Function convert_key_string_to_music21_key_string 

pub fn convert_key_string_to_music21_key_string(text: &str) -> String

music21’s convertKeyStringToMusic21KeyString: a key written with b for flat, as in Bb or eb, rewritten with music21’s -, so Bb is B- and bb is b-. A lone b is B, and anything without a trailing b is returned as it is.