The reason that "octave=-1" is confusing is that it's dealing with
pitch, and music notation doesn't have pitch.
I agree that the transposition rules in abc were not well thought out. There are really 3 different things (pitch, notation, or both) that you might want transposed, and its not clear from the rules what the octave.. and t=.. mean, except by tradition.
If anybody can think of rules that make sense, and that won't break all existing ABC files, I'd be willing to have iabc conform to it.
