Function verticality_positions_at 

pub fn verticality_positions_at(
    stream: &Stream,
    offset: FloatType,
) -> Vec<(usize, Vec<usize>)>

verticality_at by position: for each part holding anything at the offset, the part’s index and the positions in Stream::leaves of what it holds there, for a caller that keeps something of its own beside each element.

use music21_rs::{voiceleading::verticality_positions_at, Note, Stream};

let mut stream = Stream::new();
stream.insert(0.0, Note::from_name("C4")?);
stream.insert(1.0, Note::from_name("D4")?);
assert_eq!(verticality_positions_at(&stream, 0.5), [(0, vec![0])]);