arthur/docs/lane-model.md

# The Lane Model

Revised 2026-09-30. Target design. Occurrence ownership, source playback, the
content and exposure commands, placement anywhere in a lane, a one-row cel strip
and the correction-layer evaluator are implemented; the commands that produce a
correction, overwrite, the retiming commands and the remaining views are not.
See the status note under
[Proof obligations](#proof-obligations-and-implementation-order).

This revises the Claude artifact [The Lane Model](https://claude.ai/code/artifact/cd42981d-ed08-493f-94df-b7dd6657f0e6).
Its prose and diagram source were recovered from session
`1c603f71-84eb-498e-aeaf-4c0346f1f513`; the live artifact was not accessible for
reading or editing here. This repository document is the revised design. The
original artifact has not been updated, and edits made there outside the recorded
session may not be represented here.

For the subjects covered here, this document supersedes the original artifact
and conflicting proposals in `animation-model.md`, `timing-model.md`, and
`architecture.md`. Those documents retain useful detail about the existing system.

## Goal and compatibility policy

Arthur is one animation document with several ways to see and edit it: drawing
on the stage, arranging clips, timing exposures, editing curves, and generating
motion from footage. Each view exposes relevant facts and invokes shared editing
operations. Switching views must preserve the meaning of the work.

The user explicitly requires no backward compatibility. Replace obsolete shapes,
APIs, and tests when a better model requires it. Do not retain compatibility
branches, adapters, or migrations solely to preserve the current document format.
A format marker can reject unsupported files clearly; it does not promise to
convert them. This policy does not authorize deleting existing user assets.

Simplicity means predictable composition, clear ownership, and few independent
rules. Minimizing field count is secondary to representing independent choices.

## What stays

- A symbol is the one container for authored scene nodes. A drawing can be a
  one-frame symbol; an animation uses the same container over more frames.
- Nodes have stable identities and flat parent references. Shared content is
  referenced rather than copied implicitly.
- Animatable properties are addressed by channel paths. Generated and authored
  values participate in the same evaluation machinery.
- Authored data, generated blocks, and source media remain separate. Documents
  reference immutable blocks; caches and resolver indexes remain derived.
- A pure reference evaluator specifies the result. Playback, seeking, preview,
  export, and optimized cursors must agree with it.
- Existence, visibility, and missing measured data remain distinct facts.

## Content, occurrences, lanes, and rows

These have different identities and responsibilities:

| Concept | Owns | Example |
| --- | --- | --- |
| Content | Reusable nodes and their animation | Drawing `a2`, an animated head, or a sound asset |
| Occurrence | One use of content, its interval, source playback, and local treatment | `a2` exposed on frames 12–16 |
| Lane | A sequence of occurrences and shared properties | The girl's drawings and the girl's overall transform |
| View row or column | Presentation and editor state | Timeline row, exposure-sheet column, or property curve |

Use the existing instance/node identity mechanism for occurrences. An exposure
should not acquire a second identity system just because it is shown as a cel.
Lanes group occurrences; they do not introduce another node-holding content type.
The concrete candidate below uses existing group and instance nodes; its ownership
boundaries are part of the design. It is now the implemented shape, and the field
spellings below are the ones the runtime reads.

Each occurrence has a stable ID. Moving it, changing its hold, swapping its source,
or trimming it preserves that ID. Repeating it creates a new occurrence that may
reference the same content. A split retains the original ID on the left and gives
the right piece a new ID; commands return the resulting selection explicitly.

Occurrences are the canonical authored arrangement. A source-at-time channel or
interval index may be compiled from them for evaluation, but is not a second
editable copy of the schedule. This replaces the earlier proposal that every cel
must be represented solely as a source key. Ordinary property animation still
uses lightweight keys; it does not need occurrence objects.

A sequence lane has non-overlapping half-open occurrence intervals `[start,end)`
in its own time space. Uncovered intervals are gaps. Empty lanes are valid.
Compositing and simultaneous sounds are represented by multiple lanes or ordinary
scene composition; an accidental overlap never silently selects a winner.
Transitions, if added, need explicit overlap and mixing semantics.

