BOOMS Object Oriented Music System
For my Master thesis, I have developed a music editor that is actually a structure editor that can be used for any kind of creative editing. The system is very old, and it was written in a version of Allegro lisp from 1995 that is no longer supported. I have recently found an old archive with the system that actually still runs — and managed to get a relatively small archive that actually works; I'm making it available with permission from the people at Franz.
The system and the principles behind it are documented in my Master thesis and the journal article (found on the previous page), and there's also a very brief overview I gave at a talk in CMU.
To the best of my knowledge, all of this is still not something that existing systems do. Perhaps I will revive this some time in the future...