See interpret.py. Chord alterations are handled blindly, without taking any context from the given chord. This is fine if the alteration is (#5), for example, since 5 is unambiguous. However, with 7, whether we are altering the major or minor seventh depends on context. Currently, the approach is to replace all seventh notes in the chord with a minor seventh if the alteration is (b7), and to replace with a major seventh if the alteration is (#7). However, I'm not sure this scales well to (bb7) and (##7).