See also the MEI project at http://music-encoding.org. Not quite the same thing, but still noteworthy. It's an attempt to provide a parallel to TEI (http://www.tei-c.org) for music notation. MEI is still under development, and they're supposedly working on some way to output notation (maybe through Lilypond; it's been a few months since I've kept up with the project).

Missing pedalling articulations, useful to pipe organ, harp... Also I didn't find any way to include registration change, important in pipe organ, harmonium, harpsichord, electroacoustic instruments and others.

How does this compare to LilyPond, which is Scheme-based with a vaguely TeX-like syntax?

from: https://ccrma.stanford.edu/software/cmn/cmn/cmn.html His comment is telling:

Although I put a lot of effort into making the output legible, it really isn't aimed at producing publishable scores. If you need beautiful output, use Lilypond, Score or Finale.

I see. Might be interesting to output Lilypond syntax as a separate backend or something like that... hm!

How do i install this ?

It is loadable via asdf. You can look at a very silly example here: https://github.com/grantham/clef-reading