withUnflattenedList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `fillMissingListItems`,
+
+  inputs: {
+    list: input({type: 'array'}),
+    fill: input({acceptsNull: true}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+  }) => [list ?? '#list'],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('fill')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('fill')]: fill,
+      }) => continuation({
+        ['#filled']:
+          list.map(item => item ?? fill),
+      }),
+    },
+
+    {
+      dependencies: [input.staticDependency('list'), '#filled'],
+      compute: (continuation, {
+        [input.staticDependency('list')]: list,
+        ['#filled']: filled,
+      }) => continuation({
+        [list ?? '#list']:
+          filled,
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/data/index.js b/src/data/composite/data/index.js
new file mode 100644
index 00000000..ecd05129
--- /dev/null
+++ b/src/data/composite/data/index.js
@@ -0,0 +1,8 @@
+export {default as excludeFromList} from './excludeFromList.js';
+export {default as fillMissingListItems} from './fillMissingListItems.js';
+export {default as withFlattenedList} from './withFlattenedList.js';
+export {default as withPropertiesFromList} from './withPropertiesFromList.js';
+export {default as withPropertiesFromObject} from './withPropertiesFromObject.js';
+export {default as withPropertyFromList} from './withPropertyFromList.js';
+export {default as withPropertyFromObject} from './withPropertyFromObject.js';
+export {default as withUnflattenedList} from './withUnflattenedList.js';
diff --git a/src/data/composite/data/withFlattenedList.js b/src/data/composite/data/withFlattenedList.js
new file mode 100644
index 00000000..b08edb4e
--- /dev/null
+++ b/src/data/composite/data/withFlattenedList.js
@@ -0,0 +1,47 @@
+// Flattens an array with one level of nested arrays, providing as dependencies
+// both the flattened array as well as the original starting indices of each
+// successive source array.
+//
+// See also:
+//  - withFlattenedList
+//
+// More list utilities:
+//  - excludeFromList
+//  - fillMissingListItems
+//  - withPropertyFromList
+//  - withPropertiesFromList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `withFlattenedList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+  },
+
+  outputs: ['#flattenedList', '#flattenedIndices'],
+
+  steps: () => [
+    {
+      dependencies: [input('list')],
+      compute(continuation, {
+        [input('list')]: sourceList,
+      }) {
+        const flattenedList = sourceList.flat();
+        const indices = [];
+        let lastEndIndex = 0;
+        for (const {length} of sourceList) {
+          indices.push(lastEndIndex);
+          lastEndIndex += length;
+        }
+
+        return continuation({
+          ['#flattenedList']: flattenedList,
+          ['#flattenedIndices']: indices,
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/data/withPropertiesFromList.js b/src/data/composite/data/withPropertiesFromList.js
new file mode 100644
index 00000000..76ba696c
--- /dev/null
+++ b/src/data/composite/data/withPropertiesFromList.js
@@ -0,0 +1,92 @@
+// Gets the listed properties from each of a list of objects, providing lists
+// of property values each into a dependency prefixed with the same name as the
+// list (by default).
+//
+// Like withPropertyFromList, this doesn't alter indices.
+//
+// See also:
+//  - withPropertiesFromObject
+//  - withPropertyFromList
+//
+// More list utilities:
+//  - excludeFromList
+//  - fillMissingListItems
+//  - withFlattenedList
+//  - withUnflattenedList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {isString, validateArrayItems} from '#validators';
+
+export default templateCompositeFrom({
+  annotation: `withPropertiesFromList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+
+    properties: input({
+      validate: validateArrayItems(isString),
+    }),
+
+    prefix: input.staticValue({type: 'string', defaultValue: null}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+    [input.staticValue('properties')]: properties,
+    [input.staticValue('prefix')]: prefix,
+  }) =>
+    (properties
+      ? properties.map(property =>
+          (prefix
+            ? `${prefix}.${property}`
+         : list
+            ? `${list}.${property}`
+            : `#list.${property}`))
+      : ['#lists']),
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('properties')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('properties')]: properties,
+      }) => continuation({
+        ['#lists']:
+          Object.fromEntries(
+            properties.map(property => [
+              property,
+              list.map(item => item[property] ?? null),
+            ])),
+      }),
+    },
+
+    {
+      dependencies: [
+        input.staticDependency('list'),
+        input.staticValue('properties'),
+        input.staticValue('prefix'),
+        '#lists',
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('list')]: list,
+        [input.staticValue('properties')]: properties,
+        [input.staticValue('prefix')]: prefix,
+        ['#lists']: lists,
+      }) =>
+        (properties
+          ? continuation(
+              Object.fromEntries(
+                properties.map(property => [
+                  (prefix
+                    ? `${prefix}.${property}`
+                 : list
+                    ? `${list}.${property}`
+                    : `#list.${property}`),
+                  lists[property],
+                ])))
+          : continuation({'#lists': lists})),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withPropertiesFromObject.js b/src/data/composite/data/withPropertiesFromObject.js
new file mode 100644
index 00000000..21726b58
--- /dev/null
+++ b/src/data/composite/data/withPropertiesFromObject.js
@@ -0,0 +1,87 @@
+// Gets the listed properties from some object, providing each property's value
+// as a dependency prefixed with the same name as the object (by default).
+// If the object itself is null, all provided dependencies will be null;
+// if it's missing only select properties, those will be provided as null.
+//
+// See also:
+//  - withPropertiesFromList
+//  - withPropertyFromObject
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {isString, validateArrayItems} from '#validators';
+
+export default templateCompositeFrom({
+  annotation: `withPropertiesFromObject`,
+
+  inputs: {
+    object: input({type: 'object', acceptsNull: true}),
+
+    properties: input({
+      type: 'array',
+      validate: validateArrayItems(isString),
+    }),
+
+    prefix: input.staticValue({type: 'string', defaultValue: null}),
+  },
+
+  outputs: ({
+    [input.staticDependency('object')]: object,
+    [input.staticValue('properties')]: properties,
+    [input.staticValue('prefix')]: prefix,
+  }) =>
+    (properties
+      ? properties.map(property =>
+          (prefix
+            ? `${prefix}.${property}`
+         : object
+            ? `${object}.${property}`
+            : `#object.${property}`))
+      : ['#object']),
+
+  steps: () => [
+    {
+      dependencies: [input('object'), input('properties')],
+      compute: (continuation, {
+        [input('object')]: object,
+        [input('properties')]: properties,
+      }) => continuation({
+        ['#entries']:
+          (object === null
+            ? properties.map(property => [property, null])
+            : properties.map(property => [property, object[property]])),
+      }),
+    },
+
+    {
+      dependencies: [
+        input.staticDependency('object'),
+        input.staticValue('properties'),
+        input.staticValue('prefix'),
+        '#entries',
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('object')]: object,
+        [input.staticValue('properties')]: properties,
+        [input.staticValue('prefix')]: prefix,
+        ['#entries']: entries,
+      }) =>
+        (properties
+          ? continuation(
+              Object.fromEntries(
+                entries.map(([property, value]) => [
+                  (prefix
+                    ? `${prefix}.${property}`
+                 : object
+                    ? `${object}.${property}`
+                    : `#object.${property}`),
+                  value ?? null,
+                ])))
+          : continuation({
+              ['#object']:
+                Object.fromEntries(entries),
+            })),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withPropertyFromList.js b/src/data/composite/data/withPropertyFromList.js
new file mode 100644
index 00000000..1983ebbc
--- /dev/null
+++ b/src/data/composite/data/withPropertyFromList.js
@@ -0,0 +1,82 @@
+// Gets a property from each of a list of objects (in a dependency) and
+// provides the results.
+//
+// This doesn't alter any list indices, so positions which were null in the
+// original list are kept null here. Objects which don't have the specified
+// property are retained in-place as null.
+//
+// See also:
+//  - withPropertiesFromList
+//  - withPropertyFromObject
+//
+// More list utilities:
+//  - excludeFromList
+//  - fillMissingListItems
+//  - withFlattenedList
+//  - withUnflattenedList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+function getOutputName({list, property, prefix}) {
+  if (!property) return `#values`;
+  if (prefix) return `${prefix}.${property}`;
+  if (list) return `${list}.${property}`;
+  return `#list.${property}`;
+}
+
+export default templateCompositeFrom({
+  annotation: `withPropertyFromList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+    property: input({type: 'string'}),
+    prefix: input.staticValue({type: 'string', defaultValue: null}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+    [input.staticValue('property')]: property,
+    [input.staticValue('prefix')]: prefix,
+  }) =>
+    [getOutputName({list, property, prefix})],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('property')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('property')]: property,
+      }) => continuation({
+        ['#values']:
+          list.map(item => item[property] ?? null),
+      }),
+    },
+
+    {
+      dependencies: [
+        input.staticDependency('list'),
+        input.staticValue('property'),
+        input.staticValue('prefix'),
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('list')]: list,
+        [input.staticValue('property')]: property,
+        [input.staticValue('prefix')]: prefix,
+      }) => continuation({
+        ['#outputName']:
+          getOutputName({list, property, prefix}),
+      }),
+    },
+
+    {
+      dependencies: ['#values', '#outputName'],
+      compute: (continuation, {
+        ['#values']: values,
+        ['#outputName']: outputName,
+      }) =>
+        continuation.raiseOutput({[outputName]: values}),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withPropertyFromObject.js b/src/data/composite/data/withPropertyFromObject.js
new file mode 100644
index 00000000..b31bab15
--- /dev/null
+++ b/src/data/composite/data/withPropertyFromObject.js
@@ -0,0 +1,69 @@
+// Gets a property of some object (in a dependency) and provides that value.
+// If the object itself is null, or the object doesn't have the listed property,
+// the provided dependency will also be null.
+//
+// See also:
+//  - withPropertiesFromObject
+//  - withPropertyFromList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `withPropertyFromObject`,
+
+  inputs: {
+    object: input({type: 'object', acceptsNull: true}),
+    property: input({type: 'string'}),
+  },
+
+  outputs: ({
+    [input.staticDependency('object')]: object,
+    [input.staticValue('property')]: property,
+  }) =>
+    (object && property
+      ? (object.startsWith('#')
+          ? [`${object}.${property}`]
+          : [`#${object}.${property}`])
+      : ['#value']),
+
+  steps: () => [
+    {
+      dependencies: [
+        input.staticDependency('object'),
+        input.staticValue('property'),
+      ],
+
+      compute: (continuation, {
+        [input.staticDependency('object')]: object,
+        [input.staticValue('property')]: property,
+      }) => continuation({
+        '#output':
+          (object && property
+            ? (object.startsWith('#')
+                ? `${object}.${property}`
+                : `#${object}.${property}`)
+            : '#value'),
+      }),
+    },
+
+    {
+      dependencies: [
+        '#output',
+        input('object'),
+        input('property'),
+      ],
+
+      compute: (continuation, {
+        ['#output']: output,
+        [input('object')]: object,
+        [input('property')]: property,
+      }) => continuation({
+        [output]:
+          (object === null
+            ? null
+            : object[property] ?? null),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withUnflattenedList.js b/src/data/composite/data/withUnflattenedList.js
new file mode 100644
index 00000000..3cfc247b
--- /dev/null
+++ b/src/data/composite/data/withUnflattenedList.js
@@ -0,0 +1,62 @@
+// 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).
+
+import {input, templateCompositeFrom} from '#composite';
+import {isWholeNumber, validateArrayItems} from '#validators';
+
+export default templateCompositeFrom({
+  annotation: `withUnflattenedList`,
+
+  inputs: {
+    list: input({
+      type: 'array',
+      defaultDependency: '#flattenedList',
+    }),
+
+    indices: input({
+      validate: validateArrayItems(isWholeNumber),
+      defaultDependency: '#flattenedIndices',
+    }),
+
+    filter: input({
+      type: 'boolean',
+      defaultValue: true,
+    }),
+  },
+
+  outputs: ['#unflattenedList'],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('indices'), input('filter')],
+      compute(continuation, {
+        [input('list')]: list,
+        [input('indices')]: indices,
+        [input('filter')]: filter,
+      }) {
+        const unflattenedList = [];
+
+        for (let i = 0; i < indices.length; i++) {
+          const startIndex = indices[i];
+          const endIndex =
+            (i === indices.length - 1
+              ? list.length
+              : indices[i + 1]);
+
+          const values = list.slice(startIndex, endIndex);
+          unflattenedList.push(
+            (filter
+              ? values.filter(value => value !== null && value !== undefined)
+              : values));
+        }
+
+        return continuation({
+          ['#unflattenedList']: unflattenedList,
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/things/album/index.js b/src/data/composite/things/album/index.js
new file mode 100644
index 00000000..8139f10e
--- /dev/null
+++ b/src/data/composite/things/album/index.js
@@ -0,0 +1,2 @@
+export {default as withTracks} from './withTracks.js';
+export {default as withTrackSections} from './withTrackSections.js';
diff --git a/src/data/composite/things/album/withTrackSections.js b/src/data/composite/things/album/withTrackSections.js
new file mode 100644
index 00000000..baa3cb4a
--- /dev/null
+++ b/src/data/composite/things/album/withTrackSections.js
@@ -0,0 +1,128 @@
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+import {empty, stitchArrays} from '#sugar';
+import {isTrackSectionList} from '#validators';
+import {filterMultipleArrays} from '#wiki-data';
+
+import {exitWithoutDependency, exitWithoutUpdateValue}
+  from '#composite/control-flow';
+import {withResolvedReferenceList} from '#composite/wiki-data';
+
+import {
+  fillMissingListItems,
+  withFlattenedList,
+  withPropertiesFromList,
+  withUnflattenedList,
+} from '#composite/data';
+
+export default templateCompositeFrom({
+  annotation: `withTrackSections`,
+
+  outputs: ['#trackSections'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: 'trackData',
+      value: input.value([]),
+    }),
+
+    exitWithoutUpdateValue({
+      mode: input.value('empty'),
+      value: input.value([]),
+    }),
+
+    // TODO: input.updateValue description down here is a kludge.
+    withPropertiesFromList({
+      list: input.updateValue({
+        validate: isTrackSectionList,
+      }),
+      prefix: input.value('#sections'),
+      properties: input.value([
+        'tracks',
+        'dateOriginallyReleased',
+        'isDefaultTrackSection',
+        'name',
+        'color',
+      ]),
+    }),
+
+    fillMissingListItems({
+      list: '#sections.tracks',
+      fill: input.value([]),
+    }),
+
+    fillMissingListItems({
+      list: '#sections.isDefaultTrackSection',
+      fill: input.value(false),
+    }),
+
+    fillMissingListItems({
+      list: '#sections.name',
+      fill: input.value('Unnamed Track Section'),
+    }),
+
+    fillMissingListItems({
+      list: '#sections.color',
+      fill: input.dependency('color'),
+    }),
+
+    withFlattenedList({
+      list: '#sections.tracks',
+    }).outputs({
+      ['#flattenedList']: '#trackRefs',
+      ['#flattenedIndices']: '#sections.startIndex',
+    }),
+
+    withResolvedReferenceList({
+      list: '#trackRefs',
+      data: 'trackData',
+      notFoundMode: input.value('null'),
+      find: input.value(find.track),
+    }).outputs({
+      ['#resolvedReferenceList']: '#tracks',
+    }),
+
+    withUnflattenedList({
+      list: '#tracks',
+      indices: '#sections.startIndex',
+    }).outputs({
+      ['#unflattenedList']: '#sections.tracks',
+    }),
+
+    {
+      dependencies: [
+        '#sections.tracks',
+        '#sections.name',
+        '#sections.color',
+        '#sections.dateOriginallyReleased',
+        '#sections.isDefaultTrackSection',
+        '#sections.startIndex',
+      ],
+
+      compute: (continuation, {
+        '#sections.tracks': tracks,
+        '#sections.name': name,
+        '#sections.color': color,
+        '#sections.dateOriginallyReleased': dateOriginallyReleased,
+        '#sections.isDefaultTrackSection': isDefaultTrackSection,
+        '#sections.startIndex': startIndex,
+      }) => {
+        filterMultipleArrays(
+          tracks, name, color, dateOriginallyReleased, isDefaultTrackSection, startIndex,
+          tracks => !empty(tracks));
+
+        return continuation({
+          ['#trackSections']:
+            stitchArrays({
+              tracks,
+              name,
+              color,
+              dateOriginallyReleased,
+              isDefaultTrackSection,
+              startIndex,
+            }),
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/things/album/withTracks.js b/src/data/composite/things/album/withTracks.js
new file mode 100644
index 00000000..dcea6593
--- /dev/null
+++ b/src/data/composite/things/album/withTracks.js
@@ -0,0 +1,51 @@
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+
+import {exitWithoutDependency, raiseOutputWithoutDependency}
+  from '#composite/control-flow';
+import {withResolvedReferenceList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `withTracks`,
+
+  outputs: ['#tracks'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: 'trackData',
+      value: input.value([]),
+    }),
+
+    raiseOutputWithoutDependency({
+      dependency: 'trackSections',
+      mode: input.value('empty'),
+      output: input.value({
+        ['#tracks']: [],
+      }),
+    }),
+
+    {
+      dependencies: ['trackSections'],
+      compute: (continuation, {trackSections}) =>
+        continuation({
+          '#trackRefs': trackSections
+            .flatMap(section => section.tracks ?? []),
+        }),
+    },
+
+    withResolvedReferenceList({
+      list: '#trackRefs',
+      data: 'trackData',
+      find: input.value(find.track),
+    }),
+
+    {
+      dependencies: ['#resolvedReferenceList'],
+      compute: (continuation, {
+        ['#resolvedReferenceList']: resolvedReferenceList,
+      }) => continuation({
+        ['#tracks']: resolvedReferenceList,
+      })
+    },
+  ],
+});
diff --git a/src/data/composite/things/flash/index.js b/src/data/composite/things/flash/index.js
new file mode 100644
index 00000000..63ac13da
--- /dev/null
+++ b/src/data/composite/things/flash/index.js
@@ -0,0 +1 @@
+export {default as withFlashAct} from './withFlashAct.js';
diff --git a/src/data/composite/things/flash/withFlashAct.js b/src/data/composite/things/flash/withFlashAct.js
new file mode 100644
index 00000000..ada2dcfe
--- /dev/null
+++ b/src/data/composite/things/flash/withFlashAct.js
@@ -0,0 +1,108 @@
+// Gets the flash's act. This will early exit if flashActData is missing.
+// By default, if there's no flash whose list of flashes includes this flash,
+// the output dependency will be null; set {notFoundMode: 'exit'} to early
+// exit instead.
+//
+// This step models with Flash.withAlbum.
+
+import {input, templateCompositeFrom} from '#composite';
+import {is} from '#validators';
+
+import {exitWithoutDependency, withResultOfAvailabilityCheck}
+  from '#composite/control-flow';
+import {withPropertyFromList} from '#composite/data';
+
+export default templateCompositeFrom({
+  annotation: `withFlashAct`,
+
+  inputs: {
+    notFoundMode: input({
+      validate: is('exit', 'null'),
+      defaultValue: 'null',
+    }),
+  },
+
+  outputs: ['#flashAct'],
+
+  steps: () => [
+    // null flashActData is always an early exit.
+
+    exitWithoutDependency({
+      dependency: 'flashActData',
+      mode: input.value('null'),
+    }),
+
+    // empty flashActData conditionally exits early or outputs null.
+
+    withResultOfAvailabilityCheck({
+      from: 'flashActData',
+      mode: input.value('empty'),
+    }).outputs({
+      '#availability': '#flashActDataAvailability',
+    }),
+
+    {
+      dependencies: [input('notFoundMode'), '#flashActDataAvailability'],
+      compute(continuation, {
+        [input('notFoundMode')]: notFoundMode,
+        ['#flashActDataAvailability']: flashActDataIsAvailable,
+      }) {
+        if (flashActDataIsAvailable) return continuation();
+        switch (notFoundMode) {
+          case 'exit': return continuation.exit(null);
+          case 'null': return continuation.raiseOutput({'#flashAct': null});
+        }
+      },
+    },
+
+    withPropertyFromList({
+      list: 'flashActData',
+      property: input.value('flashes'),
+    }),
+
+    {
+      dependencies: [input.myself(), '#flashActData.flashes'],
+      compute: (continuation, {
+        [input.myself()]: track,
+        ['#flashActData.flashes']: flashLists,
+      }) => continuation({
+        ['#flashActIndex']:
+          flashLists.findIndex(flashes => flashes.includes(track)),
+      }),
+    },
+
+    // album not found conditionally exits or outputs null.
+
+    withResultOfAvailabilityCheck({
+      from: '#flashActIndex',
+      mode: input.value('index'),
+    }).outputs({
+      '#availability': '#flashActAvailability',
+    }),
+
+    {
+      dependencies: [input('notFoundMode'), '#flashActAvailability'],
+      compute(continuation, {
+        [input('notFoundMode')]: notFoundMode,
+        ['#flashActAvailability']: flashActIsAvailable,
+      }) {
+        if (flashActIsAvailable) return continuation();
+        switch (notFoundMode) {
+          case 'exit': return continuation.exit(null);
+          case 'null': return continuation.raiseOutput({'#flashAct': null});
+        }
+      },
+    },
+
+    {
+      dependencies: ['flashActData', '#flashActIndex'],
+      compute: (continuation, {
+        ['flashActData']: flashActData,
+        ['#flashActIndex']: flashActIndex,
+      }) => continuation.raiseOutput({
+        ['#flashAct']:
+          flashActData[flashActIndex],
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/exitWithoutUniqueCoverArt.js b/src/data/composite/things/track/exitWithoutUniqueCoverArt.js
new file mode 100644
index 00000000..f47086d9
--- /dev/null
+++ b/src/data/composite/things/track/exitWithoutUniqueCoverArt.js
@@ -0,0 +1,26 @@
+// Shorthand for checking if the track has unique cover art and exposing a
+// fallback value if it isn't.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency} from '#composite/control-flow';
+
+import withHasUniqueCoverArt from './withHasUniqueCoverArt.js';
+
+export default templateCompositeFrom({
+  annotation: `exitWithoutUniqueCoverArt`,
+
+  inputs: {
+    value: input({defaultValue: null}),
+  },
+
+  steps: () => [
+    withHasUniqueCoverArt(),
+
+    exitWithoutDependency({
+      dependency: '#hasUniqueCoverArt',
+      mode: input.value('falsy'),
+      value: input('value'),
+    }),
+  ],
+});
diff --git a/src/data/composite/things/track/index.js b/src/data/composite/things/track/index.js
new file mode 100644
index 00000000..3354b1c4
--- /dev/null
+++ b/src/data/composite/things/track/index.js
@@ -0,0 +1,9 @@
+export {default as exitWithoutUniqueCoverArt} from './exitWithoutUniqueCoverArt.js';
+export {default as inheritFromOriginalRelease} from './inheritFromOriginalRelease.js';
+export {default as trackReverseReferenceList} from './trackReverseReferenceList.js';
+export {default as withAlbum} from './withAlbum.js';
+export {default as withAlwaysReferenceByDirectory} from './withAlwaysReferenceByDirectory.js';
+export {default as withContainingTrackSection} from './withContainingTrackSection.js';
+export {default as withHasUniqueCoverArt} from './withHasUniqueCoverArt.js';
+export {default as withOtherReleases} from './withOtherReleases.js';
+export {default as withPropertyFromAlbum} from './withPropertyFromAlbum.js';
diff --git a/src/data/composite/things/track/inheritFromOriginalRelease.js b/src/data/composite/things/track/inheritFromOriginalRelease.js
new file mode 100644
index 00000000..a9d57f86
--- /dev/null
+++ b/src/data/composite/things/track/inheritFromOriginalRelease.js
@@ -0,0 +1,43 @@
+// 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.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import withOriginalRelease from './withOriginalRelease.js';
+
+export default templateCompositeFrom({
+  annotation: `inheritFromOriginalRelease`,
+
+  inputs: {
+    property: input({type: 'string'}),
+    allowOverride: input({type: 'boolean', defaultValue: false}),
+  },
+
+  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);
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/trackReverseReferenceList.js b/src/data/composite/things/track/trackReverseReferenceList.js
new file mode 100644
index 00000000..e7bfedf3
--- /dev/null
+++ b/src/data/composite/things/track/trackReverseReferenceList.js
@@ -0,0 +1,38 @@
+// Like a normal reverse reference list ("objects which reference this object
+// under a specified property"), only excluding re-releases from the possible
+// outputs. While it's useful to travel from a re-release to the tracks it
+// references, re-releases aren't generally relevant from the perspective of
+// the tracks *being* referenced. Apart from hiding re-releases from lists on
+// the site, it also excludes keeps them from relational data processing, such
+// as on the "Tracks - by Times Referenced" listing page.
+
+import {input, templateCompositeFrom} from '#composite';
+import {withReverseReferenceList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `trackReverseReferenceList`,
+
+  compose: false,
+
+  inputs: {
+    list: input({type: 'string'}),
+  },
+
+  steps: () => [
+    withReverseReferenceList({
+      data: 'trackData',
+      list: input('list'),
+    }),
+
+    {
+      flags: {expose: true},
+      expose: {
+        dependencies: ['#reverseReferenceList'],
+        compute: ({
+          ['#reverseReferenceList']: reverseReferenceList,
+        }) =>
+          reverseReferenceList.filter(track => !track.originalReleaseTrack),
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withAlbum.js b/src/data/composite/things/track/withAlbum.js
new file mode 100644
index 00000000..cbd16dcd
--- /dev/null
+++ b/src/data/composite/things/track/withAlbum.js
@@ -0,0 +1,108 @@
+// 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.
+//
+// This step models with Flash.withFlashAct.
+
+import {input, templateCompositeFrom} from '#composite';
+import {is} from '#validators';
+
+import {exitWithoutDependency, withResultOfAvailabilityCheck}
+  from '#composite/control-flow';
+import {withPropertyFromList} from '#composite/data';
+
+export default templateCompositeFrom({
+  annotation: `withAlbum`,
+
+  inputs: {
+    notFoundMode: input({
+      validate: is('exit', 'null'),
+      defaultValue: 'null',
+    }),
+  },
+
+  outputs: ['#album'],
+
+  steps: () => [
+    // null albumData is always an early exit.
+
+    exitWithoutDependency({
+      dependency: 'albumData',
+      mode: input.value('null'),
+    }),
+
+    // empty albumData conditionally exits early or outputs null.
+
+    withResultOfAvailabilityCheck({
+      from: 'albumData',
+      mode: input.value('empty'),
+    }).outputs({
+      '#availability': '#albumDataAvailability',
+    }),
+
+    {
+      dependencies: [input('notFoundMode'), '#albumDataAvailability'],
+      compute(continuation, {
+        [input('notFoundMode')]: notFoundMode,
+        ['#albumDataAvailability']: albumDataIsAvailable,
+      }) {
+        if (albumDataIsAvailable) return continuation();
+        switch (notFoundMode) {
+          case 'exit': return continuation.exit(null);
+          case 'null': return continuation.raiseOutput({'#album': null});
+        }
+      },
+    },
+
+    withPropertyFromList({
+      list: 'albumData',
+      property: input.value('tracks'),
+    }),
+
+    {
+      dependencies: [input.myself(), '#albumData.tracks'],
+      compute: (continuation, {
+        [input.myself()]: track,
+        ['#albumData.tracks']: trackLists,
+      }) => continuation({
+        ['#albumIndex']:
+          trackLists.findIndex(tracks => tracks.includes(track)),
+      }),
+    },
+
+    // album not found conditionally exits or outputs null.
+
+    withResultOfAvailabilityCheck({
+      from: '#albumIndex',
+      mode: input.value('index'),
+    }).outputs({
+      '#availability': '#albumAvailability',
+    }),
+
+    {
+      dependencies: [input('notFoundMode'), '#albumAvailability'],
+      compute(continuation, {
+        [input('notFoundMode')]: notFoundMode,
+        ['#albumAvailability']: albumIsAvailable,
+      }) {
+        if (albumIsAvailable) return continuation();
+        switch (notFoundMode) {
+          case 'exit': return continuation.exit(null);
+          case 'null': return continuation.raiseOutput({'#album': null});
+        }
+      },
+    },
+
+    {
+      dependencies: ['albumData', '#albumIndex'],
+      compute: (continuation, {
+        ['albumData']: albumData,
+        ['#albumIndex']: albumIndex,
+      }) => continuation.raiseOutput({
+        ['#album']:
+          albumData[albumIndex],
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withAlwaysReferenceByDirectory.js b/src/data/composite/things/track/withAlwaysReferenceByDirectory.js
new file mode 100644
index 00000000..d27f7b23
--- /dev/null
+++ b/src/data/composite/things/track/withAlwaysReferenceByDirectory.js
@@ -0,0 +1,91 @@
+// 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.
+//
+// See the implementation for an important caveat about matching the original
+// track against other tracks, which uses a custom implementation pulling (and
+// duplicating) details from #find instead of using withOriginalRelease and the
+// usual withResolvedReference / find.track() utilities.
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {isBoolean} from '#validators';
+
+import {exitWithoutDependency, exposeUpdateValueOrContinue}
+  from '#composite/control-flow';
+import {withPropertyFromObject} from '#composite/data';
+
+// TODO: Kludge. (The usage of this, not so much the import.)
+import CacheableObject from '../../../things/cacheable-object.js';
+
+export default templateCompositeFrom({
+  annotation: `withAlwaysReferenceByDirectory`,
+
+  outputs: ['#alwaysReferenceByDirectory'],
+
+  steps: () => [
+    exposeUpdateValueOrContinue({
+      validate: input.value(isBoolean),
+    }),
+
+    // Remaining code is for defaulting to true if this track is a rerelease of
+    // another with the same name, so everything further depends on access to
+    // trackData as well as originalReleaseTrack.
+
+    exitWithoutDependency({
+      dependency: 'trackData',
+      mode: input.value('empty'),
+      value: input.value(false),
+    }),
+
+    exitWithoutDependency({
+      dependency: 'originalReleaseTrack',
+      value: input.value(false),
+    }),
+
+    // "Slow" / uncached, manual search from trackData (with this track
+    // excluded). Otherwise there end up being pretty bad recursion issues
+    // (track1.alwaysReferencedByDirectory depends on searching through data
+    // including track2, which depends on evaluating track2.alwaysReferenced-
+    // ByDirectory, which depends on searcing through data including track1...)
+    // That said, this is 100% a kludge, since it involves duplicating find
+    // logic on a completely unrelated context.
+    {
+      dependencies: [input.myself(), 'trackData', 'originalReleaseTrack'],
+      compute: (continuation, {
+        [input.myself()]: thisTrack,
+        ['trackData']: trackData,
+        ['originalReleaseTrack']: ref,
+      }) => continuation({
+        ['#originalRelease']:
+          (ref.startsWith('track:')
+            ? trackData.find(track => track.directory === ref.slice('track:'.length))
+            : trackData.find(track =>
+                track !== thisTrack &&
+                !CacheableObject.getUpdateValue(track, 'originalReleaseTrack') &&
+                track.name.toLowerCase() === ref.toLowerCase())),
+      })
+    },
+
+    exitWithoutDependency({
+      dependency: '#originalRelease',
+      value: input.value(false),
+    }),
+
+    withPropertyFromObject({
+      object: '#originalRelease',
+      property: input.value('name'),
+    }),
+
+    {
+      dependencies: ['name', '#originalRelease.name'],
+      compute: (continuation, {
+        name,
+        ['#originalRelease.name']: originalName,
+      }) => continuation({
+        ['#alwaysReferenceByDirectory']: name === originalName,
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withContainingTrackSection.js b/src/data/composite/things/track/withContainingTrackSection.js
new file mode 100644
index 00000000..b2e5f2b3
--- /dev/null
+++ b/src/data/composite/things/track/withContainingTrackSection.js
@@ -0,0 +1,63 @@
+// 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.
+
+import {input, templateCompositeFrom} from '#composite';
+import {is} from '#validators';
+
+import withPropertyFromAlbum from './withPropertyFromAlbum.js';
+
+export default templateCompositeFrom({
+  annotation: `withContainingTrackSection`,
+
+  inputs: {
+    notFoundMode: input({
+      validate: is('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,
+          });
+        }
+
+        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,
+          });
+        }
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withHasUniqueCoverArt.js b/src/data/composite/things/track/withHasUniqueCoverArt.js
