Free/Libre music

[Question] License for Sheet Music (Lilypond)

I have been writing some music (mostly for classical guitar) and have been engraving it with lilypond (fantastic tool for free quality engraving). I want to put all of these compositions into a git repository and release them to the world under a permissive license. I would like to pick a copyleft license a la GPLv3, but cursory ...