Properties can belong to content, one occurrence, or the lane. For example:

- Rotate the reusable drawing: all its uses change.
- Rotate one occurrence: only that exposure changes.
- Animate the lane's rotation: whichever drawing is showing follows it.

An occurrence can have its own transform, gain, corrections, and source timing
while remaining a block in the same timeline row. Independent treatment never
requires a new row or an otherwise unnecessary wrapper symbol.

### Concrete candidate: a sequence group and ordinary instances

A lane is a group node with `:layout :sequence`. Its occurrences are ordinary
instance nodes whose `:parent` points to the group. All remain in their symbol's
flat node map. The sequence constraint is document semantics; which rows the UI
expands remains editor state. Ordinary groups retain unconstrained composition.

This example is a document the runtime accepts, built and evaluated by
`frontend/test/arthur/domain/lane_test.cljs`. Times here are zero-based. Channels
use the existing representation; occurrence source references and playback have
replaced the `[:source]` channel, which no longer exists.

```clojure
;; Within :main's :nodes; referenced drawings/animations live in :symbols.
{:girl
 {:id :girl :kind :group :layout :sequence :z "b"
  :channels {[:xform :rot]
             {:animated? true :interp :linear :keys {0 0, 6 30, 12 0}}}}

 :exposure-a
 {:id :exposure-a :kind :instance :parent :girl :z "a"
  :time {:at 0 :rate 1} :span [0 4]
  :source {:symbol :drawing-a}
  :playback {:in 0 :speed 0 :end :stop}}

 :exposure-b
 {:id :exposure-b :kind :instance :parent :girl :z "b"
  :time {:at 4 :rate 1} :span [0 4]
  :source {:symbol :drawing-b}
  :playback {:in 0 :speed 0 :end :stop}
  :channels {[:xform :pos]
             {:animated? true :interp :hold :keys {0 [0 0], 1 [2 0]}}}}

 :animated-insert
 {:id :animated-insert :kind :instance :parent :girl :z "c"
  :time {:at 8 :rate 1} :span [0 4]
  :source {:symbol :wave}
  :playback {:in 3 :speed 1 :end :stop}}}
```

The lane's rotation reads lane time. Each occurrence's channels read occurrence
time. Its content reads source time. The stills sample frame 0, while the insert
samples source frames 3, 4, 5, and 6. The group transform composes with the
occurrence transform and then the content's own transform.

`:span` remains in the node's own coordinates, consistent with ordinary nodes.
The interval in lane time is derived through `:time`; do not also store parent
start/end values. Sequence children require finite intervals and positive
placement rates. Ordering and overlap checks use the mapped intervals, not `:z`.
The sequence group may contain visual occurrences or audio occurrences; its
capability must reject an incompatible mixture rather than infer it per frame.

A source reference is fixed within an occurrence. The lane changes content when
another occurrence becomes active. This is a deliberate revision of the original
diagnosis that making `:of` a channel was necessary to avoid vertical growth:
multiple instances can occupy one row when the view presents their containing
sequence. A lane-level source schedule is therefore derived, not authored twice.

## Source selection and source playback are independent

The current implementation makes framed sources play and keyed sources hold.
Retire that rule. Channel storage shape must not determine playback behavior.
Adding or removing a key must not turn a still into an animation or vice versa.

An occurrence names content and describes how its source time is sampled. In the
basic case, after mapping lane time into occurrence time:

```text
source_time = in_point + speed × occurrence_time
```

A newly created occurrence starts at local time zero. Moving it preserves this
origin relative to its content. Trimming can narrow its local support without
resetting that origin; split pieces likewise preserve the source and property
values at the cut. Trimming, slipping, and retiming are distinct operations with
explicitly different effects on the interval and the source map.

| Intent | Source playback |
| --- | --- |
| Hold a drawing | Constant source frame, equivalently speed 0 |
| Play an animated symbol | Advancing source time, normally speed 1 |
| Cut between animations | Several occurrences, each with its own in-point and speed |
| Mix stills and animation in a lane | Constant and advancing maps in the same sequence |

The source reference itself is discrete and never numerically interpolated.
Interpolation belongs to properties that support it; a property registry should
declare value types, defaults, and permitted interpolation and correction modes.
Generic key toggles must consult those capabilities rather than assume every
non-boolean value can be tweened.

