Function probability_for_one_opcode 

pub fn probability_for_one_opcode(
    step: &Opcode,
    source: &str,
    destination: &str,
) -> FloatType

How likely one of the steps that turn a rhythm into another is: music21’s differenceProbabilityForOneOpCode.

source is the measure that would do the correcting and destination the measure flagged, which is how music21 names them. A stretch read alike is PROBABILITY_ON_EQUALITY for each character, a stretch replaced is probability_on_substitute of the two, a stretch the flagged measure has not got is PROBABILITY_ON_OMISSION for each and one it has over is PROBABILITY_ON_ADDITION.