Function bass_scale_degree_from_notation 

pub fn bass_scale_degree_from_notation(degree: u8, numbers: &[u8]) -> Result<u8>

Which scale degree a figured-bass column puts in the bass, for a chord whose root stands on degree.

This is music21’s bassScaleDegreeFromNotation. It works the answer out rather than looking it up: a chord of naturals spaced by the column’s own numbers is spelled, its root found, and the bass is that many steps below the root. A column that implies no root at all — 54, say — leaves the root in the bass, which is what the degree already says.