LilyPond is the GNU Project music typesetter.  This program can print
beautiful sheet music from a music definition file.  It can also play
mechanical performances to a MIDI file.  Features include multiple
staffs, meters, clefs, keys, lyrics, versatile input language, cadenzas,
beams, slurs, triplets, named chords, transposing, formatting scores, 
part extraction.  It includes a nice font of musical symbols.
