Function voice_chord_notation 

pub fn voice_chord_notation(
    root: &Pitch,
    kind: &str,
    notation: Option<&str>,
    bass: Option<&Pitch>,
    modifications: &[ChordStepModification],
) -> Result<ChordVoicing>

The same, with the root and bass as music21 is left holding them: its _updatePitches in full.