[Edgar Aichinger]
Oh and noone has mentioned Lilypond yet, or ABC - both
being notation
"programming languages" as well, with existing GUI frontends of some
sort, Denemo for Lilypond and runabc, flabc and others for ABC.
ABC is particularly dear to me. Its use can become a bit arcane for
specialised tasks but thanks to abcm2ps getting from zero to a basic
score that's easy to read and edit is a breeze.
Tim