Comment #44 on issue 2823 by [email protected]: Patch: Fixes position of mensural c clef
http://code.google.com/p/lilypond/issues/detail?id=2823
The comment above is from Rietveld, not my current position. I've made a _very_ initial stab at a new clef, and as a work-in-progress it's on Rietveld for comment.