Define source bounds and end behavior explicitly: stop contributing outside the
source, hold an endpoint, or loop an explicit range. A still uses a valid constant
frame. A loop uses a nonempty half-open range and a defined modulo rule. Playback
never guesses these policies from whether a channel happens to have keys.

Audio shares occurrence arrangement, trimming, gain ownership, and clock mapping.
It does not inherit visual frame-hold semantics: holding one audio sample is not
an audio freeze effect. Validate supported playback policies by media capability.
Actual audio scheduling must follow active occurrences, including gaps and cuts,
rather than playing every sound reachable through a structural reference.

## Time spaces and sampling

Name the relevant space whenever an API accepts a time or range: project,
symbol/lane, occurrence, or source. Store authored frame coordinates exactly;
avoid cumulative rounding when moving through nested mappings. Quantize at a
declared sampling boundary, not at every traversal step. Audio also needs its
continuous clock/sample space rather than visual frame quantization.

A hold is an evaluable time map with no unique inverse. A loop can map many
displayed occurrences to one source time. APIs must distinguish forward sampling
from inverse editing, and expose enough context to resolve an occurrence or
explicitly refuse an ambiguous operation. Do not report a missing time map merely
because inversion is unavailable.

Separate invertible placement timing from source sampling. A zero source speed
can mean hold without making the occurrence's own edit clock non-invertible.
Reparenting through changing transforms or non-invertible timing must either
preserve the full result by an explicit bake or return a reason it cannot; a
matrix captured at one frame does not prove preservation across the animation.

The source's frame step, generated-pose sampling, and the lane's transform clock
are independent scopes. Drawing on twos must not accidentally step a smooth lane
transform. An explicit whole-subtree stepping operation can exist separately.

Share the quantization primitive where possible, but retain its units, phase,
rounding policy, and order relative to retiming and lead. The original suggestion
that exposure and picture-rate sampling are simply one floor is insufficient:
noninteger grids and source-frame quantization require specified behavior.
Identity timing can be implicit; remove `:time :mode` if it only duplicates that.

An occurrence interval is authored. Lane content extent is derived from its
occurrences, including the explicit end of the last one. A separately authored
container trim/window is legitimate when it intentionally gates children. Do not
conflate that window with occupied extent or infer a final hold from the next key
when no next key exists. A range of frame numbers alone cannot encode visibility
or a missing measurement.

## Shared editing operations

Every view issues the same domain commands. A command accepts an explicit target
and edit policy, computes a valid change, and returns the change, resulting
selection, and any refusal reason. A button and a drag must not implement two
versions of exposure extension.

An edit target identifies the symbol, occurrence path, selected entities or
properties, and the time range with its space. Navigation also distinguishes
editing shared content directly from editing it through a particular occurrence.
Crossing a source cut must not silently redirect an active drawing edit to a
different symbol: retain the explicit content target until navigation changes it.

Core commands include new drawing, reuse drawing, duplicate drawing, make unique,
blank range, split, trim, move, extend exposure, slip source, retime, and apply a
bounded property edit. Ripple/overwrite policy and the set of affected lanes are
explicit command arguments. Preview consequences before committing a gesture.

New drawing creates fresh empty content and an occurrence. Blank range removes
content coverage without inventing a hidden drawing. These are different actions.
Reuse creates another occurrence pointing at existing content. Duplicate creates
a new content identity. Make unique rebinds the selected occurrence only.

Copy semantics must specify nested sharing. A normal content copy duplicates its
owned nodes and channels while preserving references to other reusable symbols.
For a fully independent drawing assembled from nested symbols, provide an
explicit deep-copy operation with ID remapping. Never promise decoupling while
leaving the relevant edited object shared. Immutable media blocks may remain shared.

