Holding a drawing and looping a one-frame symbol are the same picture, and the
second is a case the model already carries, so the first is a special case
kept for nothing. Speed 0 goes; a cel becomes a one-frame symbol with
`:end :loop` and a span. Looping and span are already properties of the
instance and `:frames` already belongs to the symbol, so nothing moves — a
mode is deleted. Two spellings of looping collapse to one, `:time :loop?`
giving way to `:playback :end :loop`, and `extend-hold` collapses into
`resize-out`, since it exists only to refuse anything that is not frozen
before editing a span.
What it buys is one rule where there were three refusals. `nest/inside` has no
invertible clock for a hold, for `:end :hold`, or for a loop, so
shift-to-reparent refuses all three — which is most of what anybody would drag
onto. Resolving the move with the destination's map at the CURRENT frame
covers every one: a loop is affine within the period the frame falls in, a
one-frame loop is that rule with a period of one — which lands exactly where
the nesting notes argued it should from first principles — and `:end :hold` is
affine in the played part and frozen in the tail.
The trap is written down beside it, because it would be found the hard way:
`audio-tracks` expands a loop into one walk per period, and is saved today
only by the `(pos? speed)` guard that a held cel fails. Make every drawing a
loop and a drawing held for 120 frames becomes 120 walks emitting any nested
sound 120 times. An audibility precheck has to land in the same change.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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