There was some question previously about the license for this package. From the project page: "CM is released under the LLGPL (Lisp Lesser General Public License)."
For more information, see the CM web page on SourceForge or the CM mailing list.
A common-music page exists to make this ASDF-Installable.
There is a new e-book out by Heinrich Konrad Taube (creator of CM), called Notes from the Metalevel An Introduction to Computer Composition, about computer music representation and composition using CM (and also includes quite a bit of introductory Lisp).
Page in this topic: Common Music Notation
Also linked from: Changes In Autumn 2000 cm Common Lisp Music common-music Debian Rick Taube
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