Commands are atomic undo transactions, even when they touch several leaves.
Pointer movement and keyboard invocation use explicit begin/preview/commit or
cancel boundaries; a timing heuristic alone must not decide user intent.
Collaboration applies a transaction consistently, validates affected references,
and detects conflicts at the owned data being changed. One leaf per channel does
not solve simultaneous edits to different keys of that same channel; define a
conflict policy rather than claiming that granularity solves all collaboration.

### Default timing behavior: exposure edits preserve lane keys

Working default from the follow-up discussion: extending a drawing's hold changes
exposure timing, leaving lane animation at its authored times. The user raised
keeping keyframes in place as a possibility; this is the proposed predictable
default, not a claim that they selected every timing policy below.

Ownership supplies the remaining rule: properties attached to an occurrence
travel with it. Extending its end does not stretch those properties; moving it
changes where their existing local times land. No per-key attachment flag is
needed to recover ownership that the document already expresses.

For the concrete example, extend `:exposure-a` by two lane frames with ripple:

| Fact | Before | After |
| --- | --- | --- |
| Exposure A's lane interval | `[0,4)` | `[0,6)` |
| Exposure B's lane interval | `[4,8)` | `[6,10)` |
| Animated insert's lane interval | `[8,12)` | `[10,14)` |
| Girl's rotation peak | Lane frame 6 | Lane frame 6 |
| B's position change | B frame 1, lane frame 5 | B frame 1, lane frame 7 |
| Insert's first source frame | Source frame 3 | Source frame 3 |

The rotation peak now coincides with a different point in the drawing sequence.
That is the intended consequence of changing exposures underneath timed motion.
The position correction stays attached to drawing B's occurrence. Neither the
background's keys nor audio on another lane moves.

The command contract for this edit names the symbol and occurrence, a delta in
lane frames, `:ripple` behavior, and an explicit scope of exposure timing. It
extends A's local support by the delta converted through A's placement rate,
and shifts subsequent occurrence placements by that delta in lane time. It does
not modify any channel's key map, source in-point, or playback speed. Reject a
nonpositive resulting duration. Validate and commit the entire change together.

The symbol's authored end is another explicit boundary: preview an overflow and
offer to extend the symbol or cancel. A command can request that extension as
part of its transaction; it must not silently truncate later occurrences or grow
other uses of a shared symbol. In the example, a 12-frame symbol needs an explicit
extension to 14 frames or the edit must be refused without partial changes.

Retime performance is a separate operation over explicitly selected occurrences
and channels. It applies the same time transformation to their relevant clocks,
keys, and correction supports. Stretching an interval requires a defined warp
and interpolation behavior; it is not merely moving keys whose frame numbers
happen to lie inside the selection. Until supported, refuse this operation
rather than approximating it with an exposure ripple.

The initial UI should default stage transforms to the lane when drawing in a cel
workflow, so movement usually remains independent of exposure timing. The
inspector names the target: lane motion, this occurrence, or shared drawing.
Changing that scope is explicit. It changes what the edit means, not just which
panel happens to be open.

## Three-frame rotation and correction layers

A range says where an edit applies; it does not specify the motion. Offer distinct
commands for a constant adjustment, a ramp, and a return-to-start motion. For UI
frames 10–12, the internal range contains exactly three frame samples after
conversion from the displayed numbering convention.

- Constant adjustment: the same offset throughout those three samples.
- Ramp: interpolate from the specified start value to the target over the range.
- Return motion: interpolate from the starting value to a peak and back.

For a return motion sampled on three frames, the values can be `0, angle, 0`.
Outside the selected range, the underlying animation must evaluate exactly as it
did before. A range-scoped correction layer expresses this directly; blindly
inserting boundary keys can alter neighboring segments or destroy existing motion.

Implement corrections as an ordered stack over the base channel. Each correction
has stable identity, explicit support interval, blend operation, and values in a
named time space. Outside its support it is inactive. `replace` can supply a value
over an absent base; `offset` cannot offset a nonexistent value. Blend capability
depends on property type, and geometry corrections require compatible topology.

Regeneration replaces the generated base and preserves corrections. If changed
topology or removed targets make a correction incompatible, report a resolvable
conflict instead of silently dropping or misapplying it. Provenance explains
where the base came from; explicit sampling policy determines its playback.

