I've started setting a bit of mensural music, from Dufay. All of the music
uses a C clef, similar to the mensural-c. However, the gap between the
upper and lower half of the clef is _exactly_ centred on the c staff line.
Compare this with the lilypond mensural C clef - it's centred on the B
below. This seems wrong to me. All the other C clefs are centred on the C.
Do we know why?