new file mode 100644
index 00000000..96078d5f
--- /dev/null
+++ b/src/data/composite/things/track/withHasUniqueCoverArt.js
@@ -0,0 +1,61 @@
+// Whether or not the track has "unique" cover artwork - a cover which is
+// specifically associated with this track in particular, rather than with
+// the track's album as a whole. This is typically used to select between
+// displaying the track artwork and a fallback, such as the album artwork
+// or a placeholder. (This property is named hasUniqueCoverArt instead of
+// the usual hasCoverArt to emphasize that it does not inherit from the
+// album.)
+
+import {input, templateCompositeFrom} from '#composite';
+import {empty} from '#sugar';
+
+import {withResolvedContribs} from '#composite/wiki-data';
+
+import withPropertyFromAlbum from './withPropertyFromAlbum.js';
+
+export default templateCompositeFrom({
+  annotation: 'withHasUniqueCoverArt',
+
+  outputs: ['#hasUniqueCoverArt'],
+
+  steps: () => [
+    {
+      dependencies: ['disableUniqueCoverArt'],
+      compute: (continuation, {disableUniqueCoverArt}) =>
+        (disableUniqueCoverArt
+          ? continuation.raiseOutput({
+              ['#hasUniqueCoverArt']: false,
+            })
+          : continuation()),
+    },
+
+    withResolvedContribs({from: 'coverArtistContribs'}),
+
+    {
+      dependencies: ['#resolvedContribs'],
+      compute: (continuation, {
+        ['#resolvedContribs']: contribsFromTrack,
+      }) =>
+        (empty(contribsFromTrack)
+          ? continuation()
+          : continuation.raiseOutput({
+              ['#hasUniqueCoverArt']: true,
+            })),
+    },
+
+    withPropertyFromAlbum({
+      property: input.value('trackCoverArtistContribs'),
+    }),
+
+    {
+      dependencies: ['#album.trackCoverArtistContribs'],
+      compute: (continuation, {
+        ['#album.trackCoverArtistContribs']: contribsFromAlbum,
+      }) =>
+        continuation.raiseOutput({
+          ['#hasUniqueCoverArt']:
+            !empty(contribsFromAlbum),
+        }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withOriginalRelease.js b/src/data/composite/things/track/withOriginalRelease.js
new file mode 100644
index 00000000..d2ee39df
--- /dev/null
+++ b/src/data/composite/things/track/withOriginalRelease.js
@@ -0,0 +1,59 @@
+// 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.
+
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+import {validateWikiData} from '#validators';
+
+import {withResolvedReference} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `withOriginalRelease`,
+
+  inputs: {
+    selfIfOriginal: input({type: 'boolean', defaultValue: false}),
+
+    data: input({
+      validate: validateWikiData({referenceType: 'track'}),
+      defaultDependency: 'trackData',
+    }),
+  },
+
+  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)),
+        }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withOtherReleases.js b/src/data/composite/things/track/withOtherReleases.js
new file mode 100644
index 00000000..84420cf8
--- /dev/null
+++ b/src/data/composite/things/track/withOtherReleases.js
@@ -0,0 +1,40 @@
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency} from '#composite/control-flow';
+
+import withOriginalRelease from './withOriginalRelease.js';
+
+export default templateCompositeFrom({
+  annotation: `withOtherReleases`,
+
+  outputs: ['#otherReleases'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: 'trackData',
+      mode: input.value('empty'),
+    }),
+
+    withOriginalRelease({
+      selfIfOriginal: input.value(true),
+    }),
+
+    {
+      dependencies: [input.myself(), '#originalRelease', 'trackData'],
+      compute: (continuation, {
+        [input.myself()]: thisTrack,
+        ['#originalRelease']: originalRelease,
+        trackData,
+      }) => continuation({
+        ['#otherReleases']:
+          (originalRelease === thisTrack
+            ? []
+            : [originalRelease])
+            .concat(trackData.filter(track =>
+              track !== originalRelease &&
+              track !== thisTrack &&
+              track.originalReleaseTrack === originalRelease)),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/things/track/withPropertyFromAlbum.js b/src/data/composite/things/track/withPropertyFromAlbum.js
new file mode 100644
index 00000000..b236a6e8
--- /dev/null
+++ b/src/data/composite/things/track/withPropertyFromAlbum.js
@@ -0,0 +1,49 @@
+// 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.
+
+import {input, templateCompositeFrom} from '#composite';
+import {is} from '#validators';
+
+import {withPropertyFromObject} from '#composite/data';
+
+import withAlbum from './withAlbum.js';
+
+export default templateCompositeFrom({
+  annotation: `withPropertyFromAlbum`,
+
+  inputs: {
+    property: input.staticValue({type: 'string'}),
+
+    notFoundMode: input({
+      validate: is('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,
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/exitWithoutContribs.js b/src/data/composite/wiki-data/exitWithoutContribs.js
new file mode 100644
index 00000000..2c8219fc
--- /dev/null
+++ b/src/data/composite/wiki-data/exitWithoutContribs.js
@@ -0,0 +1,47 @@
+// Shorthand for exiting if the contribution list (usually a property's update
+// value) resolves to empty - ensuring that the later computed results are only
+// returned if these contributions are present.
+
+import {input, templateCompositeFrom} from '#composite';
+import {isContributionList} from '#validators';
+
+import {withResultOfAvailabilityCheck} from '#composite/control-flow';
+
+import withResolvedContribs from './withResolvedContribs.js';
+
+export default templateCompositeFrom({
+  annotation: `exitWithoutContribs`,
+
+  inputs: {
+    contribs: input({
+      validate: isContributionList,
+      acceptsNull: true,
+    }),
+
+    value: input({defaultValue: null}),
+  },
+
+  steps: () => [
+    withResolvedContribs({
+      from: input('contribs'),
+    }),
+
+    // TODO: Fairly certain exitWithoutDependency would be sufficient here.
+
+    withResultOfAvailabilityCheck({
+      from: '#resolvedContribs',
+      mode: input.value('empty'),
+    }),
+
+    {
+      dependencies: ['#availability', input('value')],
+      compute: (continuation, {
+        ['#availability']: availability,
+        [input('value')]: value,
+      }) =>
+        (availability
+          ? continuation()
+          : continuation.exit(value)),
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/index.js b/src/data/composite/wiki-data/index.js
new file mode 100644
index 00000000..1d0400fc
--- /dev/null
+++ b/src/data/composite/wiki-data/index.js
@@ -0,0 +1,7 @@
+export {default as exitWithoutContribs} from './exitWithoutContribs.js';
+export {default as inputThingClass} from './inputThingClass.js';
+export {default as inputWikiData} from './inputWikiData.js';
+export {default as withResolvedContribs} from './withResolvedContribs.js';
+export {default as withResolvedReference} from './withResolvedReference.js';
+export {default as withResolvedReferenceList} from './withResolvedReferenceList.js';
+export {default as withReverseReferenceList} from './withReverseReferenceList.js';
diff --git a/src/data/composite/wiki-data/inputThingClass.js b/src/data/composite/wiki-data/inputThingClass.js
new file mode 100644
index 00000000..d70480e6
--- /dev/null
+++ b/src/data/composite/wiki-data/inputThingClass.js
@@ -0,0 +1,23 @@
+// Please note that this input, used in a variety of #composite/wiki-data
+// utilities, is basically always a kludge. Any usage of it depends on
+// referencing Thing class values defined outside of the #composite folder.
+
+import {input} from '#composite';
+import {isType} from '#validators';
+
+// TODO: Kludge.
+import Thing from '../../things/thing.js';
+
+export default function inputThingClass() {
+  return input.staticValue({
+    validate(thingClass) {
+      isType(thingClass, 'function');
+
+      if (!Object.hasOwn(thingClass, Thing.referenceType)) {
+        throw new TypeError(`Expected a Thing constructor, missing Thing.referenceType`);
+      }
+
+      return true;
+    },
+  });
+}
diff --git a/src/data/composite/wiki-data/inputWikiData.js b/src/data/composite/wiki-data/inputWikiData.js
new file mode 100644
index 00000000..cf7a7c2c
--- /dev/null
+++ b/src/data/composite/wiki-data/inputWikiData.js
@@ -0,0 +1,17 @@
+import {input} from '#composite';
+import {validateWikiData} from '#validators';
+
+// TODO: This doesn't access a class's own ThingSubclass[Thing.referenceType]
+// value because classes aren't initialized by when templateCompositeFrom gets
+// called (see: circular imports). So the reference types have to be hard-coded,
+// which somewhat defeats the point of storing them on the class in the first
+// place...
+export default function inputWikiData({
+  referenceType = '',
+  allowMixedTypes = false,
+} = {}) {
+  return input({
+    validate: validateWikiData({referenceType, allowMixedTypes}),
+    acceptsNull: true,
+  });
+}
diff --git a/src/data/composite/wiki-data/withResolvedContribs.js b/src/data/composite/wiki-data/withResolvedContribs.js
new file mode 100644
index 00000000..eda24160
--- /dev/null
+++ b/src/data/composite/wiki-data/withResolvedContribs.js
@@ -0,0 +1,77 @@
+// Resolves the contribsByRef contained in the provided dependency,
+// providing (named by the second argument) the result. "Resolving"
+// means mapping the "who" reference of each contribution to an artist
+// object, and filtering out those whose "who" doesn't match any artist.
+
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+import {stitchArrays} from '#sugar';
+import {is, isContributionList} from '#validators';
+import {filterMultipleArrays} from '#wiki-data';
+
+import {
+  raiseOutputWithoutDependency,
+} from '#composite/control-flow';
+
+import {
+  withPropertiesFromList,
+} from '#composite/data';
+
+import withResolvedReferenceList from './withResolvedReferenceList.js';
+
+export default templateCompositeFrom({
+  annotation: `withResolvedContribs`,
+
+  inputs: {
+    from: input({
+      validate: isContributionList,
+      acceptsNull: true,
+    }),
+
+    notFoundMode: input({
+      validate: is('exit', 'filter', 'null'),
+      defaultValue: 'null',
+    }),
+  },
+
+  outputs: ['#resolvedContribs'],
+
+  steps: () => [
+    raiseOutputWithoutDependency({
+      dependency: input('from'),
+      mode: input.value('empty'),
+      output: input.value({
+        ['#resolvedContribs']: [],
+      }),
+    }),
+
+    withPropertiesFromList({
+      list: input('from'),
+      properties: input.value(['who', 'what']),
+      prefix: input.value('#contribs'),
+    }),
+
+    withResolvedReferenceList({
+      list: '#contribs.who',
+      data: 'artistData',
+      find: input.value(find.artist),
+      notFoundMode: input('notFoundMode'),
+    }).outputs({
+      ['#resolvedReferenceList']: '#contribs.who',
+    }),
+
+    {
+      dependencies: ['#contribs.who', '#contribs.what'],
+
+      compute(continuation, {
+        ['#contribs.who']: who,
+        ['#contribs.what']: what,
+      }) {
+        filterMultipleArrays(who, what, (who, _what) => who);
+        return continuation({
+          ['#resolvedContribs']: stitchArrays({who, what}),
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/withResolvedReference.js b/src/data/composite/wiki-data/withResolvedReference.js
new file mode 100644
index 00000000..0fa5c554
--- /dev/null
+++ b/src/data/composite/wiki-data/withResolvedReference.js
@@ -0,0 +1,73 @@
+// 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.
+
+import {input, templateCompositeFrom} from '#composite';
+import {is} from '#validators';
+
+import {
+  exitWithoutDependency,
+  raiseOutputWithoutDependency,
+} from '#composite/control-flow';
+
+import inputWikiData from './inputWikiData.js';
+
+export default templateCompositeFrom({
+  annotation: `withResolvedReference`,
+
+  inputs: {
+    ref: input({type: 'string', acceptsNull: true}),
+
+    data: inputWikiData({allowMixedTypes: false}),
+    find: input({type: 'function'}),
+
+    notFoundMode: input({
+      validate: is('null', 'exit'),
+      defaultValue: 'null',
+    }),
+  },
+
+  outputs: ['#resolvedReference'],
+
+  steps: () => [
+    raiseOutputWithoutDependency({
+      dependency: input('ref'),
+      output: input.value({
+        ['#resolvedReference']: null,
+      }),
+    }),
+
+    exitWithoutDependency({
+      dependency: input('data'),
+    }),
+
+    {
+      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,
+        });
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/withResolvedReferenceList.js b/src/data/composite/wiki-data/withResolvedReferenceList.js
new file mode 100644
index 00000000..1d39e5b2
--- /dev/null
+++ b/src/data/composite/wiki-data/withResolvedReferenceList.js
@@ -0,0 +1,101 @@
+// Resolves a list of references, with each reference matched with provided
+// data in the same way as withResolvedReference. This will early exit if the
+// data dependency is null (even if the reference list is empty). By default
+// it will filter out references which don't match, but this can be changed
+// to early exit ({notFoundMode: 'exit'}) or leave null in place ('null').
+
+import {input, templateCompositeFrom} from '#composite';
+import {is, isString, validateArrayItems} from '#validators';
+
+import {
+  exitWithoutDependency,
+  raiseOutputWithoutDependency,
+} from '#composite/control-flow';
+
+import inputWikiData from './inputWikiData.js';
+
+export default templateCompositeFrom({
+  annotation: `withResolvedReferenceList`,
+
+  inputs: {
+    list: input({
+      validate: validateArrayItems(isString),
+      acceptsNull: true,
+    }),
+
+    data: inputWikiData({allowMixedTypes: false}),
+    find: input({type: 'function'}),
+
+    notFoundMode: input({
+      validate: is('exit', 'filter', 'null'),
+      defaultValue: 'filter',
+    }),
+  },
+
+  outputs: ['#resolvedReferenceList'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input('data'),
+      value: input.value([]),
+    }),
+
+    raiseOutputWithoutDependency({
+      dependency: input('list'),
+      mode: input.value('empty'),
+      output: input.value({
+        ['#resolvedReferenceList']: [],
+      }),
+    }),
+
+    {
+      dependencies: [input('list'), input('data'), input('find')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('data')]: data,
+        [input('find')]: findFunction,
+      }) =>
+        continuation({
+          '#matches': list.map(ref => findFunction(ref, data, {mode: 'quiet'})),
+        }),
+    },
+
+    {
+      dependencies: ['#matches'],
+      compute: (continuation, {'#matches': matches}) =>
+        (matches.every(match => match)
+          ? continuation.raiseOutput({
+              ['#resolvedReferenceList']: matches,
+            })
+          : continuation()),
+    },
+
+    {
+      dependencies: ['#matches', input('notFoundMode')],
+      compute(continuation, {
+        ['#matches']: matches,
+        [input('notFoundMode')]: notFoundMode,
+      }) {
+        switch (notFoundMode) {
+          case 'exit':
+            return continuation.exit([]);
+
+          case 'filter':
+            return continuation.raiseOutput({
+              ['#resolvedReferenceList']:
+                matches.filter(match => match),
+            });
+
+          case 'null':
+            return continuation.raiseOutput({
+              ['#resolvedReferenceList']:
+                matches.map(match => match ?? null),
+            });
+
+          default:
+            throw new TypeError(`Expected notFoundMode to be exit, filter, or null`);
+        }
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-data/withReverseReferenceList.js b/src/data/composite/wiki-data/withReverseReferenceList.js
new file mode 100644
index 00000000..a025b5ed
--- /dev/null
+++ b/src/data/composite/wiki-data/withReverseReferenceList.js
@@ -0,0 +1,41 @@
+// Check out the info on reverseReferenceList!
+// This is its composable form.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency} from '#composite/control-flow';
+
+import inputWikiData from './inputWikiData.js';
+
+export default templateCompositeFrom({
+  annotation: `withReverseReferenceList`,
+
+  inputs: {
+    data: inputWikiData({allowMixedTypes: false}),
+    list: input({type: 'string'}),
+  },
+
+  outputs: ['#reverseReferenceList'],
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: input('data'),
+      value: input.value([]),
+      mode: input.value('empty'),
+    }),
+
+    {
+      dependencies: [input.myself(), input('data'), input('list')],
+
+      compute: (continuation, {
+        [input.myself()]: thisThing,
+        [input('data')]: data,
+        [input('list')]: refListProperty,
+      }) =>
+        continuation({
+          ['#reverseReferenceList']:
+            data.filter(thing => thing[refListProperty].includes(thisThing)),
+        }),
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-properties/additionalFiles.js b/src/data/composite/wiki-properties/additionalFiles.js
new file mode 100644
index 00000000..6760527a
--- /dev/null
+++ b/src/data/composite/wiki-properties/additionalFiles.js
@@ -0,0 +1,30 @@
+// This is a somewhat more involved data structure - it's for additional
+// or "bonus" files associated with albums or tracks (or anything else).
+// It's got this form:
+//
+//   [
+//     {title: 'Booklet', files: ['Booklet.pdf']},
+//     {
+//       title: 'Wallpaper',
+//       description: 'Cool Wallpaper!',
+//       files: ['1440x900.png', '1920x1080.png']
+//     },
+//     {title: 'Alternate Covers', description: null, files: [...]},
+//     ...
+//   ]
+//
+
+import {isAdditionalFileList} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isAdditionalFileList},
+    expose: {
+      transform: (additionalFiles) =>
+        additionalFiles ?? [],
+    },
+  };
+}
diff --git a/src/data/composite/wiki-properties/color.js b/src/data/composite/wiki-properties/color.js
new file mode 100644
index 00000000..1bc9888b
--- /dev/null
+++ b/src/data/composite/wiki-properties/color.js
@@ -0,0 +1,12 @@
+// A color! This'll be some CSS-ready value.
+
+import {isColor} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isColor},
+  };
+}
diff --git a/src/data/composite/wiki-properties/commentary.js b/src/data/composite/wiki-properties/commentary.js
new file mode 100644
index 00000000..fbea9d5c
--- /dev/null
+++ b/src/data/composite/wiki-properties/commentary.js
@@ -0,0 +1,12 @@
+// Artist commentary! Generally present on tracks and albums.
+
+import {isCommentary} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isCommentary},
+  };
+}
diff --git a/src/data/composite/wiki-properties/commentatorArtists.js b/src/data/composite/wiki-properties/commentatorArtists.js
new file mode 100644
index 00000000..52aeb868
--- /dev/null
+++ b/src/data/composite/wiki-properties/commentatorArtists.js
@@ -0,0 +1,55 @@
+// 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.
+
+import {input, templateCompositeFrom} from '#composite';
+import find from '#find';
+import {unique} from '#sugar';
+
+import {exitWithoutDependency} from '#composite/control-flow';
+import {withResolvedReferenceList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `commentatorArtists`,
+
+  compose: false,
+
+  steps: () => [
+    exitWithoutDependency({
+      dependency: 'commentary',
+      mode: input.value('falsy'),
+      value: input.value([]),
+    }),
+
+    {
+      dependencies: ['commentary'],
+      compute: (continuation, {commentary}) =>
+        continuation({
+          '#artistRefs':
+            Array.from(
+              commentary
+                .replace(/<\/?b>/g, '')
+                .matchAll(/<i>(?<who>.*?):<\/i>/g))
+              .map(({groups: {who}}) => who),
+        }),
+    },
+
+    withResolvedReferenceList({
+      list: '#artistRefs',
+      data: 'artistData',
+      find: input.value(find.artist),
+    }).outputs({
+      '#resolvedReferenceList': '#artists',
+    }),
+
+    {
+      flags: {expose: true},
+
+      expose: {
+        dependencies: ['#artists'],
+        compute: ({'#artists': artists}) =>
+          unique(artists),
+      },
+    },
+  ],
+});
diff --git a/src/data/composite/wiki-properties/contribsPresent.js b/src/data/composite/wiki-properties/contribsPresent.js
new file mode 100644
index 00000000..24f302a5
--- /dev/null
+++ b/src/data/composite/wiki-properties/contribsPresent.js
@@ -0,0 +1,30 @@
+// Nice 'n simple shorthand for an exposed-only flag which is true when any
+// contributions are present in the specified property.
+
+import {input, templateCompositeFrom} from '#composite';
+import {isContributionList} from '#validators';
+
+import {exposeDependency, withResultOfAvailabilityCheck}
+  from '#composite/control-flow';
+
+export default templateCompositeFrom({
+  annotation: `contribsPresent`,
+
+  compose: false,
+
+  inputs: {
+    contribs: input.staticDependency({
+      validate: isContributionList,
+      acceptsNull: true,
+    }),
+  },
+
+  steps: () => [
+    withResultOfAvailabilityCheck({
+      from: input('contribs'),
+      mode: input.value('empty'),
+    }),
+
+    exposeDependency({dependency: '#availability'}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/contributionList.js b/src/data/composite/wiki-properties/contributionList.js
new file mode 100644
index 00000000..8fde2caa
--- /dev/null
+++ b/src/data/composite/wiki-properties/contributionList.js
@@ -0,0 +1,35 @@
+// Strong 'n sturdy contribution list, rolling a list of references (provided
+// as this property's update value) and the resolved results (as get exposed)
+// into one property. Update value will look something like this:
+//
+//   [
+//     {who: 'Artist Name', what: 'Viola'},
+//     {who: 'artist:john-cena', what: null},
+//     ...
+//   ]
+//
+// ...typically as processed from YAML, spreadsheet, or elsewhere.
+// Exposes as the same, but with the "who" replaced with matches found in
+// artistData - which means this always depends on an `artistData` property
+// also existing on this object!
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {isContributionList} from '#validators';
+
+import {exposeConstant, exposeDependencyOrContinue} from '#composite/control-flow';
+import {withResolvedContribs} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `contributionList`,
+
+  compose: false,
+
+  update: {validate: isContributionList},
+
+  steps: () => [
+    withResolvedContribs({from: input.updateValue()}),
+    exposeDependencyOrContinue({dependency: '#resolvedContribs'}),
+    exposeConstant({value: input.value([])}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/dimensions.js b/src/data/composite/wiki-properties/dimensions.js
new file mode 100644
index 00000000..57a01279
--- /dev/null
+++ b/src/data/composite/wiki-properties/dimensions.js
@@ -0,0 +1,13 @@
+// Plain ol' image dimensions. This is a two-item array of positive integers,
+// corresponding to width and height respectively.
+
+import {isDimensions} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDimensions},
+  };
+}
diff --git a/src/data/composite/wiki-properties/directory.js b/src/data/composite/wiki-properties/directory.js
new file mode 100644
index 00000000..0b2181c9
--- /dev/null
+++ b/src/data/composite/wiki-properties/directory.js
@@ -0,0 +1,23 @@
+// The all-encompassing "directory" property, used as the unique identifier for
+// almost any data object. Also corresponds to a part of the URL which pages of
+// such objects are visited at.
+
+import {isDirectory} from '#validators';
+import {getKebabCase} from '#wiki-data';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDirectory},
+    expose: {
+      dependencies: ['name'],
+      transform(directory, {name}) {
+        if (directory === null && name === null) return null;
+        else if (directory === null) return getKebabCase(name);
+        else return directory;
+      },
+    },
+  };
+}
diff --git a/src/data/composite/wiki-properties/duration.js b/src/data/composite/wiki-properties/duration.js
new file mode 100644
index 00000000..827f282d
--- /dev/null
+++ b/src/data/composite/wiki-properties/duration.js
@@ -0,0 +1,13 @@
+// Duration! This is a number of seconds, possibly floating point, always
+// at minimum zero.
+
+import {isDuration} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDuration},
+  };
+}
diff --git a/src/data/composite/wiki-properties/externalFunction.js b/src/data/composite/wiki-properties/externalFunction.js
new file mode 100644
index 00000000..c388da6c
--- /dev/null
+++ b/src/data/composite/wiki-properties/externalFunction.js
@@ -0,0 +1,11 @@
+// External function. These should only be used as dependencies for other
+// properties, so they're left unexposed.
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true},
+    update: {validate: (t) => typeof t === 'function'},
+  };
+}
diff --git a/src/data/composite/wiki-properties/fileExtension.js b/src/data/composite/wiki-properties/fileExtension.js
new file mode 100644
index 00000000..c926fa8b
--- /dev/null
+++ b/src/data/composite/wiki-properties/fileExtension.js
@@ -0,0 +1,13 @@
+// A file extension! Or the default, if provided when calling this.
+
+import {isFileExtension} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function(defaultFileExtension = null) {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isFileExtension},
+    expose: {transform: (value) => value ?? defaultFileExtension},
+  };
+}
diff --git a/src/data/composite/wiki-properties/flag.js b/src/data/composite/wiki-properties/flag.js
new file mode 100644
index 00000000..076e663f
--- /dev/null
+++ b/src/data/composite/wiki-properties/flag.js
@@ -0,0 +1,19 @@
+// Straightforward flag descriptor for a variety of property purposes.
+// Provide a default value, true or false!
+
+import {isBoolean} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+// TODO: The description is a lie. This defaults to false. Bad.
+
+export default function(defaultValue = false) {
+  if (typeof defaultValue !== 'boolean') {
+    throw new TypeError(`Always set explicit defaults for flags!`);
+  }
+
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isBoolean, default: defaultValue},
+  };
+}
diff --git a/src/data/composite/wiki-properties/index.js b/src/data/composite/wiki-properties/index.js
new file mode 100644
index 00000000..2462b047
--- /dev/null
+++ b/src/data/composite/wiki-properties/index.js
@@ -0,0 +1,20 @@
+export {default as additionalFiles} from './additionalFiles.js';
+export {default as color} from './color.js';
+export {default as commentary} from './commentary.js';
+export {default as commentatorArtists} from './commentatorArtists.js';
+export {default as contribsPresent} from './contribsPresent.js';
+export {default as contributionList} from './contributionList.js';
+export {default as dimensions} from './dimensions.js';
+export {default as directory} from './directory.js';
+export {default as duration} from './duration.js';
+export {default as externalFunction} from './externalFunction.js';
+export {default as fileExtension} from './fileExtension.js';
+export {default as flag} from './flag.js';
+export {default as name} from './name.js';
+export {default as referenceList} from './referenceList.js';
+export {default as reverseReferenceList} from './reverseReferenceList.js';
+export {default as simpleDate} from './simpleDate.js';
+export {default as simpleString} from './simpleString.js';
+export {default as singleReference} from './singleReference.js';
+export {default as urls} from './urls.js';
+export {default as wikiData} from './wikiData.js';
diff --git a/src/data/composite/wiki-properties/name.js b/src/data/composite/wiki-properties/name.js
new file mode 100644
index 00000000..5146488b
--- /dev/null
+++ b/src/data/composite/wiki-properties/name.js
@@ -0,0 +1,11 @@
+// A wiki data object's name! Its directory (i.e. unique identifier) will be
+// computed based on this value if not otherwise specified.
+
+import {isName} from '#validators';
+
+export default function(defaultName) {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isName, default: defaultName},
+  };
+}
diff --git a/src/data/composite/wiki-properties/referenceList.js b/src/data/composite/wiki-properties/referenceList.js
new file mode 100644
index 00000000..f5b6c58e
--- /dev/null
+++ b/src/data/composite/wiki-properties/referenceList.js
@@ -0,0 +1,47 @@
+// Stores and exposes a list of references to other data objects; all items
+// must be references to the same type, which is specified on the class input.
+//
+// See also:
+//  - singleReference
+//  - withResolvedReferenceList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {validateReferenceList} from '#validators';
+
+import {exposeDependency} from '#composite/control-flow';
+import {inputThingClass, inputWikiData, withResolvedReferenceList}
+  from '#composite/wiki-data';
+
+// TODO: Kludge.
+import Thing from '../../things/thing.js';
+
+export default templateCompositeFrom({
+  annotation: `referenceList`,
+
+  compose: false,
+
+  inputs: {
+    class: inputThingClass(),
+
+    data: inputWikiData({allowMixedTypes: false}),
+    find: input({type: 'function'}),
+  },
+
+  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'),
+    }),
+
+    exposeDependency({dependency: '#resolvedReferenceList'}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/reverseReferenceList.js b/src/data/composite/wiki-properties/reverseReferenceList.js
new file mode 100644
index 00000000..84ba67df
--- /dev/null
+++ b/src/data/composite/wiki-properties/reverseReferenceList.js
@@ -0,0 +1,30 @@
+// Neat little shortcut for "reversing" the reference lists stored on other
+// things - for example, tracks specify a "referenced tracks" property, and
+// you would use this to compute a corresponding "referenced *by* tracks"
+// property. Naturally, the passed ref list property is of the things in the
+// wiki data provided, not the requesting Thing itself.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exposeDependency} from '#composite/control-flow';
+import {inputWikiData, withReverseReferenceList} from '#composite/wiki-data';
+
+export default templateCompositeFrom({
+  annotation: `reverseReferenceList`,
+
+  compose: false,
+
+  inputs: {
+    data: inputWikiData({allowMixedTypes: false}),
+    list: input({type: 'string'}),
+  },
+
+  steps: () => [
+    withReverseReferenceList({
+      data: input('data'),
+      list: input('list'),
+    }),
+
+    exposeDependency({dependency: '#reverseReferenceList'}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/simpleDate.js b/src/data/composite/wiki-properties/simpleDate.js
new file mode 100644
index 00000000..f08d8323
--- /dev/null
+++ b/src/data/composite/wiki-properties/simpleDate.js
@@ -0,0 +1,14 @@
+// General date type, used as the descriptor for a bunch of properties.
+// This isn't dynamic though - it won't inherit from a date stored on
+// another object, for example.
+
+import {isDate} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isDate},
+  };
+}
diff --git a/src/data/composite/wiki-properties/simpleString.js b/src/data/composite/wiki-properties/simpleString.js
new file mode 100644
index 00000000..18d65146
--- /dev/null
+++ b/src/data/composite/wiki-properties/simpleString.js
@@ -0,0 +1,14 @@
+// General string type. This should probably generally be avoided in favor
+// of more specific validation, but using it makes it easy to find where we
+// might want to improve later, and it's a useful shorthand meanwhile.
+
+import {isString} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: isString},
+  };
+}
diff --git a/src/data/composite/wiki-properties/singleReference.js b/src/data/composite/wiki-properties/singleReference.js
new file mode 100644
index 00000000..34bd2e6d
--- /dev/null
+++ b/src/data/composite/wiki-properties/singleReference.js
@@ -0,0 +1,47 @@
+// Stores and exposes one connection, or reference, to another data object.
+// The reference must be to a specific type, which is specified on the class
+// input.
+//
+// See also:
+//  - referenceList
+//  - withResolvedReference
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {validateReference} from '#validators';
+
+import {exposeDependency} from '#composite/control-flow';
+import {inputThingClass, inputWikiData, withResolvedReference}
+  from '#composite/wiki-data';
+
+// TODO: Kludge.
+import Thing from '../../things/thing.js';
+
+export default templateCompositeFrom({
+  annotation: `singleReference`,
+
+  compose: false,
+
+  inputs: {
+    class: inputThingClass(),
+    find: input({type: 'function'}),
+    data: inputWikiData({allowMixedTypes: false}),
+  },
+
+  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'),
+    }),
+
+    exposeDependency({dependency: '#resolvedReference'}),
+  ],
+});
diff --git a/src/data/composite/wiki-properties/urls.js b/src/data/composite/wiki-properties/urls.js
new file mode 100644
index 00000000..3160a0bf
--- /dev/null
+++ b/src/data/composite/wiki-properties/urls.js
@@ -0,0 +1,14 @@
+// A list of URLs! This will always be present on the data object, even if set
+// to an empty array or null.
+
+import {isURL, validateArrayItems} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+export default function() {
+  return {
+    flags: {update: true, expose: true},
+    update: {validate: validateArrayItems(isURL)},
+    expose: {transform: value => value ?? []},
+  };
+}
diff --git a/src/data/composite/wiki-properties/wikiData.js b/src/data/composite/wiki-properties/wikiData.js
new file mode 100644
index 00000000..4ea47785
--- /dev/null
+++ b/src/data/composite/wiki-properties/wikiData.js
@@ -0,0 +1,17 @@
+// General purpose wiki data constructor, for properties like artistData,
+// trackData, etc.
+
+import {validateArrayItems, validateInstanceOf} from '#validators';
+
+// TODO: Not templateCompositeFrom.
+
+// TODO: This should validate with validateWikiData.
+
+export default function(thingClass) {
+  return {
+    flags: {update: true},
+    update: {
+      validate: validateArrayItems(validateInstanceOf(thingClass)),
+    },
+  };
+}
diff --git a/src/data/things/album.js b/src/data/things/album.js
index c012c243..546fda3b 100644
--- a/src/data/things/album.js
+++ b/src/data/things/album.js
@@ -1,163 +1,143 @@
-import {empty} from '#sugar';
+import {input} from '#composite';
 import find from '#find';
+import {isDate} from '#validators';
+
+import {exposeDependency, exposeUpdateValueOrContinue}
+  from '#composite/control-flow';
+import {exitWithoutContribs} from '#composite/wiki-data';
+
+import {
+  additionalFiles,
+  commentary,
+  color,
+  commentatorArtists,
+  contribsPresent,
+  contributionList,
+  dimensions,
+  directory,
+  fileExtension,
+  flag,
+  name,
+  referenceList,
+  simpleDate,
+  simpleString,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
+import {
+  withTracks,
+  withTrackSections,
+} from '#composite/things/album';
 