This is core to the workflow: generate motion, correct it by hand, adjust the
generator, and keep the corrections. It should be proven before adding many views.

## Other unifications worth keeping

Pose choices, tracing-frame choices, and ordinary held values should share the
channel evaluator and cursor infrastructure. Preserve their different ownership,
fallback behavior, and sampling scope. A pose choice must address the relevant
content/feature explicitly; switching to another symbol must not accidentally
reuse a track just because both symbols contain a node with the same local name.

Keep the two animation idioms distinct: keyed geometry modifies one mark over
time; drawing substitution selects content that may have different structure.
Linear geometry interpolation requires compatible vertex correspondence, not
merely two drawings that happen to look related.

Derived library grouping may collect drawings used by a single lane. This is a
convenience, not ownership or deletion authority. Reference discovery for cycle
validation, copying, and deletion examines all structural references, including
currently inactive occurrences. Authored folders, favorites, and labels remain
legitimate user data even when the UI could have suggested defaults.

## UX: location, selection, and controls

The breadcrumb sits above the timeline and states the editing location, shared
content identity, and occurrence context when applicable. Show local time and
its project context where a useful mapping exists. Holds and loops need an honest
description instead of a fictitious unique global frame.

Creation controls next to the breadcrumb act in that explicit location. Selection
does not secretly change where a new symbol goes. A shared drawing indicates its
reuse and offers Make this occurrence unique. Names help identify content;
linked-use indicators must rely on IDs, because different drawings can share names.

| Surface | Primary scope and controls |
| --- | --- |
| Topbar | Project name, save/open/export, project rate and stage size |
| Location bar | Breadcrumb, add lane/content, shared-content context |
| Cel action strip | New drawing, duplicate drawing, hold longer/shorter, blank range |
| Lane header | Lane selection, lock, mute/solo where applicable, onion settings, expansion |
| Stage tools | Drawing and transform modes, active target and scope |
| Inspector | Selected content/occurrence/lane properties and valid key controls |

Cel actions have visible contextual buttons, shortcuts, a context menu, and
command-palette entries. These are different entrances to the same commands.
Shortcut names from the original sketch (`N`, `D`, `H`, `B`, `K`) are provisional;
their meanings must match the visible labels and avoid tool conflicts.

The inspector normally edits values and the timeline normally edits timing, but
this is an organizational default. Numeric duration and in-point controls are
useful inspector edits to the same domain facts. Do not ban a convenient control
just to preserve a visual division.

Default nesting navigation enters content; expanding a lane reveals properties.
Other views may show hierarchies differently without changing the document.
Tabs can pin explicit locations. Zoom, expansion, onion preferences, and current
selection are editor state rather than animation content. Persistent workspace
preferences can be saved separately.

## A session, revised

1. In `main`, create a girl lane and a new drawing. Draw; use New drawing (`N`)
   to create the next one with the previous exposure ghosted behind it.
2. Use Duplicate drawing (`D`) when the current shapes are the starting point.
   Use Reuse drawing for a deliberately linked exposure. The UI shows the
   difference before an edit can change other uses.
3. Time the performance. Hold longer (`H`) extends the selected occurrence and
   ripples later occurrences in the explicitly targeted lane. A trim gesture
   can use overwrite instead. The preview shows which boundaries will move.
4. Choose a two-frame default exposure for newly created drawings, or run a
   separate Retime exposures command on a selected range. This does not quantize
   lane transforms or silently retime already authored exposures.
5. Place the background in a lane below. Its source holds one frame throughout
   its occurrence. Key the lane's X position at the beginning and end and choose
   linear interpolation. The background slides while the girl's drawings cut.
6. Select three frames on the girl's lane, choose Return motion, and rotate to
   the desired peak. A bounded rotation correction affects the girl across any
   drawing boundaries in that range. Existing motion survives outside it.
7. Insert a playing animated symbol among the girl's held drawings. Set that
   occurrence's source playback to advance. No lane conversion is required.

