Package: Lilypond
In Debian: yes
Web: http://lilypond.org/development/
Description
Lilypond is a language for writing music, using TeX.
Links
Presentation in french : http://logiciels-libres-cndp.ac-versailles.fr/article.php3?id_article=83
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Rough translation of link "Lilypond is a program for writing musical notation, differing from commercial programs in that you do not use the mouse to place notes. Instead, you write all the parts in a mark-up language similar to LaTeX or HTML. You then 'compile' it with Lilypond, viewing the results in (ghostscript or similar). Hence, you only need a text editor, an xterm, and a viewer(dvi or ps) in order to use it. This makes it easy to try.
Lilypond produces very high quality results. If the interface seems too harsh, having to look at source code, a GUI is working, called denemo http://denemo.sourceforge.net. A screenshot is at the foot of the page." (just seen length of page-- source code/screenshots are self-explanatory.)