 import Thing from './thing.js';
 
 export class Album extends Thing {
   static [Thing.referenceType] = 'album';
 
-  static [Thing.getPropertyDescriptors] = ({
-    ArtTag,
-    Artist,
-    Group,
-    Track,
-
-    validators: {
-      isDate,
-      isDimensions,
-      isTrackSectionList,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({ArtTag, Artist, Group, Track}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Album'),
-    color: Thing.common.color(),
-    directory: Thing.common.directory(),
-    urls: Thing.common.urls(),
-
-    date: Thing.common.simpleDate(),
-    trackArtDate: Thing.common.simpleDate(),
-    dateAddedToWiki: Thing.common.simpleDate(),
-
-    coverArtDate: {
-      flags: {update: true, expose: true},
-
-      update: {validate: isDate},
-
-      expose: {
-        dependencies: ['date', 'coverArtistContribsByRef'],
-        transform: (coverArtDate, {
-          coverArtistContribsByRef,
-          date,
-        }) =>
-          (!empty(coverArtistContribsByRef)
-            ? coverArtDate ?? date ?? null
-            : null),
-      },
-    },
-
-    artistContribsByRef: Thing.common.contribsByRef(),
-    coverArtistContribsByRef: Thing.common.contribsByRef(),
-    trackCoverArtistContribsByRef: Thing.common.contribsByRef(),
-    wallpaperArtistContribsByRef: Thing.common.contribsByRef(),
-    bannerArtistContribsByRef: Thing.common.contribsByRef(),
-
-    groupsByRef: Thing.common.referenceList(Group),
-    artTagsByRef: Thing.common.referenceList(ArtTag),
-
-    trackSections: {
-      flags: {update: true, expose: true},
-
-      update: {
-        validate: isTrackSectionList,
-      },
-
-      expose: {
-        dependencies: ['color', 'trackData'],
-        transform(trackSections, {
-          color: albumColor,
-          trackData,
-        }) {
-          let startIndex = 0;
-          return trackSections?.map(section => ({
-            name: section.name ?? null,
-            color: section.color ?? albumColor ?? null,
-            dateOriginallyReleased: section.dateOriginallyReleased ?? null,
-            isDefaultTrackSection: section.isDefaultTrackSection ?? false,
-
-            startIndex: (
-              startIndex += section.tracksByRef.length,
-              startIndex - section.tracksByRef.length
-            ),
-
-            tracksByRef: section.tracksByRef ?? [],
-            tracks:
-              (trackData && section.tracksByRef
-                ?.map(ref => find.track(ref, trackData, {mode: 'quiet'}))
-                .filter(Boolean)) ??
-              [],
-          }));
-        },
-      },
-    },
-
-    coverArtFileExtension: Thing.common.fileExtension('jpg'),
-    trackCoverArtFileExtension: Thing.common.fileExtension('jpg'),
-
-    wallpaperStyle: Thing.common.simpleString(),
-    wallpaperFileExtension: Thing.common.fileExtension('jpg'),
-
-    bannerStyle: Thing.common.simpleString(),
-    bannerFileExtension: Thing.common.fileExtension('jpg'),
-    bannerDimensions: {
-      flags: {update: true, expose: true},
-      update: {validate: isDimensions},
-    },
-
-    hasTrackNumbers: Thing.common.flag(true),
-    isListedOnHomepage: Thing.common.flag(true),
-    isListedInGalleries: Thing.common.flag(true),
-
-    commentary: Thing.common.commentary(),
-    additionalFiles: Thing.common.additionalFiles(),
+    name: name('Unnamed Album'),
+    color: color(),
+    directory: directory(),
+    urls: urls(),
+
+    date: simpleDate(),
+    trackArtDate: simpleDate(),
+    dateAddedToWiki: simpleDate(),
+
+    coverArtDate: [
+      exitWithoutContribs({contribs: 'coverArtistContribs'}),
+
+      exposeUpdateValueOrContinue({
+        validate: input.value(isDate),
+      }),
+
+      exposeDependency({dependency: 'date'}),
+    ],
+
+    coverArtFileExtension: [
+      exitWithoutContribs({contribs: 'coverArtistContribs'}),
+      fileExtension('jpg'),
+    ],
+
+    trackCoverArtFileExtension: fileExtension('jpg'),
+
+    wallpaperFileExtension: [
+      exitWithoutContribs({contribs: 'wallpaperArtistContribs'}),
+      fileExtension('jpg'),
+    ],
+
+    bannerFileExtension: [
+      exitWithoutContribs({contribs: 'bannerArtistContribs'}),
+      fileExtension('jpg'),
+    ],
+
+    wallpaperStyle: [
+      exitWithoutContribs({contribs: 'wallpaperArtistContribs'}),
+      simpleString(),
+    ],
+
+    bannerStyle: [
+      exitWithoutContribs({contribs: 'bannerArtistContribs'}),
+      simpleString(),
+    ],
+
+    bannerDimensions: [
+      exitWithoutContribs({contribs: 'bannerArtistContribs'}),
+      dimensions(),
+    ],
+
+    hasTrackNumbers: flag(true),
+    isListedOnHomepage: flag(true),
+    isListedInGalleries: flag(true),
+
+    commentary: commentary(),
+    additionalFiles: additionalFiles(),
+
+    trackSections: [
+      withTrackSections(),
+      exposeDependency({dependency: '#trackSections'}),
+    ],
+
+    artistContribs: contributionList(),
+    coverArtistContribs: contributionList(),
+    trackCoverArtistContribs: contributionList(),
+    wallpaperArtistContribs: contributionList(),
+    bannerArtistContribs: contributionList(),
+
+    groups: referenceList({
+      class: input.value(Group),
+      find: input.value(find.group),
+      data: 'groupData',
+    }),
+
+    artTags: referenceList({
+      class: input.value(ArtTag),
+      find: input.value(find.artTag),
+      data: 'artTagData',
+    }),
 
     // Update only
 
-    artistData: Thing.common.wikiData(Artist),
-    artTagData: Thing.common.wikiData(ArtTag),
-    groupData: Thing.common.wikiData(Group),
-    trackData: Thing.common.wikiData(Track),
+    artistData: wikiData(Artist),
+    artTagData: wikiData(ArtTag),
+    groupData: wikiData(Group),
+    trackData: wikiData(Track),
 
     // Expose only
 
-    artistContribs: Thing.common.dynamicContribs('artistContribsByRef'),
-    coverArtistContribs: Thing.common.dynamicContribs('coverArtistContribsByRef'),
-    trackCoverArtistContribs: Thing.common.dynamicContribs('trackCoverArtistContribsByRef'),
-    wallpaperArtistContribs: Thing.common.dynamicContribs('wallpaperArtistContribsByRef'),
-    bannerArtistContribs: Thing.common.dynamicContribs('bannerArtistContribsByRef'),
-
-    commentatorArtists: Thing.common.commentatorArtists(),
-
-    hasCoverArt: Thing.common.contribsPresent('coverArtistContribsByRef'),
-    hasWallpaperArt: Thing.common.contribsPresent('wallpaperArtistContribsByRef'),
-    hasBannerArt: Thing.common.contribsPresent('bannerArtistContribsByRef'),
-
-    tracks: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['trackSections', 'trackData'],
-        compute: ({trackSections, trackData}) =>
-          trackSections && trackData
-            ? trackSections
-                .flatMap((section) => section.tracksByRef ?? [])
-                .map((ref) => find.track(ref, trackData, {mode: 'quiet'}))
-                .filter(Boolean)
-            : [],
-      },
-    },
-
-    groups: Thing.common.dynamicThingsFromReferenceList(
-      'groupsByRef',
-      'groupData',
-      find.group
-    ),
-
-    artTags: Thing.common.dynamicThingsFromReferenceList(
-      'artTagsByRef',
-      'artTagData',
-      find.artTag
-    ),
+    commentatorArtists: commentatorArtists(),
+
+    hasCoverArt: contribsPresent({contribs: 'coverArtistContribs'}),
+    hasWallpaperArt: contribsPresent({contribs: 'wallpaperArtistContribs'}),
+    hasBannerArt: contribsPresent({contribs: 'bannerArtistContribs'}),
+
+    tracks: [
+      withTracks(),
+      exposeDependency({dependency: '#tracks'}),
+    ],
   });
 
   static [Thing.getSerializeDescriptors] = ({
@@ -201,10 +181,12 @@ export class Album extends Thing {
 }
 
 export class TrackSectionHelper extends Thing {
+  static [Thing.friendlyName] = `Track Section`;
+
   static [Thing.getPropertyDescriptors] = () => ({
-    name: Thing.common.name('Unnamed Track Group'),
-    color: Thing.common.color(),
-    dateOriginallyReleased: Thing.common.simpleDate(),
-    isDefaultTrackGroup: Thing.common.flag(false),
+    name: name('Unnamed Track Section'),
+    color: color(),
+    dateOriginallyReleased: simpleDate(),
+    isDefaultTrackGroup: flag(false),
   })
 }
diff --git a/src/data/things/art-tag.js b/src/data/things/art-tag.js
index c103c4d5..6503beec 100644
--- a/src/data/things/art-tag.js
+++ b/src/data/things/art-tag.js
@@ -1,35 +1,47 @@
+import {input} from '#composite';
 import {sortAlbumsTracksChronologically} from '#wiki-data';
+import {isName} from '#validators';
+
+import {exposeUpdateValueOrContinue} from '#composite/control-flow';
+
+import {
+  color,
+  directory,
+  flag,
+  name,
+  wikiData,
+} from '#composite/wiki-properties';
 
 import Thing from './thing.js';
 
 export class ArtTag extends Thing {
   static [Thing.referenceType] = 'tag';
+  static [Thing.friendlyName] = `Art Tag`;
 
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-    Track,
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Album, Track}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Art Tag'),
-    directory: Thing.common.directory(),
-    color: Thing.common.color(),
-    isContentWarning: Thing.common.flag(false),
+    name: name('Unnamed Art Tag'),
+    directory: directory(),
+    color: color(),
+    isContentWarning: flag(false),
 
-    nameShort: {
-      flags: {update: true, expose: true},
+    nameShort: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isName),
+      }),
 
-      expose: {
+      {
         dependencies: ['name'],
-        transform: (value, {name}) =>
-          value ?? name.replace(/ \(.*?\)$/, ''),
+        compute: ({name}) =>
+          name.replace(/ \([^)]*?\)$/, ''),
       },
-    },
+    ],
 
     // Update only
 
-    albumData: Thing.common.wikiData(Album),
-    trackData: Thing.common.wikiData(Track),
+    albumData: wikiData(Album),
+    trackData: wikiData(Track),
 
     // Expose only
 
@@ -37,8 +49,8 @@ export class ArtTag extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['albumData', 'trackData'],
-        compute: ({albumData, trackData, [ArtTag.instance]: artTag}) =>
+        dependencies: ['this', 'albumData', 'trackData'],
+        compute: ({this: artTag, albumData, trackData}) =>
           sortAlbumsTracksChronologically(
             [...albumData, ...trackData]
               .filter(({artTags}) => artTags.includes(artTag)),
diff --git a/src/data/things/artist.js b/src/data/things/artist.js
index 6d4f4a0d..1b313db6 100644
--- a/src/data/things/artist.js
+++ b/src/data/things/artist.js
@@ -1,29 +1,33 @@
+import {input} from '#composite';
 import find from '#find';
+import {isName, validateArrayItems} from '#validators';
+
+import {
+  directory,
+  fileExtension,
+  flag,
+  name,
+  simpleString,
+  singleReference,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
 
 import Thing from './thing.js';
 
 export class Artist extends Thing {
   static [Thing.referenceType] = 'artist';
 
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-    Flash,
-    Track,
-
-    validators: {
-      isName,
-      validateArrayItems,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Album, Flash, Track}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Artist'),
-    directory: Thing.common.directory(),
-    urls: Thing.common.urls(),
-    contextNotes: Thing.common.simpleString(),
+    name: name('Unnamed Artist'),
+    directory: directory(),
+    urls: urls(),
+    contextNotes: simpleString(),
 
-    hasAvatar: Thing.common.flag(false),
-    avatarFileExtension: Thing.common.fileExtension('jpg'),
+    hasAvatar: flag(false),
+    avatarFileExtension: fileExtension('jpg'),
 
     aliasNames: {
       flags: {update: true, expose: true},
@@ -31,30 +35,23 @@ export class Artist extends Thing {
       expose: {transform: (names) => names ?? []},
     },
 
-    isAlias: Thing.common.flag(),
-    aliasedArtistRef: Thing.common.singleReference(Artist),
+    isAlias: flag(),
+
+    aliasedArtist: singleReference({
+      class: input.value(Artist),
+      find: input.value(find.artist),
+      data: 'artistData',
+    }),
 
     // Update only
 
-    albumData: Thing.common.wikiData(Album),
-    artistData: Thing.common.wikiData(Artist),
-    flashData: Thing.common.wikiData(Flash),
-    trackData: Thing.common.wikiData(Track),
+    albumData: wikiData(Album),
+    artistData: wikiData(Artist),
+    flashData: wikiData(Flash),
+    trackData: wikiData(Track),
 
     // Expose only
 
-    aliasedArtist: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['artistData', 'aliasedArtistRef'],
-        compute: ({artistData, aliasedArtistRef}) =>
-          aliasedArtistRef && artistData
-            ? find.artist(aliasedArtistRef, artistData, {mode: 'quiet'})
-            : null,
-      },
-    },
-
     tracksAsArtist:
       Artist.filterByContrib('trackData', 'artistContribs'),
     tracksAsContributor:
@@ -66,14 +63,14 @@ export class Artist extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['trackData'],
+        dependencies: ['this', 'trackData'],
 
-        compute: ({trackData, [Artist.instance]: artist}) =>
+        compute: ({this: artist, trackData}) =>
           trackData?.filter((track) =>
             [
-              ...track.artistContribs,
-              ...track.contributorContribs,
-              ...track.coverArtistContribs,
+              ...track.artistContribs ?? [],
+              ...track.contributorContribs ?? [],
+              ...track.coverArtistContribs ?? [],
             ].some(({who}) => who === artist)) ?? [],
       },
     },
@@ -82,9 +79,9 @@ export class Artist extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['trackData'],
+        dependencies: ['this', 'trackData'],
 
-        compute: ({trackData, [Artist.instance]: artist}) =>
+        compute: ({this: artist, trackData}) =>
           trackData?.filter(({commentatorArtists}) =>
             commentatorArtists.includes(artist)) ?? [],
       },
@@ -120,18 +117,16 @@ export class Artist extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['albumData'],
+        dependencies: ['this', 'albumData'],
 
-        compute: ({albumData, [Artist.instance]: artist}) =>
+        compute: ({this: artist, albumData}) =>
           albumData?.filter(({commentatorArtists}) =>
             commentatorArtists.includes(artist)) ?? [],
       },
     },
 
-    flashesAsContributor: Artist.filterByContrib(
-      'flashData',
-      'contributorContribs'
-    ),
+    flashesAsContributor:
+      Artist.filterByContrib('flashData', 'contributorContribs'),
   });
 
   static [Thing.getSerializeDescriptors] = ({
@@ -165,15 +160,15 @@ export class Artist extends Thing {
     flags: {expose: true},
 
     expose: {
-      dependencies: [thingDataProperty],
+      dependencies: ['this', thingDataProperty],
 
       compute: ({
+        this: artist,
         [thingDataProperty]: thingData,
-        [Artist.instance]: artist
       }) =>
         thingData?.filter(thing =>
           thing[contribsProperty]
-            .some(contrib => contrib.who === artist)) ?? [],
+            ?.some(contrib => contrib.who === artist)) ?? [],
     },
   });
 }
diff --git a/src/data/things/cacheable-object.js b/src/data/things/cacheable-object.js
index ea705a61..9fda865e 100644
--- a/src/data/things/cacheable-object.js
+++ b/src/data/things/cacheable-object.js
@@ -76,28 +76,24 @@
 
 import {inspect as nodeInspect} from 'node:util';
 
-import {color, ENABLE_COLOR} from '#cli';
+import {colors, ENABLE_COLOR} from '#cli';
 
 function inspect(value) {
   return nodeInspect(value, {colors: ENABLE_COLOR});
 }
 
 export default class CacheableObject {
-  static instance = Symbol('CacheableObject `this` instance');
-
   #propertyUpdateValues = Object.create(null);
   #propertyUpdateCacheInvalidators = Object.create(null);
 
-  /*
-    // Note the constructor doesn't take an initial data source. Due to a quirk
-    // of JavaScript, private members can't be accessed before the superclass's
-    // constructor is finished processing - so if we call the overridden
-    // update() function from inside this constructor, it will error when
-    // writing to private members. Pretty bad!
-    //
-    // That means initial data must be provided by following up with update()
-    // after constructing the new instance of the Thing (sub)class.
-    */
+  // Note the constructor doesn't take an initial data source. Due to a quirk
+  // of JavaScript, private members can't be accessed before the superclass's
+  // constructor is finished processing - so if we call the overridden
+  // update() function from inside this constructor, it will error when
+  // writing to private members. Pretty bad!
+  //
+  // That means initial data must be provided by following up with update()
+  // after constructing the new instance of the Thing (sub)class.
 
   constructor() {
     this.#defineProperties();
@@ -143,7 +139,7 @@ export default class CacheableObject {
 
       const definition = {
         configurable: false,
-        enumerable: true,
+        enumerable: flags.expose,
       };
 
       if (flags.update) {
@@ -183,13 +179,8 @@ export default class CacheableObject {
           } else if (result !== true) {
             throw new TypeError(`Validation failed for value ${newValue}`);
           }
-        } catch (error) {
-          error.message = [
-            `Property ${color.green(property)}`,
-            `(${inspect(this[property])} -> ${inspect(newValue)}):`,
-            error.message
-          ].join(' ');
-          throw error;
+        } catch (caughtError) {
+          throw new CacheableObjectPropertyValueError(property, this[property], newValue, caughtError);
         }
       }
 
@@ -250,20 +241,27 @@ export default class CacheableObject {
 
     let getAllDependencies;
 
-    const dependencyKeys = expose.dependencies;
-    if (dependencyKeys?.length > 0) {
-      const reflectionEntry = [this.constructor.instance, this];
-      const dependencyGetters = dependencyKeys
-        .map(key => () => [key, this.#propertyUpdateValues[key]]);
+    if (expose.dependencies?.length > 0) {
+      const dependencyKeys = expose.dependencies.slice();
+      const shouldReflect = dependencyKeys.includes('this');
+
+      getAllDependencies = () => {
+        const dependencies = Object.create(null);
+
+        for (const key of dependencyKeys) {
+          dependencies[key] = this.#propertyUpdateValues[key];
+        }
+
+        if (shouldReflect) {
+          dependencies.this = this;
+        }
 