The timeline shows named exposure blocks with property marks and optional curve
subrows. The exposure sheet shows the same occurrences by frame and lane. The
graph editor edits the same properties; the stage resolves the same document.
Onion skin is configurable and counts neighboring exposure events, skipping gaps
by default; a long hold does not consume the budget. Repeated uses of the same
drawing remain distinct events. Deduplicating identical ghosts is a display option.

## Proof obligations and implementation order

The source-channel prototype has been removed: an occurrence names one symbol
and carries its own playback clock, and `node/problems` rejects the old
`[:source]` channel. What a lane IS lives in `arthur.domain.symbol` beside the
other rules about a node map; `arthur.domain.sequence` holds the commands over
one — add lane, place a drawing (new, reused or duplicated), make unique, split,
trim, move, blank and extend hold. Each is one history step, and each refuses rather than
half-applying. The timeline draws a lane's occurrences as cel blocks on the
lane's own row, and offers Make unique only where the selected exposure actually
shares its drawing.

There is ONE placement function and a position argument, so appending is not a
different operation from inserting: `:end` is a position like any other, the one
where nothing has to move. Placing ripples — occurrences at or after the
position move later by the new exposure's duration — and `:keep` versus
`:grow-symbol` still decides what happens at the shot's end. OVERWRITE is not a
policy argument yet, deliberately: taking frames away from the occurrence
already there is trimming, and until `trim` exists, placement that would need it
refuses instead of approximating it. A position inside an existing exposure
refuses too, and names `split` — one command does not quietly perform two.

Splitting turned out to cost almost nothing, which is evidence for the
representation rather than for the command. The two pieces keep ONE `:time` and
differ only in `:span`, so the right piece's own frames carry on where the
left's stopped and its source clock, keys and corrections go on meaning what
they meant — a held drawing holds the same frame either side, a playing insert
plays through the cut without a seam, and the test for it samples every frame
before and after and asserts the picture is identical. That falls out of `:span`
being in the node's own coordinates; it is not something split arranges.

Content copies are shallow by default and keep their references to other
symbols; `:deep? true` is the explicit copy that shares nothing, so the promise
of independence is only made where it is kept.

THE SHOT LENGTH IS AUTHORED, which is the decision the range commands forced.
`:frames` is the symbol's window — how long the shot IS — and the occupied
extent of its lanes is a different fact derived from the occurrences. A command
grows the window only when the caller says `:grow-symbol`, and never shrinks it:
blanking the end of a shot leaves a shot with empty frames at the end, because
that is a true statement about what somebody authored, and deriving the window
from the extent would make deleting the last drawing quietly shorten the film.
`finish` keeps the two numbers apart by name now rather than by a `max` that
read like an accident.

Trim NARROWS one edge and moves nothing else; lengthening is `extend-hold`,
which carries the ripple and shot-length policies because it needs them.
Move is one write to `:time :at` and REFUSES a destination that would overlap,
because moving a drawing and re-timing the ones around it are different
intentions — clear the room with `blank` or `trim` first, which is the
composition. Blank leaves a gap and does not close it; an exposure wholly inside
the range goes, one overlapping an end is trimmed to it, and the one spanning
the range is split. Their drawings stay in the library, since a lane does not
own its content.

All three are the same geometry as `split`: a `:span` is in the occurrence's own
frames, so moving an edge is one write and `:time` and `:playback` are never
touched. That is why trimming the front of a playing insert starts it later into
its animation instead of restarting it — the difference between trimming and
slipping, and the reason they stay separate commands.

Correction layers EVALUATE. `channel/problems` used to refuse an `:over` stack
and `value-at`/`cursor` used to throw on one; both now read it, and the
agreement test that holds the optimized cursor to the specification covers
stacked channels in forward, backward and random frame order. A layer's values
are themselves a channel, so a constant adjustment, a ramp and a return motion
are one mechanism; `:support` is half-open and a layer is inactive outside it;
and a layer has no time space of its own, because the node its channel is on
already has one. Nothing had to change in the codec — a channel is one leaf, so
a correction persists inside it — and nothing had to change in validation
plumbing, since `node/problems` already reports every channel's problems.

