A position is an argument, not another command

Everything could only be added to the end, because `append` computed its own
position — the max end of the lane — and so had no opinion to state. Insert is
not a new command; it is the argument that function was missing. `:at` takes a
lane frame or `:end`, `:end` is the position where nothing has to move, and
appending stops being a separate operation from inserting. New, reused and
duplicated drawings all take it, because there was only ever one placement rule.

Placing ripples: occurrences at or after the position move later by the new
exposure's duration, and `:keep` against `:grow-symbol` still decides what
happens at the shot's end. OVERWRITE is deliberately not a policy argument yet.
Taking frames away from the occurrence already there is TRIMMING, and an
argument whose second value is unimplemented is worse than an argument that is
not there. A position strictly inside an existing exposure refuses and names
`split`, rather than splitting on the quiet: one command performing two is how
a command stops being predictable.

Then split, which turned out to cost almost nothing, and that is the
interesting part. The two pieces keep ONE `:time` and differ only in `:span`.
The right piece's own frames therefore carry on exactly where the left's
stopped, so its source clock, its keys and its corrections go on meaning what
they meant: a held drawing holds the same frame either side of the cut, and a
playing insert plays through it without a seam. There is no arithmetic on
in-points to get wrong, and no shot-length question, since the pieces occupy
the frames the one exposure occupied. The test samples every frame before and
after and asserts the picture is identical — for a hold, for an exposure with a
correction of its own, and for a playing insert.

That is not a clever split. It is `:span` being in the node's OWN coordinates,
which was decided long before there were lanes, paying for something it was not
designed for. The same property is why extending a hold leaves lane keys alone.

Both new commands act at the playhead, which needed `lane-frame` — the symbol's
frame as a frame of the lane's own time, nil through a stepped or looping lane
where one is not the other. Nil refuses; it does not snap to a nearby frame.

Two smaller things found while doing it. `placeable` promised "a whole lane
frame" in its refusal and then accepted 2.5, so both it and `split` now require
an integer, as `extend-hold` already did for its delta. And `lane-end` is
private: `:end` is the only way to ask for it.

401 tests, 5,612 assertions. The browser flow now splits an exposure at the
playhead and puts a drawing in the gap, and checks that six exposures are still
one row.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/lane-model.md

@@ -1,9 +1,9 @@
 # The Lane Model
 
 Revised 2026-09-30. Target design. Occurrence ownership, source playback, the
-content and exposure commands and a one-row cel strip are implemented;
-correction layers, the range and retiming commands, and the remaining views are
-not. See the status note under
+content and exposure commands, placement anywhere in a lane, and a one-row cel
+strip are implemented; correction layers, overwrite and 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).
@@ -442,23 +442,43 @@ 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, append drawing, reuse drawing, duplicate drawing, make unique,
-and extend hold with explicit ripple and shot-length policy. 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.
+one — add lane, place a drawing (new, reused or duplicated), make unique, split,
+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.
 
-Correction layers, the range and retiming commands (blank, split, trim, move,
-slip source, retime) and the exposure-sheet view are not implemented; a refusal
-is the current behavior where the model demands an explicit choice nobody has
-made yet. The suite stands at 397 tests and 5,561 assertions, with
+Correction layers, overwrite, and the range and retiming commands (blank, trim,
+move, slip source, retime) and the exposure-sheet view are not implemented; a
+refusal is the current behavior where the model demands an explicit choice
+nobody has made yet. The suite stands at 401 tests and 5,612 assertions, with
 `frontend/test/browser/sequence.mjs` driving the editor through create, hold,
-overflow, undo, reuse, make unique and duplicate. Rewrite tests that encode
-superseded behavior rather than preserving behavior to keep them green.
+overflow, undo, reuse, make unique, duplicate, split and insert. 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: