Mullein

Copperbox revision 570.

More work on the ABC output pretty printers and a new module started for LilyPond pretty printers.

For both formats I'm only going for a small subset of output combinators - ABC really only needs fields and only a subset of the available ones. LilyPond needs quite a lot more, but it seems likely that the most appropriate way of generating all but the simplest LilyPond tune is to generate named melodies with Mullein and include them in a handwritten top level .ly file.