-      getAllDependencies = () =>
-        Object.fromEntries(dependencyGetters
-          .map(f => f())
-          .concat([reflectionEntry]));
+        return dependencies;
+      };
     } else {
-      const allDependencies = {[this.constructor.instance]: this};
-      Object.freeze(allDependencies);
-      getAllDependencies = () => allDependencies;
+      const dependencies = Object.create(null);
+      Object.freeze(dependencies);
+      getAllDependencies = () => dependencies;
     }
 
     if (flags.update) {
@@ -347,4 +345,22 @@ export default class CacheableObject {
       console.log(` - ${line}`);
     }
   }
+
+  static getUpdateValue(object, key) {
+    if (!Object.hasOwn(object, key)) {
+      return undefined;
+    }
+
+    return object.#propertyUpdateValues[key] ?? null;
+  }
+}
+
+export class CacheableObjectPropertyValueError extends Error {
+  constructor(property, oldValue, newValue, error) {
+    super(
+      `Error setting ${colors.green(property)} (${inspect(oldValue)} -> ${inspect(newValue)})`,
+      {cause: error});
+
+    this.property = property;
+  }
 }
diff --git a/src/data/things/composite.js b/src/data/things/composite.js
new file mode 100644
index 00000000..51525bc1
--- /dev/null
+++ b/src/data/things/composite.js
@@ -0,0 +1,1301 @@
+import {inspect} from 'node:util';
+
+import {colors} from '#cli';
+import {TupleMap} from '#wiki-data';
+import {a} from '#validators';
+
+import {
+  decorateErrorWithIndex,
+  empty,
+  filterProperties,
+  openAggregate,
+  stitchArrays,
+  typeAppearance,
+  unique,
+  withAggregate,
+} from '#sugar';
+
+const globalCompositeCache = {};
+
+const _valueIntoToken = shape =>
+  (value = null) =>
+    (value === null
+      ? Symbol.for(`hsmusic.composite.${shape}`)
+   : typeof value === 'string'
+      ? Symbol.for(`hsmusic.composite.${shape}:${value}`)
+      : {
+          symbol: Symbol.for(`hsmusic.composite.input`),
+          shape,
+          value,
+        });
+
+export const input = _valueIntoToken('input');
+input.symbol = Symbol.for('hsmusic.composite.input');
+
+input.value = _valueIntoToken('input.value');
+input.dependency = _valueIntoToken('input.dependency');
+
+input.myself = () => Symbol.for(`hsmusic.composite.input.myself`);
+
+input.updateValue = _valueIntoToken('input.updateValue');
+
+input.staticDependency = _valueIntoToken('input.staticDependency');
+input.staticValue = _valueIntoToken('input.staticValue');
+
+function isInputToken(token) {
+  if (token === null) {
+    return false;
+  } else if (typeof token === 'object') {
+    return token.symbol === Symbol.for('hsmusic.composite.input');
+  } else if (typeof token === 'symbol') {
+    return token.description.startsWith('hsmusic.composite.input');
+  } else {
+    return false;
+  }
+}
+
+function getInputTokenShape(token) {
+  if (!isInputToken(token)) {
+    throw new TypeError(`Expected an input token, got ${typeAppearance(token)}`);
+  }
+
+  if (typeof token === 'object') {
+    return token.shape;
+  } else {
+    return token.description.match(/hsmusic\.composite\.(input.*?)(:|$)/)[1];
+  }
+}
+
+function getInputTokenValue(token) {
+  if (!isInputToken(token)) {
+    throw new TypeError(`Expected an input token, got ${typeAppearance(token)}`);
+  }
+
+  if (typeof token === 'object') {
+    return token.value;
+  } else {
+    return token.description.match(/hsmusic\.composite\.input.*?:(.*)/)?.[1] ?? null;
+  }
+}
+
+function getStaticInputMetadata(inputOptions) {
+  const metadata = {};
+
+  for (const [name, token] of Object.entries(inputOptions)) {
+    if (typeof token === 'string') {
+      metadata[input.staticDependency(name)] = token;
+      metadata[input.staticValue(name)] = null;
+    } else if (isInputToken(token)) {
+      const tokenShape = getInputTokenShape(token);
+      const tokenValue = getInputTokenValue(token);
+
+      metadata[input.staticDependency(name)] =
+        (tokenShape === 'input.dependency'
+          ? tokenValue
+          : null);
+
+      metadata[input.staticValue(name)] =
+        (tokenShape === 'input.value'
+          ? tokenValue
+          : null);
+    } else {
+      metadata[input.staticDependency(name)] = null;
+      metadata[input.staticValue(name)] = null;
+    }
+  }
+
+  return metadata;
+}
+
+function getCompositionName(description) {
+  return (
+    (description.annotation
+      ? description.annotation
+      : `unnamed composite`));
+}
+
+function validateInputValue(value, description) {
+  const tokenValue = getInputTokenValue(description);
+
+  const {acceptsNull, defaultValue, type, validate} = tokenValue || {};
+
+  if (value === null || value === undefined) {
+    if (acceptsNull || defaultValue === null) {
+      return true;
+    } else {
+      throw new TypeError(
+        (type
+          ? `Expected ${a(type)}, got ${typeAppearance(value)}`
+          : `Expected a value, got ${typeAppearance(value)}`));
+    }
+  }
+
+  if (type) {
+    // Note: null is already handled earlier in this function, so it won't
+    // cause any trouble here.
+    const typeofValue =
+      (typeof value === 'object'
+        ? Array.isArray(value) ? 'array' : 'object'
+        : typeof value);
+
+    if (typeofValue !== type) {
+      throw new TypeError(`Expected ${a(type)}, got ${typeAppearance(value)}`);
+    }
+  }
+
+  if (validate) {
+    validate(value);
+  }
+
+  return true;
+}
+
+export function templateCompositeFrom(description) {
+  const compositionName = getCompositionName(description);
+
+  withAggregate({message: `Errors in description for ${compositionName}`}, ({map, nest, push}) => {
+    if ('steps' in description) {
+      if (Array.isArray(description.steps)) {
+        push(new TypeError(`Wrap steps array in a function`));
+      } else if (typeof description.steps !== 'function') {
+        push(new TypeError(`Expected steps to be a function (returning an array)`));
+      }
+    }
+
+    validateInputs:
+    if ('inputs' in description) {
+      if (
+        Array.isArray(description.inputs) ||
+        typeof description.inputs !== 'object'
+      ) {
+        push(new Error(`Expected inputs to be object, got ${typeAppearance(description.inputs)}`));
+        break validateInputs;
+      }
+
+      nest({message: `Errors in static input descriptions for ${compositionName}`}, ({push}) => {
+        const missingCallsToInput = [];
+        const wrongCallsToInput = [];
+
+        for (const [name, value] of Object.entries(description.inputs)) {
+          if (!isInputToken(value)) {
+            missingCallsToInput.push(name);
+            continue;
+          }
+
+          if (!['input', 'input.staticDependency', 'input.staticValue'].includes(getInputTokenShape(value))) {
+            wrongCallsToInput.push(name);
+          }
+        }
+
+        for (const name of missingCallsToInput) {
+          push(new Error(`${name}: Missing call to input()`));
+        }
+
+        for (const name of wrongCallsToInput) {
+          const shape = getInputTokenShape(description.inputs[name]);
+          push(new Error(`${name}: Expected call to input, input.staticDependency, or input.staticValue, got ${shape}`));
+        }
+      });
+    }
+
+    validateOutputs:
+    if ('outputs' in description) {
+      if (
+        !Array.isArray(description.outputs) &&
+        typeof description.outputs !== 'function'
+      ) {
+        push(new Error(`Expected outputs to be array or function, got ${typeAppearance(description.outputs)}`));
+        break validateOutputs;
+      }
+
+      if (Array.isArray(description.outputs)) {
+        map(
+          description.outputs,
+          decorateErrorWithIndex(value => {
+            if (typeof value !== 'string') {
+              throw new Error(`${value}: Expected string, got ${typeAppearance(value)}`)
+            } else if (!value.startsWith('#')) {
+              throw new Error(`${value}: Expected "#" at start`);
+            }
+          }),
+          {message: `Errors in output descriptions for ${compositionName}`});
+      }
+    }
+  });
+
+  const expectedInputNames =
+    (description.inputs
+      ? Object.keys(description.inputs)
+      : []);
+
+  const instantiate = (inputOptions = {}) => {
+    withAggregate({message: `Errors in input options passed to ${compositionName}`}, ({push}) => {
+      const providedInputNames = Object.keys(inputOptions);
+
+      const misplacedInputNames =
+        providedInputNames
+          .filter(name => !expectedInputNames.includes(name));
+
+      const missingInputNames =
+        expectedInputNames
+          .filter(name => !providedInputNames.includes(name))
+          .filter(name => {
+            const inputDescription = getInputTokenValue(description.inputs[name]);
+            if (!inputDescription) return true;
+            if ('defaultValue' in inputDescription) return false;
+            if ('defaultDependency' in inputDescription) return false;
+            return true;
+          });
+
+      const wrongTypeInputNames = [];
+
+      const expectedStaticValueInputNames = [];
+      const expectedStaticDependencyInputNames = [];
+      const expectedValueProvidingTokenInputNames = [];
+
+      const validateFailedErrors = [];
+
+      for (const [name, value] of Object.entries(inputOptions)) {
+        if (misplacedInputNames.includes(name)) {
+          continue;
+        }
+
+        if (typeof value !== 'string' && !isInputToken(value)) {
+          wrongTypeInputNames.push(name);
+          continue;
+        }
+
+        const descriptionShape = getInputTokenShape(description.inputs[name]);
+
+        const tokenShape = (isInputToken(value) ? getInputTokenShape(value) : null);
+        const tokenValue = (isInputToken(value) ? getInputTokenValue(value) : null);
+
+        switch (descriptionShape) {
+          case'input.staticValue':
+            if (tokenShape !== 'input.value') {
+              expectedStaticValueInputNames.push(name);
+              continue;
+            }
+            break;
+
+          case 'input.staticDependency':
+            if (typeof value !== 'string' && tokenShape !== 'input.dependency') {
+              expectedStaticDependencyInputNames.push(name);
+              continue;
+            }
+            break;
+
+          case 'input':
+            if (typeof value !== 'string' && ![
+              'input',
+              'input.value',
+              'input.dependency',
+              'input.myself',
+              'input.updateValue',
+            ].includes(tokenShape)) {
+              expectedValueProvidingTokenInputNames.push(name);
+              continue;
+            }
+            break;
+        }
+
+        if (tokenShape === 'input.value') {
+          try {
+            validateInputValue(tokenValue, description.inputs[name]);
+          } catch (error) {
+            error.message = `${name}: ${error.message}`;
+            validateFailedErrors.push(error);
+          }
+        }
+      }
+
+      if (!empty(misplacedInputNames)) {
+        push(new Error(`Unexpected input names: ${misplacedInputNames.join(', ')}`));
+      }
+
+      if (!empty(missingInputNames)) {
+        push(new Error(`Required these inputs: ${missingInputNames.join(', ')}`));
+      }
+
+      const inputAppearance = name =>
+        (isInputToken(inputOptions[name])
+          ? `${getInputTokenShape(inputOptions[name])}() call`
+          : `dependency name`);
+
+      for (const name of expectedStaticDependencyInputNames) {
+        const appearance = inputAppearance(name);
+        push(new Error(`${name}: Expected dependency name, got ${appearance}`));
+      }
+
+      for (const name of expectedStaticValueInputNames) {
+        const appearance = inputAppearance(name)
+        push(new Error(`${name}: Expected input.value() call, got ${appearance}`));
+      }
+
+      for (const name of expectedValueProvidingTokenInputNames) {
+        const appearance = getInputTokenShape(inputOptions[name]);
+        push(new Error(`${name}: Expected dependency name or value-providing input() call, got ${appearance}`));
+      }
+
+      for (const name of wrongTypeInputNames) {
+        const type = typeAppearance(inputOptions[name]);
+        push(new Error(`${name}: Expected dependency name or input() call, got ${type}`));
+      }
+
+      for (const error of validateFailedErrors) {
+        push(error);
+      }
+    });
+
+    const inputMetadata = getStaticInputMetadata(inputOptions);
+
+    const expectedOutputNames =
+      (Array.isArray(description.outputs)
+        ? description.outputs
+     : typeof description.outputs === 'function'
+        ? description.outputs(inputMetadata)
+            .map(name =>
+              (name.startsWith('#')
+                ? name
+                : '#' + name))
+        : []);
+
+    const ownUpdateDescription =
+      (typeof description.update === 'object'
+        ? description.update
+     : typeof description.update === 'function'
+        ? description.update(inputMetadata)
+        : null);
+
+    const outputOptions = {};
+
+    const instantiatedTemplate = {
+      symbol: templateCompositeFrom.symbol,
+
+      outputs(providedOptions) {
+        withAggregate({message: `Errors in output options passed to ${compositionName}`}, ({push}) => {
+          const misplacedOutputNames = [];
+          const wrongTypeOutputNames = [];
+
+          for (const [name, value] of Object.entries(providedOptions)) {
+            if (!expectedOutputNames.includes(name)) {
+              misplacedOutputNames.push(name);
+              continue;
+            }
+
+            if (typeof value !== 'string') {
+              wrongTypeOutputNames.push(name);
+              continue;
+            }
+          }
+
+          if (!empty(misplacedOutputNames)) {
+            push(new Error(`Unexpected output names: ${misplacedOutputNames.join(', ')}`));
+          }
+
+          for (const name of wrongTypeOutputNames) {
+            const appearance = typeAppearance(providedOptions[name]);
+            push(new Error(`${name}: Expected string, got ${appearance}`));
+          }
+        });
+
+        Object.assign(outputOptions, providedOptions);
+        return instantiatedTemplate;
+      },
+
+      toDescription() {
+        const finalDescription = {};
+
+        if ('annotation' in description) {
+          finalDescription.annotation = description.annotation;
+        }
+
+        if ('compose' in description) {
+          finalDescription.compose = description.compose;
+        }
+
+        if (ownUpdateDescription) {
+          finalDescription.update = ownUpdateDescription;
+        }
+
+        if ('inputs' in description) {
+          const inputMapping = {};
+
+          for (const [name, token] of Object.entries(description.inputs)) {
+            const tokenValue = getInputTokenValue(token);
+            if (name in inputOptions) {
+              if (typeof inputOptions[name] === 'string') {
+                inputMapping[name] = input.dependency(inputOptions[name]);
+              } else {
+                inputMapping[name] = inputOptions[name];
+              }
+            } else if (tokenValue.defaultValue) {
+              inputMapping[name] = input.value(tokenValue.defaultValue);
+            } else if (tokenValue.defaultDependency) {
+              inputMapping[name] = input.dependency(tokenValue.defaultDependency);
+            } else {
+              inputMapping[name] = input.value(null);
+            }
+          }
+
+          finalDescription.inputMapping = inputMapping;
+          finalDescription.inputDescriptions = description.inputs;
+        }
+
+        if ('outputs' in description) {
+          const finalOutputs = {};
+
+          for (const name of expectedOutputNames) {
+            if (name in outputOptions) {
+              finalOutputs[name] = outputOptions[name];
+            } else {
+              finalOutputs[name] = name;
+            }
+          }
+
+          finalDescription.outputs = finalOutputs;
+        }
+
+        if ('steps' in description) {
+          finalDescription.steps = description.steps;
+        }
+
+        return finalDescription;
+      },
+
+      toResolvedComposition() {
+        const ownDescription = instantiatedTemplate.toDescription();
+
+        const finalDescription = {...ownDescription};
+
+        const aggregate = openAggregate({message: `Errors resolving ${compositionName}`});
+
+        const steps = ownDescription.steps();
+
+        const resolvedSteps =
+          aggregate.map(
+            steps,
+            decorateErrorWithIndex(step =>
+              (step.symbol === templateCompositeFrom.symbol
+                ? compositeFrom(step.toResolvedComposition())
+                : step)),
+            {message: `Errors resolving steps`});
+
+        aggregate.close();
+
+        finalDescription.steps = resolvedSteps;
+
+        return finalDescription;
+      },
+    };
+
+    return instantiatedTemplate;
+  };
+
+  instantiate.inputs = instantiate;
+
+  return instantiate;
+}
+
+templateCompositeFrom.symbol = Symbol();
+
+export const continuationSymbol = Symbol.for('compositeFrom: continuation symbol');
+export const noTransformSymbol = Symbol.for('compositeFrom: no-transform symbol');
+
+export function compositeFrom(description) {
+  const {annotation} = description;
+  const compositionName = getCompositionName(description);
+
+  const debug = fn => {
+    if (compositeFrom.debug === true) {
+      const label =
+        (annotation
+          ? colors.dim(`[composite: ${annotation}]`)
+          : colors.dim(`[composite]`));
+      const result = fn();
+      if (Array.isArray(result)) {
+        console.log(label, ...result.map(value =>
+          (typeof value === 'object'
+            ? inspect(value, {depth: 1, colors: true, compact: true, breakLength: Infinity})
+            : value)));
+      } else {
+        console.log(label, result);
+      }
+    }
+  };
+
+  if (!Array.isArray(description.steps)) {
+    throw new TypeError(
+      `Expected steps to be array, got ${typeAppearance(description.steps)}` +
+      (annotation ? ` (${annotation})` : ''));
+  }
+
+  const composition =
+    description.steps.map(step =>
+      ('toResolvedComposition' in step
+        ? compositeFrom(step.toResolvedComposition())
+        : step));
+
+  const inputMetadata = getStaticInputMetadata(description.inputMapping ?? {});
+
+  function _mapDependenciesToOutputs(providedDependencies) {
+    if (!description.outputs) {
+      return {};
+    }
+
+    if (!providedDependencies) {
+      return {};
+    }
+
+    return (
+      Object.fromEntries(
+        Object.entries(description.outputs)
+          .map(([continuationName, outputName]) => [
+            outputName,
+            (continuationName in providedDependencies
+              ? providedDependencies[continuationName]
+              : providedDependencies[continuationName.replace(/^#/, '')]),
+          ])));
+  }
+
+  // These dependencies were all provided by the composition which this one is
+  // nested inside, so input('name')-shaped tokens are going to be evaluated
+  // in the context of the containing composition.
+  const dependenciesFromInputs =
+    Object.values(description.inputMapping ?? {})
+      .map(token => {
+        const tokenShape = getInputTokenShape(token);
+        const tokenValue = getInputTokenValue(token);
+        switch (tokenShape) {
+          case 'input.dependency':
+            return tokenValue;
+          case 'input':
+          case 'input.updateValue':
+            return token;
+          case 'input.myself':
+            return 'this';
+          default:
+            return null;
+        }
+      })
+      .filter(Boolean);
+
+  const anyInputsUseUpdateValue =
+    dependenciesFromInputs
+      .filter(dependency => isInputToken(dependency))
+      .some(token => getInputTokenShape(token) === 'input.updateValue');
+
+  const inputNames =
+    Object.keys(description.inputMapping ?? {});
+
+  const inputSymbols =
+    inputNames.map(name => input(name));
+
+  const inputsMayBeDynamicValue =
+    stitchArrays({
+      mappingToken: Object.values(description.inputMapping ?? {}),
+      descriptionToken: Object.values(description.inputDescriptions ?? {}),
+    }).map(({mappingToken, descriptionToken}) => {
+        if (getInputTokenShape(descriptionToken) === 'input.staticValue') return false;
+        if (getInputTokenShape(mappingToken) === 'input.value') return false;
+        return true;
+      });
+
+  const inputDescriptions =
+    Object.values(description.inputDescriptions ?? {});
+
+  /*
+  const inputsAcceptNull =
+    Object.values(description.inputDescriptions ?? {})
+      .map(token => {
+        const tokenValue = getInputTokenValue(token);
+        if (!tokenValue) return false;
+        if ('acceptsNull' in tokenValue) return tokenValue.acceptsNull;
+        if ('defaultValue' in tokenValue) return tokenValue.defaultValue === null;
+        return false;
+      });
+  */
+
+  // Update descriptions passed as the value in an input.updateValue() token,
+  // as provided as inputs for this composition.
+  const inputUpdateDescriptions =
+    Object.values(description.inputMapping ?? {})
+      .map(token =>
+        (getInputTokenShape(token) === 'input.updateValue'
+          ? getInputTokenValue(token)
+          : null))
+      .filter(Boolean);
+
+  const base = composition.at(-1);
+  const steps = composition.slice();
+
+  const aggregate = openAggregate({
+    message:
+      `Errors preparing composition` +
+      (annotation ? ` (${annotation})` : ''),
+  });
+
+  const compositionNests = description.compose ?? true;
+
+  // Steps default to exposing if using a shorthand syntax where flags aren't
+  // specified at all.
+  const stepsExpose =
+    steps
+      .map(step =>
+        (step.flags
+          ? step.flags.expose ?? false
+          : true));
+
+  // Steps default to composing if using a shorthand syntax where flags aren't
+  // specified at all - *and* aren't the base (final step), unless the whole
+  // composition is nestable.
+  const stepsCompose =
+    steps
+      .map((step, index, {length}) =>
+        (step.flags
+          ? step.flags.compose ?? false
+          : (index === length - 1
+              ? compositionNests
+              : true)));
+
+  // Steps update if the corresponding flag is explicitly set, if a transform
+  // function is provided, or if the dependencies include an input.updateValue
+  // token.
+  const stepsUpdate =
+    steps
+      .map(step =>
+        (step.flags
+          ? step.flags.update ?? false
+          : !!step.transform ||
+            !!step.dependencies?.some(dependency =>
+                isInputToken(dependency) &&
+                getInputTokenShape(dependency) === 'input.updateValue')));
+
+  // The expose description for a step is just the entire step object, when
+  // using the shorthand syntax where {flags: {expose: true}} is left implied.
+  const stepExposeDescriptions =
+    steps
+      .map((step, index) =>
+        (stepsExpose[index]
+          ? (step.flags
+              ? step.expose ?? null
+              : step)
+          : null));
+
+  // The update description for a step, if present at all, is always set
+  // explicitly. There may be multiple per step - namely that step's own
+  // {update} description, and any descriptions passed as the value in an
+  // input.updateValue({...}) token.
+  const stepUpdateDescriptions =
+    steps
+      .map((step, index) =>
+        (stepsUpdate[index]
+          ? [
+              step.update ?? null,
+              ...(stepExposeDescriptions[index]?.dependencies ?? [])
+                .filter(dependency => isInputToken(dependency))
+                .filter(token => getInputTokenShape(token) === 'input.updateValue')
+                .map(token => getInputTokenValue(token)),
+            ].filter(Boolean)
+          : []));
+
+  // Indicates presence of a {compute} function on the expose description.
+  const stepsCompute =
+    stepExposeDescriptions
+      .map(expose => !!expose?.compute);
+
+  // Indicates presence of a {transform} function on the expose description.
+  const stepsTransform =
+    stepExposeDescriptions
+      .map(expose => !!expose?.transform);
+
+  const dependenciesFromSteps =
+    unique(
+      stepExposeDescriptions
+        .flatMap(expose => expose?.dependencies ?? [])
+        .map(dependency => {
+          if (typeof dependency === 'string')
+            return (dependency.startsWith('#') ? null : dependency);
+
+          const tokenShape = getInputTokenShape(dependency);
+          const tokenValue = getInputTokenValue(dependency);
+          switch (tokenShape) {
+            case 'input.dependency':
+              return (tokenValue.startsWith('#') ? null : tokenValue);
+            case 'input.myself':
+              return 'this';
+            default:
+              return null;
+          }
+        })
+        .filter(Boolean));
+
+  const anyStepsUseUpdateValue =
+    stepExposeDescriptions
+      .some(expose =>
+        (expose?.dependencies
+          ? expose.dependencies.includes(input.updateValue())
+          : false));
+
+  const anyStepsExpose =
+    stepsExpose.includes(true);
+
+  const anyStepsUpdate =
+    stepsUpdate.includes(true);
+
+  const anyStepsCompute =
+    stepsCompute.includes(true);
+
+  const anyStepsTransform =
+    stepsTransform.includes(true);
+
+  const compositionExposes =
+    anyStepsExpose;
+
+  const compositionUpdates =
+    'update' in description ||
+    anyInputsUseUpdateValue ||
+    anyStepsUseUpdateValue ||
+    anyStepsUpdate;
+
+  const stepEntries = stitchArrays({
+    step: steps,
+    stepComposes: stepsCompose,
+    stepComputes: stepsCompute,
+    stepTransforms: stepsTransform,
+  });
+
+  for (let i = 0; i < stepEntries.length; i++) {
+    const {
+      step,
+      stepComposes,
+      stepComputes,
+      stepTransforms,
+    } = stepEntries[i];
+
+    const isBase = i === stepEntries.length - 1;
+    const message =
+      `Errors in step #${i + 1}` +
+      (isBase ? ` (base)` : ``) +
+      (step.annotation ? ` (${step.annotation})` : ``);
+
+    aggregate.nest({message}, ({push}) => {
+      if (isBase && stepComposes !== compositionNests) {
+        return push(new TypeError(
+          (compositionNests
+            ? `Base must compose, this composition is nestable`
+            : `Base must not compose, this composition isn't nestable`)));
+      } else if (!isBase && !stepComposes) {
+        return push(new TypeError(
+          (compositionNests
+            ? `All steps must compose`
+            : `All steps (except base) must compose`)));
+      }
+
+      if (
+        !compositionNests && !compositionUpdates &&
+        stepTransforms && !stepComputes
+      ) {
+        return push(new TypeError(
+          `Steps which only transform can't be used in a composition that doesn't update`));
+      }
+    });
+  }
+
+  if (!compositionNests && !anyStepsCompute && !anyStepsTransform) {
+    aggregate.push(new TypeError(`Expected at least one step to compute or transform`));
+  }
+
+  aggregate.close();
+
+  function _prepareContinuation(callingTransformForThisStep) {
+    const continuationStorage = {
+      returnedWith: null,
+      providedDependencies: undefined,
+      providedValue: undefined,
+    };
+
+    const continuation =
+      (callingTransformForThisStep
+        ? (providedValue, providedDependencies = null) => {
+            continuationStorage.returnedWith = 'continuation';
+            continuationStorage.providedDependencies = providedDependencies;
+            continuationStorage.providedValue = providedValue;
+            return continuationSymbol;
+          }
+        : (providedDependencies = null) => {
+            continuationStorage.returnedWith = 'continuation';
+            continuationStorage.providedDependencies = providedDependencies;
+            return continuationSymbol;
+          });
+
+    continuation.exit = (providedValue) => {
+      continuationStorage.returnedWith = 'exit';
+      continuationStorage.providedValue = providedValue;
+      return continuationSymbol;
+    };
+
+    if (compositionNests) {
+      const makeRaiseLike = returnWith =>
+        (callingTransformForThisStep
+          ? (providedValue, providedDependencies = null) => {
+              continuationStorage.returnedWith = returnWith;
+              continuationStorage.providedDependencies = providedDependencies;
+              continuationStorage.providedValue = providedValue;
+              return continuationSymbol;
+            }
+          : (providedDependencies = null) => {
+              continuationStorage.returnedWith = returnWith;
+              continuationStorage.providedDependencies = providedDependencies;
+              return continuationSymbol;
+            });
+
+      continuation.raiseOutput = makeRaiseLike('raiseOutput');
+      continuation.raiseOutputAbove = makeRaiseLike('raiseOutputAbove');
+    }
+
+    return {continuation, continuationStorage};
+  }
+
+  function _computeOrTransform(initialValue, continuationIfApplicable, initialDependencies) {
+    const expectingTransform = initialValue !== noTransformSymbol;
+
+    let valueSoFar =
+      (expectingTransform
+        ? initialValue
+        : undefined);
+
+    const availableDependencies = {...initialDependencies};
+
+    const inputValues =
+      Object.values(description.inputMapping ?? {})
+        .map(token => {
+          const tokenShape = getInputTokenShape(token);
+          const tokenValue = getInputTokenValue(token);
+          switch (tokenShape) {
+            case 'input.dependency':
+              return initialDependencies[tokenValue];
+            case 'input.value':
+              return tokenValue;
+            case 'input.updateValue':
+              if (!expectingTransform)
+                throw new Error(`Unexpected input.updateValue() accessed on non-transform call`);
+              return valueSoFar;
+            case 'input.myself':
+              return initialDependencies['this'];
+            case 'input':
+              return initialDependencies[token];
+            default:
+              throw new TypeError(`Unexpected input shape ${tokenShape}`);
+          }
+        });
+
+    withAggregate({message: `Errors in input values provided to ${compositionName}`}, ({push}) => {
+      for (const {dynamic, name, value, description} of stitchArrays({
+        dynamic: inputsMayBeDynamicValue,
+        name: inputNames,
+        value: inputValues,
+        description: inputDescriptions,
+      })) {
+        if (!dynamic) continue;
+        try {
+          validateInputValue(value, description);
+        } catch (error) {
+          error.message = `${name}: ${error.message}`;
+          push(error);
+        }
+      }
+    });
+
+    if (expectingTransform) {
+      debug(() => [colors.bright(`begin composition - transforming from:`), initialValue]);
+    } else {
+      debug(() => colors.bright(`begin composition - not transforming`));
+    }
+
+    for (let i = 0; i < steps.length; i++) {
+      const step = steps[i];
+      const isBase = i === steps.length - 1;
+
+      debug(() => [
+        `step #${i+1}` +
+        (isBase
+          ? ` (base):`
+          : ` of ${steps.length}:`),
+        step]);
+
+      const expose =
+        (step.flags
+          ? step.expose
+          : step);
+
+      if (!expose) {
+        if (!isBase) {
+          debug(() => `step #${i+1} - no expose description, nothing to do for this step`);
+          continue;
+        }
+
+        if (expectingTransform) {
+          debug(() => `step #${i+1} (base) - no expose description, returning so-far update value:`, valueSoFar);
+          if (continuationIfApplicable) {
+            debug(() => colors.bright(`end composition - raise (inferred - composing)`));
+            return continuationIfApplicable(valueSoFar);
+          } else {
+            debug(() => colors.bright(`end composition - exit (inferred - not composing)`));
+            return valueSoFar;
+          }
+        } else {
+          debug(() => `step #${i+1} (base) - no expose description, nothing to continue with`);
+          if (continuationIfApplicable) {
+            debug(() => colors.bright(`end composition - raise (inferred - composing)`));
+            return continuationIfApplicable();
+          } else {
+            debug(() => colors.bright(`end composition - exit (inferred - not composing)`));
+            return null;
+          }
+        }
+      }
+
+      const callingTransformForThisStep =
+        expectingTransform && expose.transform;
+
+      let continuationStorage;
+
+      const inputDictionary =
+        Object.fromEntries(
+          stitchArrays({symbol: inputSymbols, value: inputValues})
+            .map(({symbol, value}) => [symbol, value]));
+
+      const filterableDependencies = {
+        ...availableDependencies,
+        ...inputMetadata,
+        ...inputDictionary,
+        ...
+          (expectingTransform
+            ? {[input.updateValue()]: valueSoFar}
+            : {}),
+        [input.myself()]: initialDependencies?.['this'] ?? null,
+      };
+
+      const selectDependencies =
+        (expose.dependencies ?? []).map(dependency => {
+          if (!isInputToken(dependency)) return dependency;
+          const tokenShape = getInputTokenShape(dependency);
+          const tokenValue = getInputTokenValue(dependency);
+          switch (tokenShape) {
+            case 'input':
+            case 'input.staticDependency':
+            case 'input.staticValue':
+              return dependency;
+            case 'input.myself':
+              return input.myself();
+            case 'input.dependency':
+              return tokenValue;
+            case 'input.updateValue':
+              return input.updateValue();
+            default:
+              throw new Error(`Unexpected token ${tokenShape} as dependency`);
+          }
+        })
+
+      const filteredDependencies =
+        filterProperties(filterableDependencies, selectDependencies);
+
+      debug(() => [
+        `step #${i+1} - ${callingTransformForThisStep ? 'transform' : 'compute'}`,
+        `with dependencies:`, filteredDependencies,
+        `selecting:`, selectDependencies,
+        `from available:`, filterableDependencies,
+        ...callingTransformForThisStep ? [`from value:`, valueSoFar] : []]);
+
+      let result;
+
+      const getExpectedEvaluation = () =>
+        (callingTransformForThisStep
+          ? (filteredDependencies
+              ? ['transform', valueSoFar, continuationSymbol, filteredDependencies]
+              : ['transform', valueSoFar, continuationSymbol])
+          : (filteredDependencies
+              ? ['compute', continuationSymbol, filteredDependencies]
+              : ['compute', continuationSymbol]));
+
+      const naturalEvaluate = () => {
+        const [name, ...argsLayout] = getExpectedEvaluation();
+
+        let args;
+
+        if (isBase && !compositionNests) {
+          args =
+            argsLayout.filter(arg => arg !== continuationSymbol);
+        } else {
+          let continuation;
+
+          ({continuation, continuationStorage} =
+            _prepareContinuation(callingTransformForThisStep));
+
+          args =
+            argsLayout.map(arg =>
+              (arg === continuationSymbol
+                ? continuation
+                : arg));
+        }
+
+        return expose[name](...args);
+      }
+
+      switch (step.cache) {
+        // Warning! Highly WIP!
+        case 'aggressive': {
+          const hrnow = () => {
+            const hrTime = process.hrtime();
+            return hrTime[0] * 1000000000 + hrTime[1];
+          };
+
+          const [name, ...args] = getExpectedEvaluation();
+
+          let cache = globalCompositeCache[step.annotation];
+          if (!cache) {
+            cache = globalCompositeCache[step.annotation] = {
+              transform: new TupleMap(),
+              compute: new TupleMap(),
+              times: {
+                read: [],
+                evaluate: [],
+              },
+            };
+          }
+
+          const tuplefied = args
+            .flatMap(arg => [
+              Symbol.for('compositeFrom: tuplefied arg divider'),
+              ...(typeof arg !== 'object' || Array.isArray(arg)
+                ? [arg]
+                : Object.entries(arg).flat()),
+            ]);
+
+          const readTime = hrnow();
+          const cacheContents = cache[name].get(tuplefied);
+          cache.times.read.push(hrnow() - readTime);
+
+          if (cacheContents) {
+            ({result, continuationStorage} = cacheContents);
+          } else {
+            const evaluateTime = hrnow();
+            result = naturalEvaluate();
+            cache.times.evaluate.push(hrnow() - evaluateTime);
+            cache[name].set(tuplefied, {result, continuationStorage});
+          }
+
+          break;
+        }
+
+        default: {
+          result = naturalEvaluate();
+          break;
+        }
+      }
+
+      if (result !== continuationSymbol) {
+        debug(() => [`step #${i+1} - result: exit (inferred) ->`, result]);
+
+        if (compositionNests) {
+          throw new TypeError(`Inferred early-exit is disallowed in nested compositions`);
+        }
+
+        debug(() => colors.bright(`end composition - exit (inferred)`));
+
+        return result;
+      }
+
+      const {returnedWith} = continuationStorage;
+
+      if (returnedWith === 'exit') {
+        const {providedValue} = continuationStorage;
+
+        debug(() => [`step #${i+1} - result: exit (explicit) ->`, providedValue]);
+        debug(() => colors.bright(`end composition - exit (explicit)`));
+
+        if (compositionNests) {
+          return continuationIfApplicable.exit(providedValue);
+        } else {
+          return providedValue;
+        }
+      }
+
+      const {providedValue, providedDependencies} = continuationStorage;
+
+      const continuationArgs = [];
+      if (expectingTransform) {
+        continuationArgs.push(
+          (callingTransformForThisStep
+            ? providedValue ?? null
+            : valueSoFar ?? null));
+      }
+
+      debug(() => {
+        const base = `step #${i+1} - result: ` + returnedWith;
+        const parts = [];
+
+        if (callingTransformForThisStep) {
+          parts.push('value:', providedValue);
+        }
+
+        if (providedDependencies !== null) {
+          parts.push(`deps:`, providedDependencies);
+        } else {
+          parts.push(`(no deps)`);
+        }
+
+        if (empty(parts)) {
+          return base;
+        } else {
+          return [base + ' ->', ...parts];
+        }
+      });
+
+      switch (returnedWith) {
+        case 'raiseOutput':
+          debug(() =>
+            (isBase
+              ? colors.bright(`end composition - raiseOutput (base: explicit)`)
+              : colors.bright(`end composition - raiseOutput`)));
+          continuationArgs.push(_mapDependenciesToOutputs(providedDependencies));
+          return continuationIfApplicable(...continuationArgs);
+
+        case 'raiseOutputAbove':
+          debug(() => colors.bright(`end composition - raiseOutputAbove`));
+          continuationArgs.push(_mapDependenciesToOutputs(providedDependencies));
+          return continuationIfApplicable.raiseOutput(...continuationArgs);
+
+        case 'continuation':
+          if (isBase) {
+            debug(() => colors.bright(`end composition - raiseOutput (inferred)`));
+            continuationArgs.push(_mapDependenciesToOutputs(providedDependencies));
+            return continuationIfApplicable(...continuationArgs);
+          } else {
+            Object.assign(availableDependencies, providedDependencies);
+            if (callingTransformForThisStep && providedValue !== null) {
+              valueSoFar = providedValue;
+            }
+            break;
+          }
+      }
+    }
+  }
+
+  const constructedDescriptor = {};
+
+  if (annotation) {
+    constructedDescriptor.annotation = annotation;
+  }
+
+  constructedDescriptor.flags = {
+    update: compositionUpdates,
+    expose: compositionExposes,
+    compose: compositionNests,
+  };
+
+  if (compositionUpdates) {
+    // TODO: This is a dumb assign statement, and it could probably do more
+    // interesting things, like combining validation functions.
+    constructedDescriptor.update =
+      Object.assign(
+        {...description.update ?? {}},
+        ...inputUpdateDescriptions,
+        ...stepUpdateDescriptions.flat());
+  }
+
+  if (compositionExposes) {
+    const expose = constructedDescriptor.expose = {};
+
+    expose.dependencies =
+      unique([
+        ...dependenciesFromInputs,
+        ...dependenciesFromSteps,
+      ]);
+
+    const _wrapper = (...args) => {
+      try {
+        return _computeOrTransform(...args);
+      } catch (thrownError) {
+        const error = new Error(
+          `Error computing composition` +
+          (annotation ? ` ${annotation}` : ''));
+        error.cause = thrownError;
+        throw error;
+      }
+    };
+
+    if (compositionNests) {
+      if (compositionUpdates) {
+        expose.transform = (value, continuation, dependencies) =>
+          _wrapper(value, continuation, dependencies);
+      }
+
+      if (anyStepsCompute && !anyStepsUseUpdateValue && !anyInputsUseUpdateValue) {
+        expose.compute = (continuation, dependencies) =>
+          _wrapper(noTransformSymbol, continuation, dependencies);
+      }
+
+      if (base.cacheComposition) {
+        expose.cache = base.cacheComposition;
+      }
+    } else if (compositionUpdates) {
+      expose.transform = (value, dependencies) =>
+        _wrapper(value, null, dependencies);
+    } else {
+      expose.compute = (dependencies) =>
+        _wrapper(noTransformSymbol, null, dependencies);
+    }
+  }
+
+  return constructedDescriptor;
+}
+
+export function displayCompositeCacheAnalysis() {
+  const showTimes = (cache, key) => {
+    const times = cache.times[key].slice().sort();
+
+    const all = times;
+    const worst10pc = times.slice(-times.length / 10);
+    const best10pc = times.slice(0, times.length / 10);
+    const middle50pc = times.slice(times.length / 4, -times.length / 4);
+    const middle80pc = times.slice(times.length / 10, -times.length / 10);
+
+    const fmt = val => `${(val / 1000).toFixed(2)}ms`.padStart(9);
+    const avg = times => times.reduce((a, b) => a + b, 0) / times.length;
+
+    const left = ` - ${key}: `;
+    const indn = ' '.repeat(left.length);
+    console.log(left + `${fmt(avg(all))} (all ${all.length})`);
+    console.log(indn + `${fmt(avg(worst10pc))} (worst 10%)`);
+    console.log(indn + `${fmt(avg(best10pc))} (best 10%)`);
+    console.log(indn + `${fmt(avg(middle80pc))} (middle 80%)`);
+    console.log(indn + `${fmt(avg(middle50pc))} (middle 50%)`);
+  };
+
+  for (const [annotation, cache] of Object.entries(globalCompositeCache)) {
+    console.log(`Cached ${annotation}:`);
+    showTimes(cache, 'evaluate');
+    showTimes(cache, 'read');
+  }
+}
+
+// Evaluates a function with composite debugging enabled, turns debugging
+// off again, and returns the result of the function. This is mostly syntax
+// sugar, but also helps avoid unit tests avoid accidentally printing debug
+// info for a bunch of unrelated composites (due to property enumeration
+// when displaying an unexpected result). Use as so:
+//
+//   Without debugging:
+//     t.same(thing.someProp, value)
+//
+//   With debugging:
+//     t.same(debugComposite(() => thing.someProp), value)
+//
+export function debugComposite(fn) {
+  compositeFrom.debug = true;
+  const value = fn();
+  compositeFrom.debug = false;
+  return value;
+}
diff --git a/src/data/things/flash.js b/src/data/things/flash.js
index 6eb5234f..e2afcef4 100644
--- a/src/data/things/flash.js
+++ b/src/data/things/flash.js
@@ -1,25 +1,42 @@
+import {input} from '#composite';
 import find from '#find';
 
