Optionaltimeline?: Timeline | ChordTimelineThe harmony to write against, as a Timeline or the plain chord
timeline the analysis layer hands out. A timeline carries its own segment
boundaries, so a chord change anywhere — including off the generator's
half-beat probe grid — is seen; prefer it over chordAt.
How a counter line is written: what CounterMelodyOptions asks for minus the settings the composer already holds, with the harmony taken the way Composer.bass takes it.