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The key identifies another event which is in the same voice. This could be Post.hand_key, but it could also match polos to sangsih, since they form a composite part.

Conceptually, all notes move together until they bump into each other. Or, they move without restriction, and then go to midway of the overlap. But the note's start is a hard lower or upper limit, so one note moving can never cause another note to move, it can just cause it to not move as much as it wanted.

TODO actually "half of the overlap" is not the same as "all move together". For the latter, the overlap split depends on how far the note moved to get there. So instead of overlap2 it's 'max 0 (overlap - n) 2', where n is the imbalance between their move offsets.

TODO this is still broken if an offset causes an note to skip over another. But that should be stopped by the next event, right?

Merge notes with Flags.strong and Flags.weak. The rules are that strong notes merge with weaker ones, in the order strong, normal, weak.

Previously I considered multiple weaks or strongs ambiguous, but it turns out I get multiple strongs with two hand strokes at the end of a block, and I might as well allow the rest too, for simplicity.

Handle Flags.infer_duration for notes merged together. This is the case where a final note replaces a coincident initial note. The strong note gets the duration of the longest weak notes, if there are any. If there are no weaks, then there are no coincedent notes to merge, so return the event unchanged so infer_duration_single can handle it.