+import {
+  isColor,
+  isDirectory,
+  isNumber,
+  isString,
+  oneOf,
+} from '#validators';
+
+import {exposeDependency, exposeUpdateValueOrContinue}
+  from '#composite/control-flow';
+import {withPropertyFromObject} from '#composite/data';
+
+import {
+  color,
+  contributionList,
+  directory,
+  fileExtension,
+  name,
+  referenceList,
+  simpleDate,
+  simpleString,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
+import {withFlashAct} from '#composite/things/flash';
+
 import Thing from './thing.js';
 
 export class Flash extends Thing {
   static [Thing.referenceType] = 'flash';
 
-  static [Thing.getPropertyDescriptors] = ({
-    Artist,
-    Track,
-    FlashAct,
-
-    validators: {
-      isDirectory,
-      isNumber,
-      isString,
-      oneOf,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Artist, Track, FlashAct}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Flash'),
+    name: name('Unnamed Flash'),
 
     directory: {
       flags: {update: true, expose: true},
@@ -47,51 +64,51 @@ export class Flash extends Thing {
       },
     },
 
-    date: Thing.common.simpleDate(),
-
-    coverArtFileExtension: Thing.common.fileExtension('jpg'),
+    color: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isColor),
+      }),
 
-    contributorContribsByRef: Thing.common.contribsByRef(),
+      withFlashAct(),
 
-    featuredTracksByRef: Thing.common.referenceList(Track),
+      withPropertyFromObject({
+        object: '#flashAct',
+        property: input.value('color'),
+      }),
 
-    urls: Thing.common.urls(),
+      exposeDependency({dependency: '#flashAct.color'}),
+    ],
 
-    // Update only
+    date: simpleDate(),
 
-    artistData: Thing.common.wikiData(Artist),
-    trackData: Thing.common.wikiData(Track),
-    flashActData: Thing.common.wikiData(FlashAct),
+    coverArtFileExtension: fileExtension('jpg'),
 
-    // Expose only
+    contributorContribs: contributionList(),
 
-    contributorContribs: Thing.common.dynamicContribs('contributorContribsByRef'),
+    featuredTracks: referenceList({
+      class: input.value(Track),
+      find: input.value(find.track),
+      data: 'trackData',
+    }),
 
-    featuredTracks: Thing.common.dynamicThingsFromReferenceList(
-      'featuredTracksByRef',
-      'trackData',
-      find.track
-    ),
+    urls: urls(),
 
-    act: {
-      flags: {expose: true},
+    // Update only
 
-      expose: {
-        dependencies: ['flashActData'],
+    artistData: wikiData(Artist),
+    trackData: wikiData(Track),
+    flashActData: wikiData(FlashAct),
 
-        compute: ({flashActData, [Flash.instance]: flash}) =>
-          flashActData.find((act) => act.flashes.includes(flash)) ?? null,
-      },
-    },
+    // Expose only
 
-    color: {
+    act: {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['flashActData'],
+        dependencies: ['this', 'flashActData'],
 
-        compute: ({flashActData, [Flash.instance]: flash}) =>
-          flashActData.find((act) => act.flashes.includes(flash))?.color ?? null,
+        compute: ({this: flash, flashActData}) =>
+          flashActData.find((act) => act.flashes.includes(flash)) ?? null,
       },
     },
   });
@@ -111,17 +128,18 @@ export class Flash extends Thing {
 }
 
 export class FlashAct extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    validators: {
-      isColor,
-    },
-  }) => ({
+  static [Thing.referenceType] = 'flash-act';
+  static [Thing.friendlyName] = `Flash Act`;
+
+  static [Thing.getPropertyDescriptors] = () => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Flash Act'),
-    color: Thing.common.color(),
-    anchor: Thing.common.simpleString(),
-    jump: Thing.common.simpleString(),
+    name: name('Unnamed Flash Act'),
+    directory: directory(),
+    color: color(),
+    listTerminology: simpleString(),
+
+    jump: simpleString(),
 
     jumpColor: {
       flags: {update: true, expose: true},
@@ -133,18 +151,14 @@ export class FlashAct extends Thing {
       }
     },
 
-    flashesByRef: Thing.common.referenceList(Flash),
+    flashes: referenceList({
+      class: input.value(Flash),
+      find: input.value(find.flash),
+      data: 'flashData',
+    }),
 
     // Update only
 
-    flashData: Thing.common.wikiData(Flash),
-
-    // Expose only
-
-    flashes: Thing.common.dynamicThingsFromReferenceList(
-      'flashesByRef',
-      'flashData',
-      find.flash
-    ),
+    flashData: wikiData(Flash),
   })
 }
diff --git a/src/data/things/group.js b/src/data/things/group.js
index ba339b3e..8764a9db 100644
--- a/src/data/things/group.js
+++ b/src/data/things/group.js
@@ -1,33 +1,44 @@
+import {input} from '#composite';
 import find from '#find';
 
+import {
+  color,
+  directory,
+  name,
+  referenceList,
+  simpleString,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
 import Thing from './thing.js';
 
 export class Group extends Thing {
   static [Thing.referenceType] = 'group';
 
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Album}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Group'),
-    directory: Thing.common.directory(),
+    name: name('Unnamed Group'),
+    directory: directory(),
 
-    description: Thing.common.simpleString(),
+    description: simpleString(),
 
-    urls: Thing.common.urls(),
+    urls: urls(),
 
-    featuredAlbumsByRef: Thing.common.referenceList(Album),
+    featuredAlbums: referenceList({
+      class: input.value(Album),
+      find: input.value(find.album),
+      data: 'albumData',
+    }),
 
     // Update only
 
-    albumData: Thing.common.wikiData(Album),
-    groupCategoryData: Thing.common.wikiData(GroupCategory),
+    albumData: wikiData(Album),
+    groupCategoryData: wikiData(GroupCategory),
 
     // Expose only
 
-    featuredAlbums: Thing.common.dynamicThingsFromReferenceList('featuredAlbumsByRef', 'albumData', find.album),
-
     descriptionShort: {
       flags: {expose: true},
 
@@ -41,8 +52,8 @@ export class Group extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['albumData'],
-        compute: ({albumData, [Group.instance]: group}) =>
+        dependencies: ['this', 'albumData'],
+        compute: ({this: group, albumData}) =>
           albumData?.filter((album) => album.groups.includes(group)) ?? [],
       },
     },
@@ -51,9 +62,8 @@ export class Group extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['groupCategoryData'],
-
-        compute: ({groupCategoryData, [Group.instance]: group}) =>
+        dependencies: ['this', 'groupCategoryData'],
+        compute: ({this: group, groupCategoryData}) =>
           groupCategoryData.find((category) => category.groups.includes(group))
             ?.color,
       },
@@ -63,8 +73,8 @@ export class Group extends Thing {
       flags: {expose: true},
 
       expose: {
-        dependencies: ['groupCategoryData'],
-        compute: ({groupCategoryData, [Group.instance]: group}) =>
+        dependencies: ['this', 'groupCategoryData'],
+        compute: ({this: group, groupCategoryData}) =>
           groupCategoryData.find((category) => category.groups.includes(group)) ??
           null,
       },
@@ -73,26 +83,22 @@ export class Group extends Thing {
 }
 
 export class GroupCategory extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    Group,
-  }) => ({
+  static [Thing.friendlyName] = `Group Category`;
+
+  static [Thing.getPropertyDescriptors] = ({Group}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Group Category'),
-    color: Thing.common.color(),
+    name: name('Unnamed Group Category'),
+    color: color(),
 
-    groupsByRef: Thing.common.referenceList(Group),
+    groups: referenceList({
+      class: input.value(Group),
+      find: input.value(find.group),
+      data: 'groupData',
+    }),
 
     // Update only
 
-    groupData: Thing.common.wikiData(Group),
-
-    // Expose only
-
-    groups: Thing.common.dynamicThingsFromReferenceList(
-      'groupsByRef',
-      'groupData',
-      find.group
-    ),
+    groupData: wikiData(Group),
   });
 }
diff --git a/src/data/things/homepage-layout.js b/src/data/things/homepage-layout.js
index ec9e9556..bfa971ca 100644
--- a/src/data/things/homepage-layout.js
+++ b/src/data/things/homepage-layout.js
@@ -1,20 +1,37 @@
+import {input} from '#composite';
 import find from '#find';
 
+import {
+  is,
+  isCountingNumber,
+  isString,
+  isStringNonEmpty,
+  oneOf,
+  validateArrayItems,
+  validateInstanceOf,
+  validateReference,
+} from '#validators';
+
+import {exposeDependency} from '#composite/control-flow';
+import {withResolvedReference} from '#composite/wiki-data';
+
+import {
+  color,
+  name,
+  referenceList,
+  simpleString,
+  wikiData,
+} from '#composite/wiki-properties';
+
 import Thing from './thing.js';
 
 export class HomepageLayout extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    HomepageLayoutRow,
+  static [Thing.friendlyName] = `Homepage Layout`;
 
-    validators: {
-      isStringNonEmpty,
-      validateArrayItems,
-      validateInstanceOf,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({HomepageLayoutRow}) => ({
     // Update & expose
 
-    sidebarContent: Thing.common.simpleString(),
+    sidebarContent: simpleString(),
 
     navbarLinks: {
       flags: {update: true, expose: true},
@@ -32,13 +49,12 @@ export class HomepageLayout extends Thing {
 }
 
 export class HomepageLayoutRow extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-    Group,
-  }) => ({
+  static [Thing.friendlyName] = `Homepage Row`;
+
+  static [Thing.getPropertyDescriptors] = ({Album, Group}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Homepage Row'),
+    name: name('Unnamed Homepage Row'),
 
     type: {
       flags: {update: true, expose: true},
@@ -50,30 +66,22 @@ export class HomepageLayoutRow extends Thing {
       },
     },
 
-    color: Thing.common.color(),
+    color: color(),
 
     // Update only
 
     // These aren't necessarily used by every HomepageLayoutRow subclass, but
     // for convenience of providing this data, every row accepts all wiki data
     // arrays depended upon by any subclass's behavior.
-    albumData: Thing.common.wikiData(Album),
-    groupData: Thing.common.wikiData(Group),
+    albumData: wikiData(Album),
+    groupData: wikiData(Group),
   });
 }
 
 export class HomepageLayoutAlbumsRow extends HomepageLayoutRow {
-  static [Thing.getPropertyDescriptors] = (opts, {
-    Album,
-    Group,
-
-    validators: {
-      is,
-      isCountingNumber,
-      isString,
-      validateArrayItems,
-    },
-  } = opts) => ({
+  static [Thing.friendlyName] = `Homepage Albums Row`;
+
+  static [Thing.getPropertyDescriptors] = (opts, {Album, Group} = opts) => ({
     ...HomepageLayoutRow[Thing.getPropertyDescriptors](opts),
 
     // Update & expose
@@ -104,8 +112,39 @@ export class HomepageLayoutAlbumsRow extends HomepageLayoutRow {
       },
     },
 
-    sourceGroupByRef: Thing.common.singleReference(Group),
-    sourceAlbumsByRef: Thing.common.referenceList(Album),
+    sourceGroup: [
+      {
+        flags: {expose: true, update: true, compose: true},
+
+        update: {
+          validate:
+            oneOf(
+              is('new-releases', 'new-additions'),
+              validateReference(Group[Thing.referenceType])),
+        },
+
+        expose: {
+          transform: (value, continuation) =>
+            (value === 'new-releases' || value === 'new-additions'
+              ? value
+              : continuation(value)),
+        },
+      },
+
+      withResolvedReference({
+        ref: input.updateValue(),
+        data: 'groupData',
+        find: input.value(find.group),
+      }),
+
+      exposeDependency({dependency: '#resolvedReference'}),
+    ],
+
+    sourceAlbums: referenceList({
+      class: input.value(Album),
+      find: input.value(find.album),
+      data: 'albumData',
+    }),
 
     countAlbumsFromGroup: {
       flags: {update: true, expose: true},
@@ -116,19 +155,5 @@ export class HomepageLayoutAlbumsRow extends HomepageLayoutRow {
       flags: {update: true, expose: true},
       update: {validate: validateArrayItems(isString)},
     },
-
-    // Expose only
-
-    sourceGroup: Thing.common.dynamicThingFromSingleReference(
-      'sourceGroupByRef',
-      'groupData',
-      find.group
-    ),
-
-    sourceAlbums: Thing.common.dynamicThingsFromReferenceList(
-      'sourceAlbumsByRef',
-      'albumData',
-      find.album
-    ),
   });
 }
diff --git a/src/data/things/index.js b/src/data/things/index.js
index 591cdc3b..4ea1f007 100644
--- a/src/data/things/index.js
+++ b/src/data/things/index.js
@@ -2,9 +2,9 @@ import * as path from 'node:path';
 import {fileURLToPath} from 'node:url';
 
 import {logError} from '#cli';
+import {compositeFrom} from '#composite';
 import * as serialize from '#serialize';
 import {openAggregate, showAggregate} from '#sugar';
-import * as validators from '#validators';
 
 import Thing from './thing.js';
 
@@ -21,7 +21,11 @@ import * as trackClasses from './track.js';
 import * as wikiInfoClasses from './wiki-info.js';
 
 export {default as Thing} from './thing.js';
-export {default as CacheableObject} from './cacheable-object.js';
+
+export {
+  default as CacheableObject,
+  CacheableObjectPropertyValueError,
+} from './cacheable-object.js';
 
 const allClassLists = {
   'album.js': albumClasses,
@@ -82,6 +86,8 @@ function errorDuplicateClassNames() {
 function flattenClassLists() {
   for (const classes of Object.values(allClassLists)) {
     for (const [name, constructor] of Object.entries(classes)) {
+      if (typeof constructor !== 'function') continue;
+      if (!(constructor.prototype instanceof Thing)) continue;
       allClasses[name] = constructor;
     }
   }
@@ -119,7 +125,7 @@ function descriptorAggregateHelper({
 }
 
 function evaluatePropertyDescriptors() {
-  const opts = {...allClasses, validators};
+  const opts = {...allClasses};
 
   return descriptorAggregateHelper({
     message: `Errors evaluating Thing class property descriptors`,
@@ -129,8 +135,21 @@ function evaluatePropertyDescriptors() {
         throw new Error(`Missing [Thing.getPropertyDescriptors] function`);
       }
 
-      constructor.propertyDescriptors =
-        constructor[Thing.getPropertyDescriptors](opts);
+      const results = constructor[Thing.getPropertyDescriptors](opts);
+
+      for (const [key, value] of Object.entries(results)) {
+        if (Array.isArray(value)) {
+          results[key] = compositeFrom({
+            annotation: `${constructor.name}.${key}`,
+            compose: false,
+            steps: value,
+          });
+        } else if (value.toResolvedComposition) {
+          results[key] = compositeFrom(value.toResolvedComposition());
+        }
+      }
+
+      constructor.propertyDescriptors = results;
     },
 
     showFailedClasses(failedClasses) {
diff --git a/src/data/things/language.js b/src/data/things/language.js
index 7755c505..646eb6d1 100644
--- a/src/data/things/language.js
+++ b/src/data/things/language.js
@@ -1,11 +1,17 @@
+import {Tag} from '#html';
+import {isLanguageCode} from '#validators';
+
+import {
+  externalFunction,
+  flag,
+  simpleString,
+} from '#composite/wiki-properties';
+
+import CacheableObject from './cacheable-object.js';
 import Thing from './thing.js';
 
 export class Language extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    validators: {
-      isLanguageCode,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = () => ({
     // Update & expose
 
     // General language code. This is used to identify the language distinctly
@@ -18,7 +24,7 @@ export class Language extends Thing {
 
     // Human-readable name. This should be the language's own native name, not
     // localized to any other language.
-    name: Thing.common.simpleString(),
+    name: simpleString(),
 
     // Language code specific to JavaScript's Internationalization (Intl) API.
     // Usually this will be the same as the language's general code, but it
@@ -40,7 +46,7 @@ export class Language extends Thing {
     // with languages that are currently in development and not ready for
     // formal release, or which are just kept hidden as "experimental zones"
     // for wiki development or content testing.
-    hidden: Thing.common.flag(false),
+    hidden: flag(false),
 
     // Mapping of translation keys to values (strings). Generally, don't
     // access this object directly - use methods instead.
@@ -68,7 +74,7 @@ export class Language extends Thing {
 
     // Update only
 
-    escapeHTML: Thing.common.externalFunction(),
+    escapeHTML: externalFunction(),
 
     // Expose only
 
@@ -95,6 +101,7 @@ export class Language extends Thing {
       },
     },
 
+    // TODO: This currently isn't used. Is it still needed?
     strings_htmlEscaped: {
       flags: {expose: true},
       expose: {
@@ -124,8 +131,8 @@ export class Language extends Thing {
     };
   }
 
-  $(key, args = {}) {
-    return this.formatString(key, args);
+  $(...args) {
+    return this.formatString(...args);
   }
 
   assertIntlAvailable(property) {
@@ -139,20 +146,22 @@ export class Language extends Thing {
     return this.intl_pluralCardinal.select(value);
   }
 
-  formatString(key, args = {}) {
-    if (this.strings && !this.strings_htmlEscaped) {
-      throw new Error(`HTML-escaped strings unavailable - please ensure escapeHTML function is provided`);
-    }
+  formatString(...args) {
+    const hasOptions =
+      typeof args.at(-1) === 'object' &&
+      args.at(-1) !== null;
 
-    return this.formatStringHelper(this.strings_htmlEscaped, key, args);
-  }
+    const key =
+      (hasOptions ? args.slice(0, -1) : args)
+        .filter(Boolean)
+        .join('.');
 
-  formatStringNoHTMLEscape(key, args = {}) {
-    return this.formatStringHelper(this.strings, key, args);
-  }
+    const options =
+      (hasOptions
+        ? args.at(-1)
+        : null);
 
-  formatStringHelper(strings, key, args = {}) {
-    if (!strings) {
+    if (!this.strings) {
       throw new Error(`Strings unavailable`);
     }
 
@@ -160,30 +169,94 @@ export class Language extends Thing {
       throw new Error(`Invalid key ${key} accessed`);
     }
 
-    const template = strings[key];
-
-    // Convert the keys on the args dict from camelCase to CONSTANT_CASE.
-    // (This isn't an OUTRAGEOUSLY versatile algorithm for doing that, 8ut
-    // like, who cares, dude?) Also, this is an array, 8ecause it's handy
-    // for the iterating we're a8out to do.
-    const processedArgs = Object.entries(args).map(([k, v]) => [
-      k.replace(/[A-Z]/g, '_$&').toUpperCase(),
-      v,
-    ]);
-
-    // Replacement time! Woot. Reduce comes in handy here!
-    const output = processedArgs.reduce(
-      (x, [k, v]) => x.replaceAll(`{${k}}`, v),
-      template
-    );
+    const template = this.strings[key];
+
+    let output;
+
+    if (hasOptions) {
+      // Convert the keys on the options dict from camelCase to CONSTANT_CASE.
+      // (This isn't an OUTRAGEOUSLY versatile algorithm for doing that, 8ut
+      // like, who cares, dude?) Also, this is an array, 8ecause it's handy
+      // for the iterating we're a8out to do. Also strip HTML from arguments
+      // that are literal strings - real HTML content should always be proper
+      // HTML objects (see html.js).
+      const processedOptions =
+        Object.entries(options).map(([k, v]) => [
+          k.replace(/[A-Z]/g, '_$&').toUpperCase(),
+          this.#sanitizeStringArg(v),
+        ]);
+
+      // Replacement time! Woot. Reduce comes in handy here!
+      output =
+        processedOptions.reduce(
+          (x, [k, v]) => x.replaceAll(`{${k}}`, v),
+          template);
+    } else {
+      // Without any options provided, just use the template as-is. This will
+      // still error if the template expected arguments, and otherwise will be
+      // the right value.
+      output = template;
+    }
 
     // Post-processing: if any expected arguments *weren't* replaced, that
     // is almost definitely an error.
-    if (output.match(/\{[A-Z_]+\}/)) {
+    if (output.match(/\{[A-Z][A-Z0-9_]*\}/)) {
       throw new Error(`Args in ${key} were missing - output: ${output}`);
     }
 
