Black mensural notation

[Lukas Pietsch]

Hi, I'm quite new to Lilypond programming, but I thought I'd jump in at what
seemed to me to be pretty much the deep end, and try if I could get black
mensural notation implemented. Here's what I've come up with so far: 
http://lukas-pietsch.de/Music/blackmensural.ly (source file)
http://lukas-pietsch.de/Music/blackmensural.pdf (doc)

It's all just scheme functions and embedded postscript, and works for me under
the current stable 2.12.3.

The biggest stumbling block turned out to be black ligatures, which I found no
other way of doing than rewriting from scratch in a rather hackish way,
sidestepping the normal ligature engraver completely; and horizontal spacing,
for which I had to figure out some rather ugly workarounds (probably far from
optimal).

If anybody finds it useful, please feel free to use or modify.

Lukas