Attempt merging patches in callback sequencer

[ddworak]

TBH I'm not a fan of both solutions right now. In a test case you've pointed out I'd expect single patch emitted with:

• 5 elements removed: (1, 2), (3, 4), (7, 8), (9, 10), (13, 14),
• 4 elements added: (3, 4), (7, 8), (9, 10), (13, 14).

If we can't implement a structure listener this way, I'd prefer previous behaviour (emitting an "intermediate" patch with defaultA value used, and then a "final" one) than current (emitting the "final" patch, and removing/adding the same afterwards). However we should fire a structure listener once IMO if both elements are removed from numbers within a CallbackSequencer.

Originally posted by @bgrochal in #735 (comment)