-    return output;
+    // Last caveat: Wrap the output in an HTML tag so that it doesn't get
+    // treated as unsanitized HTML if *it* gets passed as an argument to
+    // *another* formatString call.
+    return this.#wrapSanitized(output);
+  }
+
+  // Escapes HTML special characters so they're displayed as-are instead of
+  // treated by the browser as a tag. This does *not* have an effect on actual
+  // html.Tag objects, which are treated as sanitized by default (so that they
+  // can be nested inside strings at all).
+  #sanitizeStringArg(arg) {
+    const escapeHTML = CacheableObject.getUpdateValue(this, 'escapeHTML');
+
+    if (!escapeHTML) {
+      throw new Error(`escapeHTML unavailable`);
+    }
+
+    if (typeof arg !== 'string') {
+      return arg.toString();
+    }
+
+    return escapeHTML(arg);
+  }
+
+  // Wraps the output of a formatting function in a no-name-nor-attributes
+  // HTML tag, which will indicate to other calls to formatString that this
+  // content is a string *that may contain HTML* and doesn't need to
+  // sanitized any further. It'll still .toString() to just the string
+  // contents, if needed.
+  #wrapSanitized(output) {
+    return new Tag(null, null, output);
+  }
+
+  // Similar to the above internal methods, but this one is public.
+  // It should be used when embedding content that may not have previously
+  // been sanitized directly into an HTML tag or template's contents.
+  // The templating engine usually handles this on its own, as does passing
+  // a value (sanitized or not) directly as an argument to formatString,
+  // but if you used a custom validation function ({validate: v => v.isHTML}
+  // instead of {type: 'string'} / {type: 'html'}) and are embedding the
+  // contents of a slot directly, it should be manually sanitized with this
+  // function first.
+  sanitize(arg) {
+    const escapeHTML = CacheableObject.getUpdateValue(this, 'escapeHTML');
+
+    if (!escapeHTML) {
+      throw new Error(`escapeHTML unavailable`);
+    }
+
+    return (
+      (typeof arg === 'string'
+        ? new Tag(null, null, escapeHTML(arg))
+        : arg));
   }
 
   formatDate(date) {
@@ -252,19 +325,32 @@ export class Language extends Thing {
   // Conjunction list: A, B, and C
   formatConjunctionList(array) {
     this.assertIntlAvailable('intl_listConjunction');
-    return this.intl_listConjunction.format(array.map(arr => arr.toString()));
+    return this.#wrapSanitized(
+      this.intl_listConjunction.format(
+        array.map(item => this.#sanitizeStringArg(item))));
   }
 
   // Disjunction lists: A, B, or C
   formatDisjunctionList(array) {
     this.assertIntlAvailable('intl_listDisjunction');
-    return this.intl_listDisjunction.format(array.map(arr => arr.toString()));
+    return this.#wrapSanitized(
+      this.intl_listDisjunction.format(
+        array.map(item => this.#sanitizeStringArg(item))));
   }
 
   // Unit lists: A, B, C
   formatUnitList(array) {
     this.assertIntlAvailable('intl_listUnit');
-    return this.intl_listUnit.format(array.map(arr => arr.toString()));
+    return this.#wrapSanitized(
+      this.intl_listUnit.format(
+        array.map(item => this.#sanitizeStringArg(item))));
+  }
+
+  // Lists without separator: A B C
+  formatListWithoutSeparator(array) {
+    return this.#wrapSanitized(
+      array.map(item => this.#sanitizeStringArg(item))
+        .join(' '));
   }
 
   // File sizes: 42.5 kB, 127.2 MB, 4.13 GB, 998.82 TB
diff --git a/src/data/things/news-entry.js b/src/data/things/news-entry.js
index 43911410..36da0299 100644
--- a/src/data/things/news-entry.js
+++ b/src/data/things/news-entry.js
@@ -1,16 +1,24 @@
+import {
+  directory,
+  name,
+  simpleDate,
+  simpleString,
+} from '#composite/wiki-properties';
+
 import Thing from './thing.js';
 
 export class NewsEntry extends Thing {
   static [Thing.referenceType] = 'news-entry';
+  static [Thing.friendlyName] = `News Entry`;
 
   static [Thing.getPropertyDescriptors] = () => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed News Entry'),
-    directory: Thing.common.directory(),
-    date: Thing.common.simpleDate(),
+    name: name('Unnamed News Entry'),
+    directory: directory(),
+    date: simpleDate(),
 
-    content: Thing.common.simpleString(),
+    content: simpleString(),
 
     // Expose only
 
diff --git a/src/data/things/static-page.js b/src/data/things/static-page.js
index 3d8d474c..ab9c5f98 100644
--- a/src/data/things/static-page.js
+++ b/src/data/things/static-page.js
@@ -1,16 +1,21 @@
+import {isName} from '#validators';
+
+import {
+  directory,
+  name,
+  simpleString,
+} from '#composite/wiki-properties';
+
 import Thing from './thing.js';
 
 export class StaticPage extends Thing {
   static [Thing.referenceType] = 'static';
+  static [Thing.friendlyName] = `Static Page`;
 
-  static [Thing.getPropertyDescriptors] = ({
-    validators: {
-      isName,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = () => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Static Page'),
+    name: name('Unnamed Static Page'),
 
     nameShort: {
       flags: {update: true, expose: true},
@@ -22,8 +27,8 @@ export class StaticPage extends Thing {
       },
     },
 
-    directory: Thing.common.directory(),
-    content: Thing.common.simpleString(),
-    stylesheet: Thing.common.simpleString(),
+    directory: directory(),
+    content: simpleString(),
+    stylesheet: simpleString(),
   });
 }
diff --git a/src/data/things/thing.js b/src/data/things/thing.js
index c2876f56..def7e914 100644
--- a/src/data/things/thing.js
+++ b/src/data/things/thing.js
@@ -1,399 +1,19 @@
-// Thing: base class for wiki data types, providing wiki-specific utility
-// functions on top of essential CacheableObject behavior.
+// Thing: base class for wiki data types, providing interfaces generally useful
+// to all wiki data objects on top of foundational CacheableObject behavior.
 
 import {inspect} from 'node:util';
 
-import {color} from '#cli';
-import find from '#find';
-import {empty} from '#sugar';
-import {getKebabCase} from '#wiki-data';
-
-import {
-  isAdditionalFileList,
-  isBoolean,
-  isCommentary,
-  isColor,
-  isContributionList,
-  isDate,
-  isDirectory,
-  isFileExtension,
-  isName,
-  isString,
-  isURL,
-  validateArrayItems,
-  validateInstanceOf,
-  validateReference,
-  validateReferenceList,
-} from '#validators';
+import {colors} from '#cli';
 
 import CacheableObject from './cacheable-object.js';
 
 export default class Thing extends CacheableObject {
-  static referenceType = Symbol('Thing.referenceType');
+  static referenceType = Symbol.for('Thing.referenceType');
+  static friendlyName = Symbol.for(`Thing.friendlyName`);
 
   static getPropertyDescriptors = Symbol('Thing.getPropertyDescriptors');
   static getSerializeDescriptors = Symbol('Thing.getSerializeDescriptors');
 
-  // Regularly reused property descriptors, for ease of access and generally
-  // duplicating less code across wiki data types. These are specialized utility
-  // functions, so check each for how its own arguments behave!
-  static common = {
-    name: (defaultName) => ({
-      flags: {update: true, expose: true},
-      update: {validate: isName, default: defaultName},
-    }),
-
-    color: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: isColor},
-    }),
-
-    directory: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: isDirectory},
-      expose: {
-        dependencies: ['name'],
-        transform(directory, {name}) {
-          if (directory === null && name === null) return null;
-          else if (directory === null) return getKebabCase(name);
-          else return directory;
-        },
-      },
-    }),
-
-    urls: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: validateArrayItems(isURL)},
-      expose: {transform: (value) => value ?? []},
-    }),
-
-    // A file extension! Or the default, if provided when calling this.
-    fileExtension: (defaultFileExtension = null) => ({
-      flags: {update: true, expose: true},
-      update: {validate: isFileExtension},
-      expose: {transform: (value) => value ?? defaultFileExtension},
-    }),
-
-    // Straightforward flag descriptor for a variety of property purposes.
-    // Provide a default value, true or false!
-    flag: (defaultValue = false) => {
-      if (typeof defaultValue !== 'boolean') {
-        throw new TypeError(`Always set explicit defaults for flags!`);
-      }
-
-      return {
-        flags: {update: true, expose: true},
-        update: {validate: isBoolean, default: defaultValue},
-      };
-    },
-
-    // General date type, used as the descriptor for a bunch of properties.
-    // This isn't dynamic though - it won't inherit from a date stored on
-    // another object, for example.
-    simpleDate: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: isDate},
-    }),
-
-    // General string type. This should probably generally be avoided in favor
-    // of more specific validation, but using it makes it easy to find where we
-    // might want to improve later, and it's a useful shorthand meanwhile.
-    simpleString: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: isString},
-    }),
-
-    // External function. These should only be used as dependencies for other
-    // properties, so they're left unexposed.
-    externalFunction: () => ({
-      flags: {update: true},
-      update: {validate: (t) => typeof t === 'function'},
-    }),
-
-    // Super simple "contributions by reference" list, used for a variety of
-    // properties (Artists, Cover Artists, etc). This is the property which is
-    // externally provided, in the form:
-    //
-    //     [
-    //         {who: 'Artist Name', what: 'Viola'},
-    //         {who: 'artist:john-cena', what: null},
-    //         ...
-    //     ]
-    //
-    // ...processed from YAML, spreadsheet, or any other kind of input.
-    contribsByRef: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: isContributionList},
-    }),
-
-    // Artist commentary! Generally present on tracks and albums.
-    commentary: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: isCommentary},
-    }),
-
-    // This is a somewhat more involved data structure - it's for additional
-    // or "bonus" files associated with albums or tracks (or anything else).
-    // It's got this form:
-    //
-    //     [
-    //         {title: 'Booklet', files: ['Booklet.pdf']},
-    //         {
-    //             title: 'Wallpaper',
-    //             description: 'Cool Wallpaper!',
-    //             files: ['1440x900.png', '1920x1080.png']
-    //         },
-    //         {title: 'Alternate Covers', description: null, files: [...]},
-    //         ...
-    //     ]
-    //
-    additionalFiles: () => ({
-      flags: {update: true, expose: true},
-      update: {validate: isAdditionalFileList},
-      expose: {
-        transform: (additionalFiles) =>
-          additionalFiles ?? [],
-      },
-    }),
-
-    // A reference list! Keep in mind this is for general references to wiki
-    // objects of (usually) other Thing subclasses, not specifically leitmotif
-    // references in tracks (although that property uses referenceList too!).
-    //
-    // The underlying function validateReferenceList expects a string like
-    // 'artist' or 'track', but this utility keeps from having to hard-code the
-    // string in multiple places by referencing the value saved on the class
-    // instead.
-    referenceList: (thingClass) => {
-      const {[Thing.referenceType]: referenceType} = thingClass;
-      if (!referenceType) {
-        throw new Error(`The passed constructor ${thingClass.name} doesn't define Thing.referenceType!`);
-      }
-
-      return {
-        flags: {update: true, expose: true},
-        update: {validate: validateReferenceList(referenceType)},
-      };
-    },
-
-    // Corresponding function for a single reference.
-    singleReference: (thingClass) => {
-      const {[Thing.referenceType]: referenceType} = thingClass;
-      if (!referenceType) {
-        throw new Error(`The passed constructor ${thingClass.name} doesn't define Thing.referenceType!`);
-      }
-
-      return {
-        flags: {update: true, expose: true},
-        update: {validate: validateReference(referenceType)},
-      };
-    },
-
-    // Corresponding dynamic property to referenceList, which takes the values
-    // in the provided property and searches the specified wiki data for
-    // matching actual Thing-subclass objects.
-    dynamicThingsFromReferenceList: (
-      referenceListProperty,
-      thingDataProperty,
-      findFn
-    ) => ({
-      flags: {expose: true},
-
-      expose: {
-        dependencies: [referenceListProperty, thingDataProperty],
-        compute: ({
-          [referenceListProperty]: refs,
-          [thingDataProperty]: thingData,
-        }) =>
-          refs && thingData
-            ? refs
-                .map((ref) => findFn(ref, thingData, {mode: 'quiet'}))
-                .filter(Boolean)
-            : [],
-      },
-    }),
-
-    // Corresponding function for a single reference.
-    dynamicThingFromSingleReference: (
-      singleReferenceProperty,
-      thingDataProperty,
-      findFn
-    ) => ({
-      flags: {expose: true},
-
-      expose: {
-        dependencies: [singleReferenceProperty, thingDataProperty],
-        compute: ({
-          [singleReferenceProperty]: ref,
-          [thingDataProperty]: thingData,
-        }) => (ref && thingData ? findFn(ref, thingData, {mode: 'quiet'}) : null),
-      },
-    }),
-
-    // Corresponding dynamic property to contribsByRef, which takes the values
-    // in the provided property and searches the object's artistData for
-    // matching actual Artist objects. The computed structure has the same form
-    // as contribsByRef, but with Artist objects instead of string references:
-    //
-    //     [
-    //         {who: (an Artist), what: 'Viola'},
-    //         {who: (an Artist), what: null},
-    //         ...
-    //     ]
-    //
-    // Contributions whose "who" values don't match anything in artistData are
-    // filtered out. (So if the list is all empty, chances are that either the
-    // reference list is somehow messed up, or artistData isn't being provided
-    // properly.)
-    dynamicContribs: (contribsByRefProperty) => ({
-      flags: {expose: true},
-      expose: {
-        dependencies: ['artistData', contribsByRefProperty],
-        compute: ({artistData, [contribsByRefProperty]: contribsByRef}) =>
-          contribsByRef && artistData
-            ? contribsByRef
-                .map(({who: ref, what}) => ({
-                  who: find.artist(ref, artistData),
-                  what,
-                }))
-                .filter(({who}) => who)
-            : [],
-      },
-    }),
-
-    // Dynamically inherit a contribution list from some other object, if it
-    // hasn't been overridden on this object. This is handy for solo albums
-    // where all tracks have the same artist, for example.
-    dynamicInheritContribs: (
-      // If this property is explicitly false, the contribution list returned
-      // will always be empty.
-      nullerProperty,
-
-      // Property holding contributions on the current object.
-      contribsByRefProperty,
-
-      // Property holding corresponding "default" contributions on the parent
-      // object, which will fallen back to if the object doesn't have its own
-      // contribs.
-      parentContribsByRefProperty,
-
-      // Data array to search in and "find" function to locate parent object
-      // (which will be passed the child object and the wiki data array).
-      thingDataProperty,
-      findFn
-    ) => ({
-      flags: {expose: true},
-      expose: {
-        dependencies: [
-          contribsByRefProperty,
-          thingDataProperty,
-          nullerProperty,
-          'artistData',
-        ].filter(Boolean),
-
-        compute({
-          [Thing.instance]: thing,
-          [nullerProperty]: nuller,
-          [contribsByRefProperty]: contribsByRef,
-          [thingDataProperty]: thingData,
-          artistData,
-        }) {
-          if (!artistData) return [];
-          if (nuller === false) return [];
-          const refs =
-            contribsByRef ??
-            findFn(thing, thingData, {mode: 'quiet'})?.[parentContribsByRefProperty];
-          if (!refs) return [];
-          return refs
-            .map(({who: ref, what}) => ({
-              who: find.artist(ref, artistData),
-              what,
-            }))
-            .filter(({who}) => who);
-        },
-      },
-    }),
-
-    // Nice 'n simple shorthand for an exposed-only flag which is true when any
-    // contributions are present in the specified property.
-    contribsPresent: (contribsByRefProperty) => ({
-      flags: {expose: true},
-      expose: {
-        dependencies: [contribsByRefProperty],
-        compute({
-          [contribsByRefProperty]: contribsByRef,
-        }) {
-          return !empty(contribsByRef);
-        },
-      }
-    }),
-
-    // Neat little shortcut for "reversing" the reference lists stored on other
-    // things - for example, tracks specify a "referenced tracks" property, and
-    // you would use this to compute a corresponding "referenced *by* tracks"
-    // property. Naturally, the passed ref list property is of the things in the
-    // wiki data provided, not the requesting Thing itself.
-    reverseReferenceList: (thingDataProperty, referencerRefListProperty) => ({
-      flags: {expose: true},
-
-      expose: {
-        dependencies: [thingDataProperty],
-
-        compute: ({[thingDataProperty]: thingData, [Thing.instance]: thing}) =>
-          thingData?.filter(t => t[referencerRefListProperty].includes(thing)) ?? [],
-      },
-    }),
-
-    // Corresponding function for single references. Note that the return value
-    // is still a list - this is for matching all the objects whose single
-    // reference (in the given property) matches this Thing.
-    reverseSingleReference: (thingDataProperty, referencerRefListProperty) => ({
-      flags: {expose: true},
-
-      expose: {
-        dependencies: [thingDataProperty],
-
-        compute: ({[thingDataProperty]: thingData, [Thing.instance]: thing}) =>
-          thingData?.filter((t) => t[referencerRefListProperty] === thing) ?? [],
-      },
-    }),
-
-    // General purpose wiki data constructor, for properties like artistData,
-    // trackData, etc.
-    wikiData: (thingClass) => ({
-      flags: {update: true},
-      update: {
-        validate: validateArrayItems(validateInstanceOf(thingClass)),
-      },
-    }),
-
-    // This one's kinda tricky: it parses artist "references" from the
-    // commentary content, and finds the matching artist for each reference.
-    // This is mostly useful for credits and listings on artist pages.
-    commentatorArtists: () => ({
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['artistData', 'commentary'],
-
-        compute: ({artistData, commentary}) =>
-          artistData && commentary
-            ? Array.from(
-                new Set(
-                  Array.from(
-                    commentary
-                      .replace(/<\/?b>/g, '')
-                      .matchAll(/<i>(?<who>.*?):<\/i>/g)
-                  ).map(({groups: {who}}) =>
-                    find.artist(who, artistData, {mode: 'quiet'})
-                  )
-                )
-              )
-            : [],
-      },
-    }),
-  };
-
   // Default custom inspect function, which may be overridden by Thing
   // subclasses. This will be used when displaying aggregate errors and other
   // command-line logging - it's the place to provide information useful in
@@ -402,8 +22,8 @@ export default class Thing extends CacheableObject {
     const cname = this.constructor.name;
 
     return (
-      (this.name ? `${cname} ${color.green(`"${this.name}"`)}` : `${cname}`) +
-      (this.directory ? ` (${color.blue(Thing.getReference(this))})` : '')
+      (this.name ? `${cname} ${colors.green(`"${this.name}"`)}` : `${cname}`) +
+      (this.directory ? ` (${colors.blue(Thing.getReference(this))})` : '')
     );
   }
 
diff --git a/src/data/things/track.js b/src/data/things/track.js
index e176acb4..db325a17 100644
--- a/src/data/things/track.js
+++ b/src/data/things/track.js
@@ -1,460 +1,330 @@
 import {inspect} from 'node:util';
 
-import {color} from '#cli';
+import {colors} from '#cli';
+import {input} from '#composite';
 import find from '#find';
-import {empty} from '#sugar';
 
+import {
+  isColor,
+  isContributionList,
+  isDate,
+  isFileExtension,
+} from '#validators';
+
+import {withPropertyFromObject} from '#composite/data';
+import {withResolvedContribs} from '#composite/wiki-data';
+
+import {
+  exitWithoutDependency,
+  exposeConstant,
+  exposeDependency,
+  exposeDependencyOrContinue,
+  exposeUpdateValueOrContinue,
+} from '#composite/control-flow';
+
+import {
+  additionalFiles,
+  commentary,
+  commentatorArtists,
+  contributionList,
+  directory,
+  duration,
+  flag,
+  name,
+  referenceList,
+  reverseReferenceList,
+  simpleDate,
+  singleReference,
+  simpleString,
+  urls,
+  wikiData,
+} from '#composite/wiki-properties';
+
+import {
+  exitWithoutUniqueCoverArt,
+  inheritFromOriginalRelease,
+  trackReverseReferenceList,
+  withAlbum,
+  withAlwaysReferenceByDirectory,
+  withContainingTrackSection,
+  withHasUniqueCoverArt,
+  withOtherReleases,
+  withPropertyFromAlbum,
+} from '#composite/things/track';
+
+import CacheableObject from './cacheable-object.js';
 import Thing from './thing.js';
 
 export class Track extends Thing {
   static [Thing.referenceType] = 'track';
 
-  static [Thing.getPropertyDescriptors] = ({
-    Album,
-    ArtTag,
-    Artist,
-    Flash,
-
-    validators: {
-      isBoolean,
-      isColor,
-      isDate,
-      isDuration,
-      isFileExtension,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Album, ArtTag, Artist, Flash}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Track'),
-    directory: Thing.common.directory(),
-
-    duration: {
-      flags: {update: true, expose: true},
-      update: {validate: isDuration},
-    },
-
-    urls: Thing.common.urls(),
-    dateFirstReleased: Thing.common.simpleDate(),
-
-    artistContribsByRef: Thing.common.contribsByRef(),
-    contributorContribsByRef: Thing.common.contribsByRef(),
-    coverArtistContribsByRef: Thing.common.contribsByRef(),
-
-    referencedTracksByRef: Thing.common.referenceList(Track),
-    sampledTracksByRef: Thing.common.referenceList(Track),
-    artTagsByRef: Thing.common.referenceList(ArtTag),
-
-    hasCoverArt: {
-      flags: {update: true, expose: true},
-
-      update: {
-        validate(value) {
-          if (value !== false) {
-            throw new TypeError(`Expected false or null`);
-          }
-
-          return true;
-        },
-      },
-
-      expose: {
-        dependencies: ['albumData', 'coverArtistContribsByRef'],
-        transform: (hasCoverArt, {
-          albumData,
-          coverArtistContribsByRef,
-          [Track.instance]: track,
-        }) =>
-          Track.hasCoverArt(
-            track,
-            albumData,
-            coverArtistContribsByRef,
-            hasCoverArt
-          ),
-      },
-    },
-
-    coverArtFileExtension: {
-      flags: {update: true, expose: true},
-
-      update: {validate: isFileExtension},
-
-      expose: {
-        dependencies: ['albumData', 'coverArtistContribsByRef'],
-        transform: (coverArtFileExtension, {
-          albumData,
-          coverArtistContribsByRef,
-          hasCoverArt,
-          [Track.instance]: track,
-        }) =>
-          coverArtFileExtension ??
-          (Track.hasCoverArt(
-            track,
-            albumData,
-            coverArtistContribsByRef,
-            hasCoverArt
-          )
-            ? Track.findAlbum(track, albumData)?.trackCoverArtFileExtension
-            : Track.findAlbum(track, albumData)?.coverArtFileExtension) ??
-          'jpg',
-      },
-    },
-
-    originalReleaseTrackByRef: Thing.common.singleReference(Track),
-
-    dataSourceAlbumByRef: Thing.common.singleReference(Album),
-
-    commentary: Thing.common.commentary(),
-    lyrics: Thing.common.simpleString(),
-    additionalFiles: Thing.common.additionalFiles(),
-    sheetMusicFiles: Thing.common.additionalFiles(),
-    midiProjectFiles: Thing.common.additionalFiles(),
+    name: name('Unnamed Track'),
+    directory: directory(),
+
+    duration: duration(),
+    urls: urls(),
+    dateFirstReleased: simpleDate(),
+
+    color: [
+      exposeUpdateValueOrContinue({
+        validate: input.value(isColor),
+      }),
+
+      withContainingTrackSection(),
+
+      withPropertyFromObject({
+        object: '#trackSection',
+        property: input.value('color'),
+      }),
+
+      exposeDependencyOrContinue({dependency: '#trackSection.color'}),
+
+      withPropertyFromAlbum({
+        property: input.value('color'),
+      }),
+
+      exposeDependency({dependency: '#album.color'}),
+    ],
+
+    alwaysReferenceByDirectory: [
+      withAlwaysReferenceByDirectory(),
+      exposeDependency({dependency: '#alwaysReferenceByDirectory'}),
+    ],
+
+    // Disables presenting the track as though it has its own unique artwork.
+    // This flag should only be used in select circumstances, i.e. to override
+    // an album's trackCoverArtists. This flag supercedes that property, as well
+    // as the track's own coverArtists.
+    disableUniqueCoverArt: flag(),
+
+    // File extension for track's corresponding media file. This represents the
+    // track's unique cover artwork, if any, and does not inherit the extension
+    // of the album's main artwork. It does inherit trackCoverArtFileExtension,
+    // if present on the album.
+    coverArtFileExtension: [
+      exitWithoutUniqueCoverArt(),
+
+      exposeUpdateValueOrContinue({
+        validate: input.value(isFileExtension),
+      }),
+
+      withPropertyFromAlbum({
+        property: input.value('trackCoverArtFileExtension'),
+      }),
+
+      exposeDependencyOrContinue({dependency: '#album.trackCoverArtFileExtension'}),
+
+      exposeConstant({
+        value: input.value('jpg'),
+      }),
+    ],
+
+    // Date of cover art release. Like coverArtFileExtension, this represents
+    // only the track's own unique cover artwork, if any. This exposes only as
+    // the track's own coverArtDate or its album's trackArtDate, so if neither
+    // is specified, this value is null.
+    coverArtDate: [
+      withHasUniqueCoverArt(),
+
+      exitWithoutDependency({
+        dependency: '#hasUniqueCoverArt',
+        mode: input.value('falsy'),
+      }),
+
+      exposeUpdateValueOrContinue({
+        validate: input.value(isDate),
+      }),
+
+      withPropertyFromAlbum({
+        property: input.value('trackArtDate'),
+      }),
+
+      exposeDependency({dependency: '#album.trackArtDate'}),
+    ],
+
+    commentary: commentary(),
+    lyrics: simpleString(),
+
+    additionalFiles: additionalFiles(),
+    sheetMusicFiles: additionalFiles(),
+    midiProjectFiles: additionalFiles(),
+
+    originalReleaseTrack: singleReference({
+      class: input.value(Track),
+      find: input.value(find.track),
+      data: 'trackData',
+    }),
+
+    // Internal use only - for directly identifying an album inside a track's
+    // util.inspect display, if it isn't indirectly available (by way of being
+    // included in an album's track list).
+    dataSourceAlbum: singleReference({
+      class: input.value(Album),
+      find: input.value(find.album),
+      data: 'albumData',
+    }),
+
+    artistContribs: [
+      inheritFromOriginalRelease({
+        property: input.value('artistContribs'),
+      }),
+
+      withResolvedContribs({
+        from: input.updateValue({validate: isContributionList}),
+      }).outputs({
+        '#resolvedContribs': '#artistContribs',
+      }),
+
+      exposeDependencyOrContinue({
+        dependency: '#artistContribs',
+        mode: input.value('empty'),
+      }),
+
+      withPropertyFromAlbum({
+        property: input.value('artistContribs'),
+      }),
+
+      exposeDependency({dependency: '#album.artistContribs'}),
+    ],
+
+    contributorContribs: [
+      inheritFromOriginalRelease({
+        property: input.value('contributorContribs'),
+      }),
+
+      contributionList(),
+    ],
+
+    // Cover artists aren't inherited from the original release, since it
+    // typically varies by release and isn't defined by the musical qualities
+    // of the track.
+    coverArtistContribs: [
+      exitWithoutUniqueCoverArt({
+        value: input.value([]),
+      }),
+
+      withResolvedContribs({
+        from: input.updateValue({validate: isContributionList}),
+      }).outputs({
+        '#resolvedContribs': '#coverArtistContribs',
+      }),
+
+      exposeDependencyOrContinue({
+        dependency: '#coverArtistContribs',
+        mode: input.value('empty'),
+      }),
+
+      withPropertyFromAlbum({
+        property: input.value('trackCoverArtistContribs'),
+      }),
+
+      exposeDependency({dependency: '#album.trackCoverArtistContribs'}),
+    ],
+
+    referencedTracks: [
+      inheritFromOriginalRelease({
+        property: input.value('referencedTracks'),
+      }),
+
+      referenceList({
+        class: input.value(Track),
+        find: input.value(find.track),
+        data: 'trackData',
+      }),
+    ],
+
+    sampledTracks: [
+      inheritFromOriginalRelease({
+        property: input.value('sampledTracks'),
+      }),
+
+      referenceList({
+        class: input.value(Track),
+        find: input.value(find.track),
+        data: 'trackData',
+      }),
+    ],
+
+    artTags: referenceList({
+      class: input.value(ArtTag),
+      find: input.value(find.artTag),
+      data: 'artTagData',
+    }),
 
     // Update only
 
-    albumData: Thing.common.wikiData(Album),
-    artistData: Thing.common.wikiData(Artist),
-    artTagData: Thing.common.wikiData(ArtTag),
-    flashData: Thing.common.wikiData(Flash),
-    trackData: Thing.common.wikiData(Track),
+    albumData: wikiData(Album),
+    artistData: wikiData(Artist),
+    artTagData: wikiData(ArtTag),
+    flashData: wikiData(Flash),
+    trackData: wikiData(Track),
 
