| By the way, now that we're on the topic, what is the purpose of the ABC
| "Hp" key? (F sharp/C sharp/G natural.) Is that just a more explicit way
| of showing A mixolydian? (Why A mixolydian, for that matter...)
It's supposed to produce "bagpipe" notation. Why "Hp" I'm not sure.
But several programs (abc2mtex, abc2ps) recognize this and produce
music that looks like what (Scottish) pipers are used to reading.