Interface CompositionUndoneEvent

Number of undone sequences (or individual keys, when undo was called with { byKey: true }). A sequence whose undoAction returned null and stopped the walk was not undone — it stays buffered (any entries already undone are dropped) and is not counted, so an undo that only truncated a sequence reports 0. In isolated mode such a call also returns null; the flat path still returns its seeded context.

Event discriminant — always "undone".