Function make_voices 

pub fn make_voices(measure: &mut Stream)

Puts a measure’s notes into voices where they overlap: music21’s makeVoices with fillGaps=False.

There are as many voices as notes sounding at once at the most, and each note goes to the first voice that has finished by the time it starts. A note no voice is free for is lost, as it is there. A measure whose notes never overlap is left alone.