Optionalinstrument?: InstrumentProfileLikeThe instrument the line is written for, as a plain StringedProfile
or an Instrument. Giving one is itself the request that the line be
playable on it: octave then says where in that instrument to aim rather
than which absolute band to use, and where the two disagree the instrument
wins — a note below the lowest string comes back an octave up, the way a
player would take it. Leave it out for a programmed part.
How a bass line is written: what BassLineOptions asks for minus the settings the composer already holds, with the instrument taken the way the rest of the class API takes one.