Function dispatch 

pub fn dispatch(track: &mut TrackState, op: SeqOp) -> SeqEffect

Apply one edit.

The single mutation surface. Everything a key, a menu or a controller can do to a sequencer arrives here, and nothing else in the project writes to a SequencerState.

A track with no sequencer on it is not an error — the same key may be live on an ordinary track — so it returns SeqEffect::NOTHING and changes nothing.