Both halves of ownership are under test at lane level: a three-frame correction
on the girl's lane reaches across the drawing boundary beneath it and leaves
every frame outside its support identical, and a correction owned by one
exposure travels with that exposure when a hold before it grows.

Regeneration keeps them, which is the obligation the layer design exists to
meet: `rebased` replaces a base and carries its corrections across, and a
correction the new base no longer fits is MARKED rather than dropped or
misapplied — `clip/conflicts` lists those for a view to offer, separately from
`problems`, because a conflict is a decision nobody has made yet and not a
document that will not load. Turning the mouth's `:verts` knob is a real
topology change and is what the test uses. Two latent faults turned up there and
are fixed: `regenerate-head` compared authored channels to measured ones
directly, so the first correction on the head would have stopped it following
re-measurement for good; and an incompatible offset threw in the read path,
which would have taken the stage down on exactly the case the model says to
report.

Still unbuilt: overwrite as a placement policy (which is now `blank` then
`place`, composed inside one transaction), slip source, retime, and deleting
reused content. A lane cannot hold AUDIO occurrences — `sequence-problems`
requires visual ones, though this document says a lane may hold either and
should reject only a mixture.

What is NOT implemented is a command that produces a layer — the doc's Constant
adjustment, Ramp and Return motion — and with it the question of how a view
offers those three over a selected range, and how it offers a conflict for
resolution. Overwrite, the range and retiming
commands (blank, trim, move, slip source, retime) and the exposure-sheet view
are also not implemented; a refusal is the current behavior where the model
demands an explicit choice nobody has made yet. The suite stands at 424 tests
and 5,749 assertions, with `frontend/test/browser/sequence.mjs` driving the
editor through create, hold, overflow, undo, reuse, make unique, duplicate,
split, insert, trim, move and blank. Rewrite tests that encode superseded
behavior rather than preserving behavior to keep them green.

Build small adversarial documents and test their domain operations before
expanding the interface:

| Scenario | Required invariant |
| --- | --- |
| Same drawing exposed twice, then one made unique | Linked edits affect both before copying and only the selected content after |
| Holds, playing inserts, nonzero in-points, and gaps on one lane | Source behavior is independent of property key count and channel encoding |
| Adjacent occurrences, final hold, split, trim, ripple, and overwrite | Exact boundaries, stable IDs, deterministic collision handling |
| Extend a hold under lane keys and occurrence-local corrections | Lane key times remain fixed; later occurrence corrections travel with their owners; source playback origins survive |
| Ripple beyond the symbol end | Explicit extent policy; refusal leaves the document unchanged; resizing and retiming undo together |
| Girl on twos over a moving background | Drawing cadence does not quantize either lane's continuous properties |
| Three-frame correction crossing a drawing boundary | Exact support, same result outside it, one undo step |
| Nested retiming, holds, loops, and fractional sampling | Explicit time spaces; ambiguous inverse edits cannot silently choose a target |
| Audio inside changing source occurrences | Only active intervals sound, with correct trim and source timing |
| Regenerate with corrections and a topology change | Compatible edits survive; incompatible ones produce actionable conflicts |
| Reference cycles and deletion of reused content | Inactive references are validated too; no dangling references |
| Save/load and command undo/redo | Identity, source maps, corrections, and evaluation round-trip |
| Timeline and exposure-sheet invocation of one command | Identical document changes and selection targets |
| Random forward/backward seeks and export | Reference and optimized evaluation agree, including defaults and absence |
| Concurrent commands on overlapping and disjoint targets | Transactions remain valid; conflicts are explicit and undo preserves others' work |

Implementation order: occurrence ownership and playback semantics; shared
commands and validation; correction layers and time-addressing contracts; then
breadcrumb, cel strip, and an exposure-sheet projection. Use those two temporal
views plus direct stage editing to prove the model before broadening the UI.

A new presentation should not require duplicate animation state. A genuinely new
authoring capability may require new domain data. The model is successful when
such additions have a clear owner and compose with existing operations, not when
it can claim that no future feature will ever need another field.