Function roman_inversion_name 

pub fn roman_inversion_name(chord: &Chord, inversion: Option<u8>) -> String

The figured-bass suffix a Roman numeral takes for a chord’s inversion: music21’s romanInversionName, 6 and 64 for a triad, 7, 65, 43 and 42 for a seventh chord, and nothing otherwise. The inversion is the chord’s own unless one is given.