Ostinato mode: only score the notes on the staff

Keep an ostinato going with timing feedback, and improvise on top without extra hits counting as mistakes.

You’d see exactly which patters pulls your time off, and could loop it until it is tight.

Per-bar option: set ostinato mode on each bar, so one exercise can be “4 bars of time, 4 bars of solo”.

Today: on a MIDI kit, hits on drum groups that aren’t in the exercise are already ignored. Overlapping groups, like a ride over a hi-hat-foot ostinato, need a pad remap.