     // Expose only
 
-    commentatorArtists: Thing.common.commentatorArtists(),
-
-    album: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['albumData'],
-        compute: ({[Track.instance]: track, albumData}) =>
-          albumData?.find((album) => album.tracks.includes(track)) ?? null,
-      },
-    },
-
-    // Note - this is an internal property used only to help identify a track.
-    // It should not be assumed in general that the album and dataSourceAlbum match
-    // (i.e. a track may dynamically be moved from one album to another, at
-    // which point dataSourceAlbum refers to where it was originally from, and is
-    // not generally relevant information). It's also not guaranteed that
-    // dataSourceAlbum is available (depending on the Track creator to optionally
-    // provide dataSourceAlbumByRef).
-    dataSourceAlbum: Thing.common.dynamicThingFromSingleReference(
-      'dataSourceAlbumByRef',
-      'albumData',
-      find.album
-    ),
-
-    date: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['albumData', 'dateFirstReleased'],
-        compute: ({albumData, dateFirstReleased, [Track.instance]: track}) =>
-          dateFirstReleased ?? Track.findAlbum(track, albumData)?.date ?? null,
-      },
-    },
-
-    color: {
-      flags: {update: true, expose: true},
-
-      update: {validate: isColor},
-
-      expose: {
-        dependencies: ['albumData'],
-
-        transform: (color, {albumData, [Track.instance]: track}) =>
-          color ??
-            Track.findAlbum(track, albumData)
-              ?.trackSections.find(({tracks}) => tracks.includes(track))
-                ?.color ?? null,
-      },
-    },
-
-    coverArtDate: {
-      flags: {update: true, expose: true},
-
-      update: {validate: isDate},
-
-      expose: {
-        dependencies: [
-          'albumData',
-          'coverArtistContribsByRef',
-          'dateFirstReleased',
-          'hasCoverArt',
-        ],
-        transform: (coverArtDate, {
-          albumData,
-          coverArtistContribsByRef,
-          dateFirstReleased,
-          hasCoverArt,
-          [Track.instance]: track,
-        }) =>
-          (Track.hasCoverArt(track, albumData, coverArtistContribsByRef, hasCoverArt)
-            ? coverArtDate ??
-              dateFirstReleased ??
-              Track.findAlbum(track, albumData)?.trackArtDate ??
-              Track.findAlbum(track, albumData)?.date ??
-              null
-            : null),
-      },
-    },
-
-    hasUniqueCoverArt: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['albumData', 'coverArtistContribsByRef', 'hasCoverArt'],
-        compute: ({
-          albumData,
-          coverArtistContribsByRef,
-          hasCoverArt,
-          [Track.instance]: track,
-        }) =>
-          Track.hasUniqueCoverArt(
-            track,
-            albumData,
-            coverArtistContribsByRef,
-            hasCoverArt
-          ),
-      },
-    },
-
-    originalReleaseTrack: Thing.common.dynamicThingFromSingleReference(
-      'originalReleaseTrackByRef',
-      'trackData',
-      find.track
-    ),
-
-    otherReleases: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['originalReleaseTrackByRef', 'trackData'],
-
-        compute: ({
-          originalReleaseTrackByRef: t1origRef,
-          trackData,
-          [Track.instance]: t1,
-        }) => {
-          if (!trackData) {
-            return [];
-          }
-
-          const t1orig = find.track(t1origRef, trackData);
-
-          return [
-            t1orig,
-            ...trackData.filter((t2) => {
-              const {originalReleaseTrack: t2orig} = t2;
-              return t2 !== t1 && t2orig && (t2orig === t1orig || t2orig === t1);
-            }),
-          ].filter(Boolean);
-        },
-      },
-    },
-
-    artistContribs:
-      Track.inheritFromOriginalRelease('artistContribs', [],
-        Thing.common.dynamicInheritContribs(
-          null,
-          'artistContribsByRef',
-          'artistContribsByRef',
-          'albumData',
-          Track.findAlbum)),
-
-    contributorContribs:
-      Track.inheritFromOriginalRelease('contributorContribs', [],
-        Thing.common.dynamicContribs('contributorContribsByRef')),
-
-    // Cover artists aren't inherited from the original release, since it
-    // typically varies by release and isn't defined by the musical qualities
-    // of the track.
-    coverArtistContribs:
-      Thing.common.dynamicInheritContribs(
-        'hasCoverArt',
-        'coverArtistContribsByRef',
-        'trackCoverArtistContribsByRef',
-        'albumData',
-        Track.findAlbum),
-
-    referencedTracks:
-      Track.inheritFromOriginalRelease('referencedTracks', [],
-        Thing.common.dynamicThingsFromReferenceList(
-          'referencedTracksByRef',
-          'trackData',
-          find.track)),
-
-    sampledTracks:
-      Track.inheritFromOriginalRelease('sampledTracks', [],
-        Thing.common.dynamicThingsFromReferenceList(
-          'sampledTracksByRef',
-          'trackData',
-          find.track)),
-
-    // Specifically exclude re-releases from this list - while it's useful to
-    // get from a re-release to the tracks it references, re-releases aren't
-    // generally relevant from the perspective of the tracks being referenced.
-    // Filtering them from data here hides them from the corresponding field
-    // on the site (obviously), and has the bonus of not counting them when
-    // counting the number of times a track has been referenced, for use in
-    // the "Tracks - by Times Referenced" listing page (or other data
-    // processing).
-    referencedByTracks: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['trackData'],
-
-        compute: ({trackData, [Track.instance]: track}) =>
-          trackData
-            ? trackData
-                .filter((t) => !t.originalReleaseTrack)
-                .filter((t) => t.referencedTracks?.includes(track))
-            : [],
-      },
-    },
-
-    // For the same reasoning, exclude re-releases from sampled tracks too.
-    sampledByTracks: {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: ['trackData'],
-
-        compute: ({trackData, [Track.instance]: track}) =>
-          trackData
-            ? trackData
-                .filter((t) => !t.originalReleaseTrack)
-                .filter((t) => t.sampledTracks?.includes(track))
-            : [],
-      },
-    },
-
-    featuredInFlashes: Thing.common.reverseReferenceList(
-      'flashData',
-      'featuredTracks'
-    ),
-
-    artTags: Thing.common.dynamicThingsFromReferenceList(
-      'artTagsByRef',
-      'artTagData',
-      find.artTag
-    ),
-  });
-
-  // This is a quick utility function for now, since the same code is reused in
-  // several places. Ideally it wouldn't be - we'd just reuse the `album`
-  // property - but support for that hasn't been coded yet :P
-  static findAlbum = (track, albumData) =>
-    albumData?.find((album) => album.tracks.includes(track));
-
-  // Another reused utility function. This one's logic is a bit more complicated.
-  static hasCoverArt(
-    track,
-    albumData,
-    coverArtistContribsByRef,
-    hasCoverArt
-  ) {
-    if (!empty(coverArtistContribsByRef)) {
-      return true;
-    }
+    commentatorArtists: commentatorArtists(),
 
-    const album = Track.findAlbum(track, albumData);
-    if (album && !empty(album.trackCoverArtistContribsByRef)) {
-      return true;
-    }
+    album: [
+      withAlbum(),
+      exposeDependency({dependency: '#album'}),
+    ],
 
-    return false;
-  }
+    date: [
+      exposeDependencyOrContinue({dependency: 'dateFirstReleased'}),
 
-  static hasUniqueCoverArt(
-    track,
-    albumData,
-    coverArtistContribsByRef,
-    hasCoverArt
-  ) {
-    if (!empty(coverArtistContribsByRef)) {
-      return true;
-    }
+      withPropertyFromAlbum({
+        property: input.value('date'),
+      }),
 
-    if (hasCoverArt === false) {
-      return false;
-    }
+      exposeDependency({dependency: '#album.date'}),
+    ],
 
-    const album = Track.findAlbum(track, albumData);
-    if (album && !empty(album.trackCoverArtistContribsByRef)) {
-      return true;
-    }
+    hasUniqueCoverArt: [
+      withHasUniqueCoverArt(),
+      exposeDependency({dependency: '#hasUniqueCoverArt'}),
+    ],
 
-    return false;
-  }
+    otherReleases: [
+      withOtherReleases(),
+      exposeDependency({dependency: '#otherReleases'}),
+    ],
 
-  static inheritFromOriginalRelease(
-    originalProperty,
-    originalMissingValue,
-    ownPropertyDescriptor
-  ) {
-    return {
-      flags: {expose: true},
-
-      expose: {
-        dependencies: [
-          ...ownPropertyDescriptor.expose.dependencies,
-          'originalReleaseTrackByRef',
-          'trackData',
-        ],
-
-        compute(dependencies) {
-          const {
-            originalReleaseTrackByRef,
-            trackData,
-          } = dependencies;
-
-          if (originalReleaseTrackByRef) {
-            if (!trackData) return originalMissingValue;
-            const original = find.track(originalReleaseTrackByRef, trackData, {mode: 'quiet'});
-            if (!original) return originalMissingValue;
-            return original[originalProperty];
-          }
-
-          return ownPropertyDescriptor.expose.compute(dependencies);
-        },
-      },
-    };
-  }
+    referencedByTracks: trackReverseReferenceList({
+      list: input.value('referencedTracks'),
+    }),
 
-  [inspect.custom]() {
-    const base = Thing.prototype[inspect.custom].apply(this);
+    sampledByTracks: trackReverseReferenceList({
+      list: input.value('sampledTracks'),
+    }),
 
-    const rereleasePart =
-      (this.originalReleaseTrackByRef
-        ? `${color.yellow('[rerelease]')} `
-        : ``);
+    featuredInFlashes: reverseReferenceList({
+      data: 'flashData',
+      list: input.value('featuredTracks'),
+    }),
+  });
 
-    const {album, dataSourceAlbum} = this;
+  [inspect.custom](depth) {
+    const parts = [];
 
-    const albumName =
-      (album
-        ? album.name
-        : dataSourceAlbum?.name);
+    parts.push(Thing.prototype[inspect.custom].apply(this));
 
-    const albumIndex =
-      albumName &&
-        (album
-          ? album.tracks.indexOf(this)
-          : dataSourceAlbum.tracks.indexOf(this));
+    if (CacheableObject.getUpdateValue(this, 'originalReleaseTrack')) {
+      parts.unshift(`${colors.yellow('[rerelease]')} `);
+    }
 
-    const trackNum =
-      albumName &&
+    let album;
+    if (depth >= 0 && (album = this.album ?? this.dataSourceAlbum)) {
+      const albumName = album.name;
+      const albumIndex = album.tracks.indexOf(this);
+      const trackNum =
         (albumIndex === -1
           ? '#?'
           : `#${albumIndex + 1}`);
+      parts.push(` (${colors.yellow(trackNum)} in ${colors.green(albumName)})`);
+    }
 
-    const albumPart =
-      albumName
-        ? ` (${color.yellow(trackNum)} in ${color.green(albumName)})`
-        : ``;
-
-    return rereleasePart + base + albumPart;
+    return parts.join('');
   }
 }
diff --git a/src/data/things/validators.js b/src/data/things/validators.js
index fc953c2a..ee301f15 100644
--- a/src/data/things/validators.js
+++ b/src/data/things/validators.js
@@ -1,7 +1,7 @@
 import {inspect as nodeInspect} from 'node:util';
 
-import {color, ENABLE_COLOR} from '#cli';
-import {withAggregate} from '#sugar';
+import {colors, ENABLE_COLOR} from '#cli';
+import {empty, typeAppearance, withAggregate} from '#sugar';
 
 function inspect(value) {
   return nodeInspect(value, {colors: ENABLE_COLOR});
@@ -9,13 +9,13 @@ function inspect(value) {
 
 // Basic types (primitives)
 
-function a(noun) {
+export function a(noun) {
   return /[aeiou]/.test(noun[0]) ? `an ${noun}` : `a ${noun}`;
 }
 
-function isType(value, type) {
+export function isType(value, type) {
   if (typeof value !== type)
-    throw new TypeError(`Expected ${a(type)}, got ${typeof value}`);
+    throw new TypeError(`Expected ${a(type)}, got ${typeAppearance(value)}`);
 
   return true;
 }
@@ -132,7 +132,7 @@ export function isObject(value) {
 
 export function isArray(value) {
   if (typeof value !== 'object' || value === null || !Array.isArray(value))
-    throw new TypeError(`Expected an array, got ${value}`);
+    throw new TypeError(`Expected an array, got ${typeAppearance(value)}`);
 
   return true;
 }
@@ -174,7 +174,8 @@ function validateArrayItemsHelper(itemValidator) {
         throw new Error(`Expected validator to return true`);
       }
     } catch (error) {
-      error.message = `(index: ${color.green(index)}, item: ${inspect(item)}) ${error.message}`;
+      error.message = `(index: ${colors.yellow(`${index}`)}, item: ${inspect(item)}) ${error.message}`;
+      error[Symbol.for('hsmusic.decorate.indexInSourceArray')] = index;
       throw error;
     }
   };
@@ -264,7 +265,7 @@ export function validateProperties(spec) {
           try {
             specValidator(value);
           } catch (error) {
-            error.message = `(key: ${color.green(specKey)}, value: ${inspect(value)}) ${error.message}`;
+            error.message = `(key: ${colors.green(specKey)}, value: ${inspect(value)}) ${error.message}`;
             throw error;
           }
         });
@@ -308,7 +309,7 @@ export const isTrackSection = validateProperties({
   color: optional(isColor),
   dateOriginallyReleased: optional(isDate),
   isDefaultTrackSection: optional(isBoolean),
-  tracksByRef: optional(validateReferenceList('track')),
+  tracks: optional(validateReferenceList('track')),
 });
 
 export const isTrackSectionList = validateArrayItems(isTrackSection);
@@ -404,6 +405,76 @@ export function validateReferenceList(type = '') {
   return validateArrayItems(validateReference(type));
 }
 
+const validateWikiData_cache = {};
+
+export function validateWikiData({
+  referenceType = '',
+  allowMixedTypes = false,
+}) {
+  if (referenceType && allowMixedTypes) {
+    throw new TypeError(`Don't specify both referenceType and allowMixedTypes`);
+  }
+
+  validateWikiData_cache[referenceType] ??= {};
+  validateWikiData_cache[referenceType][allowMixedTypes] ??= new WeakMap();
+
+  const isArrayOfObjects = validateArrayItems(isObject);
+
+  return (array) => {
+    const subcache = validateWikiData_cache[referenceType][allowMixedTypes];
+    if (subcache.has(array)) return subcache.get(array);
+
+    let OK = false;
+
+    try {
+      isArrayOfObjects(array);
+
+      if (empty(array)) {
+        OK = true; return true;
+      }
+
+      const allRefTypes =
+        new Set(array.map(object =>
+          object.constructor[Symbol.for('Thing.referenceType')]));
+
+      if (allRefTypes.has(undefined)) {
+        if (allRefTypes.size === 1) {
+          throw new TypeError(`Expected array of wiki data objects, got array of other objects`);
+        } else {
+          throw new TypeError(`Expected array of wiki data objects, got mixed items`);
+        }
+      }
+
+      if (allRefTypes.size > 1) {
+        if (allowMixedTypes) {
+          OK = true; return true;
+        }
+
+        const types = () => Array.from(allRefTypes).join(', ');
+
+        if (referenceType) {
+          if (allRefTypes.has(referenceType)) {
+            allRefTypes.remove(referenceType);
+            throw new TypeError(`Expected array of only ${referenceType}, also got other types: ${types()}`)
+          } else {
+            throw new TypeError(`Expected array of only ${referenceType}, got other types: ${types()}`);
+          }
+        }
+
+        throw new TypeError(`Expected array of unmixed reference types, got multiple: ${types()}`);
+      }
+
+      if (referenceType && !allRefTypes.has(referenceType)) {
+        throw new TypeError(`Expected array of ${referenceType}, got array of ${allRefTypes[0]}`)
+      }
+
+      OK = true; return true;
+    } finally {
+      subcache.set(array, OK);
+    }
+  };
+}
+
 // Compositional utilities
 
 export function oneOf(...checks) {
diff --git a/src/data/things/wiki-info.js b/src/data/things/wiki-info.js
index e906cab1..6286a267 100644
--- a/src/data/things/wiki-info.js
+++ b/src/data/things/wiki-info.js
@@ -1,20 +1,25 @@
+import {input} from '#composite';
 import find from '#find';
+import {isLanguageCode, isName, isURL} from '#validators';
+
+import {
+  color,
+  flag,
+  name,
+  referenceList,
+  simpleString,
+  wikiData,
+} from '#composite/wiki-properties';
 
 import Thing from './thing.js';
 
 export class WikiInfo extends Thing {
-  static [Thing.getPropertyDescriptors] = ({
-    Group,
+  static [Thing.friendlyName] = `Wiki Info`;
 
-    validators: {
-      isLanguageCode,
-      isName,
-      isURL,
-    },
-  }) => ({
+  static [Thing.getPropertyDescriptors] = ({Group}) => ({
     // Update & expose
 
-    name: Thing.common.name('Unnamed Wiki'),
+    name: name('Unnamed Wiki'),
 
     // Displayed in nav bar.
     nameShort: {
@@ -27,12 +32,12 @@ export class WikiInfo extends Thing {
       },
     },
 
-    color: Thing.common.color(),
+    color: color(),
 
     // One-line description used for <meta rel="description"> tag.
-    description: Thing.common.simpleString(),
+    description: simpleString(),
 
-    footerContent: Thing.common.simpleString(),
+    footerContent: simpleString(),
 
     defaultLanguage: {
       flags: {update: true, expose: true},
@@ -44,25 +49,21 @@ export class WikiInfo extends Thing {
       update: {validate: isURL},
     },
 
-    divideTrackListsByGroupsByRef: Thing.common.referenceList(Group),
+    divideTrackListsByGroups: referenceList({
+      class: input.value(Group),
+      find: input.value(find.group),
+      data: 'groupData',
+    }),
 
     // Feature toggles
-    enableFlashesAndGames: Thing.common.flag(false),
-    enableListings: Thing.common.flag(false),
-    enableNews: Thing.common.flag(false),
-    enableArtTagUI: Thing.common.flag(false),
-    enableGroupUI: Thing.common.flag(false),
+    enableFlashesAndGames: flag(false),
+    enableListings: flag(false),
+    enableNews: flag(false),
+    enableArtTagUI: flag(false),
+    enableGroupUI: flag(false),
 
     // Update only
 
-    groupData: Thing.common.wikiData(Group),
-
-    // Expose only
-
-    divideTrackListsByGroups: Thing.common.dynamicThingsFromReferenceList(
-      'divideTrackListsByGroupsByRef',
-      'groupData',
-      find.group
-    ),
+    groupData: wikiData(Group),
   });
 }
diff --git a/src/data/yaml.js b/src/data/yaml.js
index 35943199..f7856cb7 100644
--- a/src/data/yaml.js
+++ b/src/data/yaml.js
@@ -7,10 +7,15 @@ import {inspect as nodeInspect} from 'node:util';
 
 import yaml from 'js-yaml';
 
-import {color, ENABLE_COLOR, logInfo, logWarn} from '#cli';
+import {colors, ENABLE_COLOR, logInfo, logWarn} from '#cli';
 import find, {bindFind} from '#find';
 import {traverse} from '#node-utils';
-import T from '#things';
+
+import T, {
+  CacheableObject,
+  CacheableObjectPropertyValueError,
+  Thing,
+} from '#things';
 
 import {
   conditionallySuppressError,
@@ -59,7 +64,7 @@ export const DATA_STATIC_PAGE_DIRECTORY = 'static-page';
 // document and apply the configuration passed to makeProcessDocument in order
 // to construct a Thing subclass.
 function makeProcessDocument(
-  thingClass,
+  thingConstructor,
   {
     // Optional early step for transforming field values before providing them
     // to the Thing's update() method. This is useful when the input format
@@ -110,7 +115,7 @@ function makeProcessDocument(
     invalidFieldCombinations = [],
   }
 ) {
-  if (!thingClass) {
+  if (!thingConstructor) {
     throw new Error(`Missing Thing class`);
   }
 
@@ -137,22 +142,47 @@ function makeProcessDocument(
         const name = document[nameField];
         error.message = name
           ? `(name: ${inspect(name)}) ${error.message}`
-          : `(${color.dim(`no name found`)}) ${error.message}`;
+          : `(${colors.dim(`no name found`)}) ${error.message}`;
         throw error;
       }
     };
   };
 
   const fn = decorateErrorWithName((document) => {
+    const nameField = propertyFieldMapping['name'];
+    const namePart =
+      (nameField
+        ? (document[nameField]
+          ? ` named ${colors.green(`"${document[nameField]}"`)}`
+          : ` (name field, "${nameField}", not specified)`)
+        : ``);
+
+    const constructorPart =
+      (thingConstructor[Thing.friendlyName]
+        ? colors.green(thingConstructor[Thing.friendlyName])
+     : thingConstructor.name
+        ? colors.green(thingConstructor.name)
+        : `document`);
+
+    const aggregate = openAggregate({
+      message: `Errors processing ${constructorPart}` + namePart,
+    });
+
     const documentEntries = Object.entries(document)
       .filter(([field]) => !ignoredFields.includes(field));
 
+    const skippedFields = new Set();
+
     const unknownFields = documentEntries
       .map(([field]) => field)
       .filter((field) => !knownFields.includes(field));
 
     if (!empty(unknownFields)) {
-      throw new makeProcessDocument.UnknownFieldsError(unknownFields);
+      aggregate.push(new UnknownFieldsError(unknownFields));
+
+      for (const field of unknownFields) {
+        skippedFields.add(field);
+      }
     }
 
     const presentFields = Object.keys(document);
@@ -162,28 +192,57 @@ function makeProcessDocument(
     for (const {message, fields} of invalidFieldCombinations) {
       const fieldsPresent = presentFields.filter(field => fields.includes(field));
 
-      if (fieldsPresent.length <= 1) {
-        continue;
-      }
+      if (fieldsPresent.length >= 2) {
+        const filteredDocument =
+          filterProperties(
+            document,
+            fieldsPresent,
+            {preserveOriginalOrder: true});
 
-      fieldCombinationErrors.push(
-        new makeProcessDocument.FieldCombinationError(
-          filterProperties(document, fieldsPresent),
-          message));
+        fieldCombinationErrors.push(new FieldCombinationError(filteredDocument, message));
+
+        for (const field of Object.keys(filteredDocument)) {
+          skippedFields.add(field);
+        }
+      }
     }
 
     if (!empty(fieldCombinationErrors)) {
-      throw new makeProcessDocument.FieldCombinationsError(fieldCombinationErrors);
+      aggregate.push(new FieldCombinationAggregateError(fieldCombinationErrors));
     }
 
     const fieldValues = {};
 
-    for (const [field, value] of documentEntries) {
-      if (Object.hasOwn(fieldTransformations, field)) {
-        fieldValues[field] = fieldTransformations[field](value);
-      } else {
-        fieldValues[field] = value;
+    for (const [field, documentValue] of documentEntries) {
+      if (skippedFields.has(field)) continue;
+
+      // This variable would like to certify itself as "not into capitalism".
+      let propertyValue =
+        (Object.hasOwn(fieldTransformations, field)
+          ? fieldTransformations[field](documentValue)
+          : documentValue);
+
+      // Completely blank items in a YAML list are read as null.
+      // They're handy to have around when filling out a document and shouldn't
+      // be considered an error (or data at all).
+      if (Array.isArray(propertyValue)) {
+        const wasEmpty = empty(propertyValue);
+
+        propertyValue =
+          propertyValue.filter(item => item !== null);
+
+        const isEmpty = empty(propertyValue);
+
+        // Don't set arrays which are empty as a result of the above filter.
+        // Arrays which were originally empty, i.e. `Field: []`, are still
+        // valid data, but if it's just an array not containing any filled out
+        // items, it should be treated as a placeholder and skipped over.
+        if (isEmpty && !wasEmpty) {
+          propertyValue = null;
+        }
       }
+
+      fieldValues[field] = propertyValue;
     }
 
     const sourceProperties = {};
@@ -193,15 +252,34 @@ function makeProcessDocument(
       sourceProperties[property] = value;
     }
 
-    const thing = Reflect.construct(thingClass, []);
+    const thing = Reflect.construct(thingConstructor, []);
 
-    withAggregate({message: `Errors applying ${color.green(thingClass.name)} properties`}, ({call}) => {
-      for (const [property, value] of Object.entries(sourceProperties)) {
-        call(() => (thing[property] = value));
+    const fieldValueErrors = [];
+
+    for (const [property, value] of Object.entries(sourceProperties)) {
+      const field = propertyFieldMapping[property];
+      try {
+        thing[property] = value;
+      } catch (caughtError) {
+        skippedFields.add(field);
+        fieldValueErrors.push(new FieldValueError(field, property, value, caughtError));
       }
-    });
+    }
+
+    if (!empty(fieldValueErrors)) {
+      aggregate.push(new FieldValueAggregateError(thingConstructor, fieldValueErrors));
+    }
 
-    return thing;
+    if (skippedFields.size >= 1) {
+      aggregate.push(
+        new SkippedFieldsSummaryError(
+          filterProperties(
+            document,
+            Array.from(skippedFields),
+            {preserveOriginalOrder: true})));
+    }
+
+    return {thing, aggregate};
   });
 
   Object.assign(fn, {
@@ -212,36 +290,81 @@ function makeProcessDocument(
   return fn;
 }
 
-makeProcessDocument.UnknownFieldsError = class UnknownFieldsError extends Error {
+export class UnknownFieldsError extends Error {
   constructor(fields) {
-    super(`Unknown fields present: ${fields.join(', ')}`);
+    super(`Unknown fields ignored: ${fields.map(field => colors.red(field)).join(', ')}`);
     this.fields = fields;
   }
-};
+}
 
-makeProcessDocument.FieldCombinationsError = class FieldCombinationsError extends AggregateError {
+export class FieldCombinationAggregateError extends AggregateError {
   constructor(errors) {
-    super(errors, `Errors in combinations of fields present`);
+    super(errors, `Invalid field combinations - all involved fields ignored`);
   }
-};
+}
 
-makeProcessDocument.FieldCombinationError = class FieldCombinationError extends Error {
+export class FieldCombinationError extends Error {
   constructor(fields, message) {
     const fieldNames = Object.keys(fields);
-    const combinePart = `Don't combine ${fieldNames.map(field => color.red(field)).join(', ')}`;
 
-    const messagePart =
+    const mainMessage = `Don't combine ${fieldNames.map(field => colors.red(field)).join(', ')}`;
+
+    const causeMessage =
       (typeof message === 'function'
-        ? `: ${message(fields)}`
+        ? message(fields)
      : typeof message === 'string'
-        ? `: ${message}`
-        : ``);
+        ? message
+        : null);
+
+    super(mainMessage, {
+      cause:
+        (causeMessage
+          ? new Error(causeMessage)
+          : null),
+    });
 
-    super(combinePart + messagePart);
     this.fields = fields;
   }
 }
 
+export class FieldValueAggregateError extends AggregateError {
+  constructor(thingConstructor, errors) {
+    super(errors, `Errors processing field values for ${colors.green(thingConstructor.name)}`);
+  }
+}
+
+export class FieldValueError extends Error {
+  constructor(field, property, value, caughtError) {
+    const cause =
+      (caughtError instanceof CacheableObjectPropertyValueError
+        ? caughtError.cause
+        : caughtError);
+
+    super(
+      `Failed to set ${colors.green(`"${field}"`)} field (${colors.green(property)}) to ${inspect(value)}`,
+      {cause});
+  }
+}
+
+export class SkippedFieldsSummaryError extends Error {
+  constructor(filteredDocument) {
+    const entries = Object.entries(filteredDocument);
+
+    const lines =
+      entries.map(([field, value]) =>
+        ` - ${field}: ` +
+        inspect(value)
+          .split('\n')
+          .map((line, index) => index === 0 ? line : `   ${line}`)
+          .join('\n'));
+
+    super(
+      colors.bright(colors.yellow(`Altogether, skipped ${entries.length === 1 ? `1 field` : `${entries.length} fields`}:\n`)) +
+      lines.join('\n') + '\n' +
+      colors.bright(colors.yellow(`See above errors for details.`)));
+  }
+}
+
 export const processAlbumDocument = makeProcessDocument(T.Album, {
   fieldTransformations: {
     'Artists': parseContributors,
@@ -278,11 +401,11 @@ export const processAlbumDocument = makeProcessDocument(T.Album, {
     coverArtFileExtension: 'Cover Art File Extension',
     trackCoverArtFileExtension: 'Track Art File Extension',
 
-    wallpaperArtistContribsByRef: 'Wallpaper Artists',
+    wallpaperArtistContribs: 'Wallpaper Artists',
     wallpaperStyle: 'Wallpaper Style',
     wallpaperFileExtension: 'Wallpaper File Extension',
 
-    bannerArtistContribsByRef: 'Banner Artists',
+    bannerArtistContribs: 'Banner Artists',
     bannerStyle: 'Banner Style',
     bannerFileExtension: 'Banner File Extension',
     bannerDimensions: 'Banner Dimensions',
@@ -290,11 +413,11 @@ export const processAlbumDocument = makeProcessDocument(T.Album, {
     commentary: 'Commentary',
     additionalFiles: 'Additional Files',
 
