pub fn unwrap<'t, 'a>(
verticalities: &[Verticality<'t, 'a>],
) -> Vec<(Option<&'a Stream>, Horizontality<'t, 'a>)>Expand description
Each part’s timespans across a run of verticalities – those sounding
across or starting at the first, then those starting at the rest –
part by part in the order first met: music21’s
VerticalitySequence.unwrap.