Function probability_based_on_changes_between 

pub fn probability_based_on_changes_between(
    matching: &str,
    source: &str,
    destination: &str,
) -> FloatType

How likely the measure source was misread as destination, with the steps read between matching and source: music21’s getProbabilityBasedOnChanges with its two strings told apart.

music21 reads its steps off whatever its SequenceMatcher was set from and reads a replacement off its own hash, which are the same string in every call it makes – probability_based_on_changes is that call – but a caller may set the one and not the other.