-    artistContribsByRef: 'Artists',
-    coverArtistContribsByRef: 'Cover Artists',
-    trackCoverArtistContribsByRef: 'Default Track Cover Artists',
-    groupsByRef: 'Groups',
-    artTagsByRef: 'Art Tags',
+    artistContribs: 'Artists',
+    coverArtistContribs: 'Cover Artists',
+    trackCoverArtistContribs: 'Default Track Cover Artists',
+    groups: 'Groups',
+    artTags: 'Art Tags',
   },
 });
 
@@ -316,6 +439,10 @@ export const processTrackDocument = makeProcessDocument(T.Track, {
 
     'Date First Released': (value) => new Date(value),
     'Cover Art Date': (value) => new Date(value),
+    'Has Cover Art': (value) =>
+      (value === true ? false :
+       value === false ? true :
+       value),
 
     'Artists': parseContributors,
     'Contributors': parseContributors,
@@ -336,7 +463,9 @@ export const processTrackDocument = makeProcessDocument(T.Track, {
     dateFirstReleased: 'Date First Released',
     coverArtDate: 'Cover Art Date',
     coverArtFileExtension: 'Cover Art File Extension',
-    hasCoverArt: 'Has Cover Art',
+    disableUniqueCoverArt: 'Has Cover Art', // This gets transformed to flip true/false.
+
+    alwaysReferenceByDirectory: 'Always Reference By Directory',
 
     lyrics: 'Lyrics',
     commentary: 'Commentary',
@@ -344,13 +473,13 @@ export const processTrackDocument = makeProcessDocument(T.Track, {
     sheetMusicFiles: 'Sheet Music Files',
     midiProjectFiles: 'MIDI Project Files',
 
-    originalReleaseTrackByRef: 'Originally Released As',
-    referencedTracksByRef: 'Referenced Tracks',
-    sampledTracksByRef: 'Sampled Tracks',
-    artistContribsByRef: 'Artists',
-    contributorContribsByRef: 'Contributors',
-    coverArtistContribsByRef: 'Cover Artists',
-    artTagsByRef: 'Art Tags',
+    originalReleaseTrack: 'Originally Released As',
+    referencedTracks: 'Referenced Tracks',
+    sampledTracks: 'Sampled Tracks',
+    artistContribs: 'Artists',
+    contributorContribs: 'Contributors',
+    coverArtistContribs: 'Cover Artists',
+    artTags: 'Art Tags',
   },
 
   invalidFieldCombinations: [
@@ -415,21 +544,25 @@ export const processFlashDocument = makeProcessDocument(T.Flash, {
     name: 'Flash',
     directory: 'Directory',
     page: 'Page',
+    color: 'Color',
     urls: 'URLs',
 
     date: 'Date',
     coverArtFileExtension: 'Cover Art File Extension',
 
-    featuredTracksByRef: 'Featured Tracks',
-    contributorContribsByRef: 'Contributors',
+    featuredTracks: 'Featured Tracks',
+    contributorContribs: 'Contributors',
   },
 });
 
 export const processFlashActDocument = makeProcessDocument(T.FlashAct, {
   propertyFieldMapping: {
     name: 'Act',
+    directory: 'Directory',
+
     color: 'Color',
-    anchor: 'Anchor',
+    listTerminology: 'List Terminology',
+
     jump: 'Jump',
     jumpColor: 'Jump Color',
   },
@@ -466,7 +599,7 @@ export const processGroupDocument = makeProcessDocument(T.Group, {
     description: 'Description',
     urls: 'URLs',
 
-    featuredAlbumsByRef: 'Featured Albums',
+    featuredAlbums: 'Featured Albums',
   },
 });
 
@@ -497,7 +630,7 @@ export const processWikiInfoDocument = makeProcessDocument(T.WikiInfo, {
     footerContent: 'Footer Content',
     defaultLanguage: 'Default Language',
     canonicalBase: 'Canonical Base',
-    divideTrackListsByGroupsByRef: 'Divide Track Lists By Groups',
+    divideTrackListsByGroups: 'Divide Track Lists By Groups',
     enableFlashesAndGames: 'Enable Flashes & Games',
     enableListings: 'Enable Listings',
     enableNews: 'Enable News',
@@ -532,9 +665,9 @@ export const homepageLayoutRowTypeProcessMapping = {
   albums: makeProcessHomepageLayoutRowDocument(T.HomepageLayoutAlbumsRow, {
     propertyFieldMapping: {
       displayStyle: 'Display Style',
-      sourceGroupByRef: 'Group',
+      sourceGroup: 'Group',
       countAlbumsFromGroup: 'Count',
-      sourceAlbumsByRef: 'Albums',
+      sourceAlbums: 'Albums',
       actionLinks: 'Actions',
     },
   }),
@@ -591,33 +724,17 @@ export function parseContributors(contributors) {
     return contributors;
   }
 
-  if (contributors.length === 1 && contributors[0].startsWith('<i>')) {
-    const arr = [];
-    arr.textContent = contributors[0];
-    return arr;
-  }
-
   contributors = contributors.map((contrib) => {
-    // 8asically, the format is "Who (What)", or just "Who". 8e sure to
-    // keep in mind that "what" doesn't necessarily have a value!
+    if (typeof contrib !== 'string') return contrib;
+
     const match = contrib.match(/^(.*?)( \((.*)\))?$/);
-    if (!match) {
-      return contrib;
-    }
+    if (!match) return contrib;
+
     const who = match[1];
     const what = match[3] || null;
     return {who, what};
   });
 
-  const badContributor = contributors.find((val) => typeof val === 'string');
-  if (badContributor) {
-    throw new Error(`Incorrectly formatted contribution: "${badContributor}".`);
-  }
-
-  if (contributors.length === 1 && contributors[0].who === 'none') {
-    return null;
-  }
-
   return contributors;
 }
 
@@ -767,13 +884,13 @@ export const dataSteps = [
         let currentTrackSection = {
           name: `Default Track Section`,
           isDefaultTrackSection: true,
-          tracksByRef: [],
+          tracks: [],
         };
 
-        const albumRef = T.Thing.getReference(album);
+        const albumRef = Thing.getReference(album);
 
         const closeCurrentTrackSection = () => {
-          if (!empty(currentTrackSection.tracksByRef)) {
+          if (!empty(currentTrackSection.tracks)) {
             trackSections.push(currentTrackSection);
           }
         };
@@ -787,7 +904,7 @@ export const dataSteps = [
               color: entry.color,
               dateOriginallyReleased: entry.dateOriginallyReleased,
               isDefaultTrackSection: false,
-              tracksByRef: [],
+              tracks: [],
             };
 
             continue;
@@ -795,9 +912,9 @@ export const dataSteps = [
 
           trackData.push(entry);
 
-          entry.dataSourceAlbumByRef = albumRef;
+          entry.dataSourceAlbum = albumRef;
 
-          currentTrackSection.tracksByRef.push(T.Thing.getReference(entry));
+          currentTrackSection.tracks.push(Thing.getReference(entry));
         }
 
         closeCurrentTrackSection();
@@ -821,12 +938,12 @@ export const dataSteps = [
       const artistData = results;
 
       const artistAliasData = results.flatMap((artist) => {
-        const origRef = T.Thing.getReference(artist);
+        const origRef = Thing.getReference(artist);
         return artist.aliasNames?.map((name) => {
           const alias = new T.Artist();
           alias.name = name;
           alias.isAlias = true;
-          alias.aliasedArtistRef = origRef;
+          alias.aliasedArtist = origRef;
           alias.artistData = artistData;
           return alias;
         }) ?? [];
@@ -850,7 +967,7 @@ export const dataSteps = [
 
     save(results) {
       let flashAct;
-      let flashesByRef = [];
+      let flashRefs = [];
 
       if (results[0] && !(results[0] instanceof T.FlashAct)) {
         throw new Error(`Expected an act at top of flash data file`);
@@ -859,18 +976,18 @@ export const dataSteps = [
       for (const thing of results) {
         if (thing instanceof T.FlashAct) {
           if (flashAct) {
-            Object.assign(flashAct, {flashesByRef});
+            Object.assign(flashAct, {flashes: flashRefs});
           }
 
           flashAct = thing;
-          flashesByRef = [];
+          flashRefs = [];
         } else {
-          flashesByRef.push(T.Thing.getReference(thing));
+          flashRefs.push(Thing.getReference(thing));
         }
       }
 
       if (flashAct) {
-        Object.assign(flashAct, {flashesByRef});
+        Object.assign(flashAct, {flashes: flashRefs});
       }
 
       const flashData = results.filter((x) => x instanceof T.Flash);
@@ -893,7 +1010,7 @@ export const dataSteps = [
 
     save(results) {
       let groupCategory;
-      let groupsByRef = [];
+      let groupRefs = [];
 
       if (results[0] && !(results[0] instanceof T.GroupCategory)) {
         throw new Error(`Expected a category at top of group data file`);
@@ -902,18 +1019,18 @@ export const dataSteps = [
       for (const thing of results) {
         if (thing instanceof T.GroupCategory) {
           if (groupCategory) {
-            Object.assign(groupCategory, {groupsByRef});
+            Object.assign(groupCategory, {groups: groupRefs});
           }
 
           groupCategory = thing;
-          groupsByRef = [];
+          groupRefs = [];
         } else {
-          groupsByRef.push(T.Thing.getReference(thing));
+          groupRefs.push(Thing.getReference(thing));
         }
       }
 
       if (groupCategory) {
-        Object.assign(groupCategory, {groupsByRef});
+        Object.assign(groupCategory, {groups: groupRefs});
       }
 
       const groupData = results.filter((x) => x instanceof T.Group);
@@ -925,6 +1042,10 @@ export const dataSteps = [
 
   {
     title: `Process homepage layout file`,
+
+    // Kludge: This benefits from the same headerAndEntries style messaging as
+    // albums and tracks (for example), but that document mode is designed to
+    // support multiple files, and only one is actually getting processed here.
     files: [HOMEPAGE_LAYOUT_DATA_FILE],
 
     documentMode: documentModes.headerAndEntries,
@@ -1005,7 +1126,7 @@ export async function loadAndProcessDataDocuments({dataPath}) {
       } catch (error) {
         error.message +=
           (error.message.includes('\n') ? '\n' : ' ') +
-          `(file: ${color.bright(color.blue(path.relative(dataPath, x.file)))})`;
+          `(file: ${colors.bright(colors.blue(path.relative(dataPath, x.file)))})`;
         throw error;
       }
     };
@@ -1013,8 +1134,8 @@ export async function loadAndProcessDataDocuments({dataPath}) {
 
   for (const dataStep of dataSteps) {
     await processDataAggregate.nestAsync(
-      {message: `Errors during data step: ${dataStep.title}`},
-      async ({call, callAsync, map, mapAsync, nest}) => {
+      {message: `Errors during data step: ${colors.bright(dataStep.title)}`},
+      async ({call, callAsync, map, mapAsync, push, nest}) => {
         const {documentMode} = dataStep;
 
         if (!Object.values(documentModes).includes(documentMode)) {
@@ -1028,7 +1149,7 @@ export async function loadAndProcessDataDocuments({dataPath}) {
         // just without the callbacks. Thank you.
         const filterBlankDocuments = documents => {
           const aggregate = openAggregate({
-            message: `Found blank documents - check for extra '${color.cyan(`---`)}'`,
+            message: `Found blank documents - check for extra '${colors.cyan(`---`)}'`,
           });
 
           const filteredDocuments =
@@ -1072,10 +1193,10 @@ export async function loadAndProcessDataDocuments({dataPath}) {
 
               if (count === 1) {
                 const range = `#${start + 1}`;
-                parts.push(`${count} document (${color.yellow(range)}), `);
+                parts.push(`${count} document (${colors.yellow(range)}), `);
               } else {
                 const range = `#${start + 1}-${end + 1}`;
-                parts.push(`${count} documents (${color.yellow(range)}), `);
+                parts.push(`${count} documents (${colors.yellow(range)}), `);
               }
 
               if (previous === null) {
@@ -1085,7 +1206,7 @@ export async function loadAndProcessDataDocuments({dataPath}) {
               } else {
                 const previousDescription = Object.entries(previous).at(0).join(': ');
                 const nextDescription = Object.entries(next).at(0).join(': ');
-                parts.push(`between "${color.cyan(previousDescription)}" and "${color.cyan(nextDescription)}"`);
+                parts.push(`between "${colors.cyan(previousDescription)}" and "${colors.cyan(nextDescription)}"`);
               }
 
               aggregate.push(new Error(parts.join('')));
@@ -1139,32 +1260,52 @@ export async function loadAndProcessDataDocuments({dataPath}) {
             return;
           }
 
-          const yamlResult =
-            documentMode === documentModes.oneDocumentTotal
-              ? call(yaml.load, readResult)
-              : call(yaml.loadAll, readResult);
+          let processResults;
 
-          if (!yamlResult) {
-            return;
-          }
+          switch (documentMode) {
+            case documentModes.oneDocumentTotal: {
+              const yamlResult = call(yaml.load, readResult);
 
-          let processResults;
+              if (!yamlResult) {
+                processResults = null;
+                break;
+              }
+
+              const {thing, aggregate} =
+                dataStep.processDocument(yamlResult);
+
+              processResults = thing;
+
+              call(() => aggregate.close());
+
+              break;
+            }
+
+            case documentModes.allInOne: {
+              const yamlResults = call(yaml.loadAll, readResult);
+
+              if (!yamlResults) {
+                processResults = [];
+                return;
+              }
+
+              const {documents, aggregate: filterAggregate} =
+                filterBlankDocuments(yamlResults);
+
+              call(filterAggregate.close);
+
+              processResults = [];
 
-          if (documentMode === documentModes.oneDocumentTotal) {
-            nest({message: `Errors processing document`}, ({call}) => {
-              processResults = call(dataStep.processDocument, yamlResult);
-            });
-          } else {
-            const {documents, aggregate: aggregate1} = filterBlankDocuments(yamlResult);
-            call(aggregate1.close);
-
-            const {result, aggregate: aggregate2} = mapAggregate(
-              documents,
-              decorateErrorWithIndex(dataStep.processDocument),
-              {message: `Errors processing documents`});
-            call(aggregate2.close);
-
-            processResults = result;
+              map(documents, decorateErrorWithIndex(document => {
+                const {thing, aggregate} =
+                  dataStep.processDocument(document);
+
+                processResults.push(thing);
+                aggregate.close();
+              }), {message: `Errors processing documents`});
+
+              break;
+            }
           }
 
           if (!processResults) return;
@@ -1222,81 +1363,74 @@ export async function loadAndProcessDataDocuments({dataPath}) {
           return {file, documents: filteredDocuments};
         });
 
-        let processResults;
+        const processResults = [];
 
-        if (documentMode === documentModes.headerAndEntries) {
-          nest({message: `Errors processing data files as valid documents`}, ({call, map}) => {
-            processResults = [];
+        switch (documentMode) {
+          case documentModes.headerAndEntries:
+            map(yamlResults, decorateErrorWithFile(({documents}) => {
+              const headerDocument = documents[0];
+              const entryDocuments = documents.slice(1).filter(Boolean);
 
-            yamlResults.forEach(({file, documents}) => {
-              const [headerDocument, ...entryDocuments] = documents;
+              if (!headerDocument)
+                throw new Error(`Missing header document (empty file or erroneously starting with "---"?)`);
 
-              if (!headerDocument) {
-                call(decorateErrorWithFile(() => {
-                  throw new Error(`Missing header document (empty file or erroneously starting with "---"?)`);
-                }), {file});
-                return;
-              }
+              // This'll be decorated with the file, and groups together any
+              // errors from processing the header and entry documents.
+              const fileAggregate =
+                openAggregate({message: `Errors processing documents`});
 
-              const header = call(
-                decorateErrorWithFile(({document}) =>
-                  dataStep.processHeaderDocument(document)),
-                {file, document: headerDocument});
+              const {thing: headerObject, aggregate: headerAggregate} =
+                dataStep.processHeaderDocument(headerDocument);
 
-              // Don't continue processing files whose header
-              // document is invalid - the entire file is excempt
-              // from data in this case.
-              if (!header) {
-                return;
+              try {
+                headerAggregate.close()
+              } catch (caughtError) {
+                caughtError.message = `(${colors.yellow(`header`)}) ${caughtError.message}`;
+                fileAggregate.push(caughtError);
               }
 
-              const entries = map(
-                entryDocuments
-                  .filter(Boolean)
-                  .map((document) => ({file, document})),
-                decorateErrorWithFile(
-                  decorateErrorWithIndex(({document}) =>
-                    dataStep.processEntryDocument(document))),
-                {message: `Errors processing entry documents`});
-
-              // Entries may be incomplete (i.e. any errored
-              // documents won't have a processed output
-              // represented here) - this is intentional! By
-              // principle, partial output is preferred over
-              // erroring an entire file.
-              processResults.push({header, entries});
-            });
-          });
-        }
+              const entryObjects = [];
 
-        if (documentMode === documentModes.onePerFile) {
-          nest({message: `Errors processing data files as valid documents`}, ({call}) => {
-            processResults = [];
+              for (let index = 0; index < entryDocuments.length; index++) {
+                const entryDocument = entryDocuments[index];
 
-            yamlResults.forEach(({file, documents}) => {
-              if (documents.length > 1) {
-                call(decorateErrorWithFile(() => {
-                  throw new Error(`Only expected one document to be present per file`);
-                }), {file});
-                return;
-              } else if (empty(documents) || !documents[0]) {
-                call(decorateErrorWithFile(() => {
-                  throw new Error(`Expected a document, this file is empty`);
-                }), {file});
-              }
+                const {thing: entryObject, aggregate: entryAggregate} =
+                  dataStep.processEntryDocument(entryDocument);
 
-              const result = call(
-                decorateErrorWithFile(({document}) =>
-                  dataStep.processDocument(document)),
-                {file, document: documents[0]});
+                entryObjects.push(entryObject);
 
-              if (!result) {
-                return;
+                try {
+                  entryAggregate.close();
+                } catch (caughtError) {
+                  caughtError.message = `(${colors.yellow(`entry #${index + 1}`)}) ${caughtError.message}`;
+                  fileAggregate.push(caughtError);
+                }
               }
 
-              processResults.push(result);
-            });
-          });
+              processResults.push({
+                header: headerObject,
+                entries: entryObjects,
+              });
+
+              fileAggregate.close();
+            }), {message: `Errors processing documents in data files`});
+            break;
+
+          case documentModes.onePerFile:
+            map(yamlResults, decorateErrorWithFile(({documents}) => {
+              if (documents.length > 1)
+                throw new Error(`Only expected one document to be present per file, got ${documents.length} here`);
+
+              if (empty(documents) || !documents[0])
+                throw new Error(`Expected a document, this file is empty`);
+
+              const {thing, aggregate} =
+                dataStep.processDocument(documents[0]);
+
+              processResults.push(thing);
+              aggregate.close();
+            }), {message: `Errors processing data files as valid documents`});
+            break;
         }
 
         const saveResult = call(dataStep.save, processResults);
@@ -1316,13 +1450,27 @@ export async function loadAndProcessDataDocuments({dataPath}) {
 
 // Data linking! Basically, provide (portions of) wikiData to the Things which
 // require it - they'll expose dynamically computed properties as a result (many
-// of which are required for page HTML generation).
-export function linkWikiDataArrays(wikiData) {
+// of which are required for page HTML generation and other expected behavior).
+//
+// The XXX_decacheWikiData option should be used specifically to mark
+// points where you *aren't* replacing any of the arrays under wikiData with
+// new values, and are using linkWikiDataArrays to instead "decache" data
+// properties which depend on any of them. It's currently not possible for
+// a CacheableObject to depend directly on the value of a property exposed
+// on some other CacheableObject, so when those values change, you have to
+// manually decache before the object will realize its cache isn't valid
+// anymore.
+export function linkWikiDataArrays(wikiData, {
+  XXX_decacheWikiData = false,
+} = {}) {
   function assignWikiData(things, ...keys) {
+    if (things === undefined) return;
     for (let i = 0; i < things.length; i++) {
       const thing = things[i];
       for (let j = 0; j < keys.length; j++) {
         const key = keys[j];
+        if (!(key in wikiData)) continue;
+        if (XXX_decacheWikiData) thing[key] = [];
         thing[key] = wikiData[key];
       }
     }
@@ -1340,7 +1488,7 @@ export function linkWikiDataArrays(wikiData) {
   assignWikiData(WD.flashData, 'artistData', 'flashActData', 'trackData');
   assignWikiData(WD.flashActData, 'flashData');
   assignWikiData(WD.artTagData, 'albumData', 'trackData');
-  assignWikiData(WD.homepageLayout.rows, 'albumData', 'groupData');
+  assignWikiData(WD.homepageLayout?.rows, 'albumData', 'groupData');
 }
 
 export function sortWikiDataArrays(wikiData) {
@@ -1368,7 +1516,9 @@ export function filterDuplicateDirectories(wikiData) {
   const deduplicateSpec = [
     'albumData',
     'artTagData',
+    'artistData',
     'flashData',
+    'flashActData',
     'groupData',
     'newsData',
     'trackData',
@@ -1377,7 +1527,7 @@ export function filterDuplicateDirectories(wikiData) {
   const aggregate = openAggregate({message: `Duplicate directories found`});
   for (const thingDataProp of deduplicateSpec) {
     const thingData = wikiData[thingDataProp];
-    aggregate.nest({message: `Duplicate directories found in ${color.green('wikiData.' + thingDataProp)}`}, ({call}) => {
+    aggregate.nest({message: `Duplicate directories found in ${colors.green('wikiData.' + thingDataProp)}`}, ({call}) => {
       const directoryPlaces = Object.create(null);
       const duplicateDirectories = [];
 
@@ -1403,7 +1553,7 @@ export function filterDuplicateDirectories(wikiData) {
         const places = directoryPlaces[directory];
         call(() => {
           throw new Error(
-            `Duplicate directory ${color.green(directory)}:\n` +
+            `Duplicate directory ${colors.green(directory)}:\n` +
               places.map((thing) => ` - ` + inspect(thing)).join('\n')
           );
         });
@@ -1444,45 +1594,45 @@ export function filterDuplicateDirectories(wikiData) {
 export function filterReferenceErrors(wikiData) {
   const referenceSpec = [
     ['wikiInfo', processWikiInfoDocument, {
-      divideTrackListsByGroupsByRef: 'group',
+      divideTrackListsByGroups: 'group',
     }],
 
     ['albumData', processAlbumDocument, {
-      artistContribsByRef: '_contrib',
-      coverArtistContribsByRef: '_contrib',
-      trackCoverArtistContribsByRef: '_contrib',
-      wallpaperArtistContribsByRef: '_contrib',
-      bannerArtistContribsByRef: '_contrib',
-      groupsByRef: 'group',
-      artTagsByRef: 'artTag',
+      artistContribs: '_contrib',
+      coverArtistContribs: '_contrib',
+      trackCoverArtistContribs: '_contrib',
+      wallpaperArtistContribs: '_contrib',
+      bannerArtistContribs: '_contrib',
+      groups: 'group',
+      artTags: 'artTag',
     }],
 
     ['trackData', processTrackDocument, {
-      artistContribsByRef: '_contrib',
-      contributorContribsByRef: '_contrib',
-      coverArtistContribsByRef: '_contrib',
-      referencedTracksByRef: '_trackNotRerelease',
-      sampledTracksByRef: '_trackNotRerelease',
-      artTagsByRef: 'artTag',
-      originalReleaseTrackByRef: '_trackNotRerelease',
+      artistContribs: '_contrib',
+      contributorContribs: '_contrib',
+      coverArtistContribs: '_contrib',
+      referencedTracks: '_trackNotRerelease',
+      sampledTracks: '_trackNotRerelease',
+      artTags: 'artTag',
+      originalReleaseTrack: '_trackNotRerelease',
     }],
 
     ['groupCategoryData', processGroupCategoryDocument, {
-      groupsByRef: 'group',
+      groups: 'group',
     }],
 
     ['homepageLayout.rows', undefined, {
-      sourceGroupByRef: 'group',
-      sourceAlbumsByRef: 'album',
+      sourceGroup: '_homepageSourceGroup',
+      sourceAlbums: 'album',
     }],
 
     ['flashData', processFlashDocument, {
-      contributorContribsByRef: '_contrib',
-      featuredTracksByRef: 'track',
+      contributorContribs: '_contrib',
+      featuredTracks: 'track',
     }],
 
     ['flashActData', processFlashActDocument, {
-      flashesByRef: 'flash',
+      flashes: 'flash',
     }],
   ];
 
@@ -1498,7 +1648,7 @@ export function filterReferenceErrors(wikiData) {
   for (const [thingDataProp, providedProcessDocumentFn, propSpec] of referenceSpec) {
     const thingData = getNestedProp(wikiData, thingDataProp);
 
-    aggregate.nest({message: `Reference errors in ${color.green('wikiData.' + thingDataProp)}`}, ({nest}) => {
+    aggregate.nest({message: `Reference errors in ${colors.green('wikiData.' + thingDataProp)}`}, ({nest}) => {
       const things = Array.isArray(thingData) ? thingData : [thingData];
 
       for (const thing of things) {
@@ -1514,10 +1664,10 @@ export function filterReferenceErrors(wikiData) {
 
         nest({message: `Reference errors in ${inspect(thing)}`}, ({push, filter}) => {
           for (const [property, findFnKey] of Object.entries(propSpec)) {
-            const value = thing[property];
+            const value = CacheableObject.getUpdateValue(thing, property);
 
             if (value === undefined) {
-              push(new TypeError(`Property ${color.red(property)} isn't valid for ${color.green(thing.constructor.name)}`));
+              push(new TypeError(`Property ${colors.red(property)} isn't valid for ${colors.green(thing.constructor.name)}`));
               continue;
             }
 
@@ -1534,23 +1684,34 @@ export function filterReferenceErrors(wikiData) {
                   if (alias) {
                     // No need to check if the original exists here. Aliases are automatically
                     // created from a field on the original, so the original certainly exists.
-                    const original = find.artist(alias.aliasedArtistRef, wikiData.artistData, {mode: 'quiet'});
-                    throw new Error(`Reference ${color.red(contribRef.who)} is to an alias, should be ${color.green(original.name)}`);
+                    const original = alias.aliasedArtist;
+                    throw new Error(`Reference ${colors.red(contribRef.who)} is to an alias, should be ${colors.green(original.name)}`);
                   }
 
                   return boundFind.artist(contribRef.who);
                 };
                 break;
 
+              case '_homepageSourceGroup':
+                findFn = groupRef => {
+                  if (groupRef === 'new-additions' || groupRef === 'new-releases') {
+                    return true;
+                  }
+
+                  return boundFind.group(groupRef);
+                };
+                break;
+
               case '_trackNotRerelease':
                 findFn = trackRef => {
                   const track = find.track(trackRef, wikiData.trackData, {mode: 'error'});
+                  const originalRef = track && CacheableObject.getUpdateValue(track, 'originalReleaseTrack');
 
-                  if (track?.originalReleaseTrackByRef) {
+                  if (originalRef) {
                     // It's possible for the original to not actually exist, in this case.
                     // It should still be reported since the 'Originally Released As' field
                     // was present.
-                    const original = find.track(track.originalReleaseTrackByRef, wikiData.trackData, {mode: 'quiet'});
+                    const original = find.track(originalRef, wikiData.trackData, {mode: 'quiet'});
 
                     // Prefer references by name, but only if it's unambiguous.
                     const originalByName =
@@ -1560,12 +1721,12 @@ export function filterReferenceErrors(wikiData) {
 
                     const shouldBeMessage =
                       (originalByName
-                        ? color.green(original.name)
+                        ? colors.green(original.name)
                      : original
-                        ? color.green('track:' + original.directory)
-                        : color.green(track.originalReleaseTrackByRef));
+                        ? colors.green('track:' + original.directory)
+                        : colors.green(originalRef));
 
-                    throw new Error(`Reference ${color.red(trackRef)} is to a rerelease, should be ${shouldBeMessage}`);
+                    throw new Error(`Reference ${colors.red(trackRef)} is to a rerelease, should be ${shouldBeMessage}`);
                   }
 
                   return track;
@@ -1578,7 +1739,7 @@ export function filterReferenceErrors(wikiData) {
             }
 
             const suppress = fn => conditionallySuppressError(error => {
-              if (property === 'sampledTracksByRef') {
+              if (property === 'sampledTracks') {
                 // Suppress "didn't match anything" errors in particular, just for samples.
                 // In hsmusic-data we have a lot of "stub" sample data which don't have
                 // corresponding tracks yet, so it won't be useful to report such reference
@@ -1596,13 +1757,13 @@ export function filterReferenceErrors(wikiData) {
 
             const fieldPropertyMessage =
               (processDocumentFn?.propertyFieldMapping?.[property]
-                ? ` in field ${color.green(processDocumentFn.propertyFieldMapping[property])}`
-                : ` in property ${color.green(property)}`);
+                ? ` in field ${colors.green(processDocumentFn.propertyFieldMapping[property])}`
+                : ` in property ${colors.green(property)}`);
 
             const findFnMessage =
               (findFnKey.startsWith('_')
                 ? ``
-                : ` (${color.green('find.' + findFnKey)})`);
+                : ` (${colors.green('find.' + findFnKey)})`);
 
             const errorMessage =
               (Array.isArray(value)