feature proposal: /partcombineUp and /partcombineDown

[Mark Polesky]

(this is a copy of an unanswered post on lilypond-devel:
http://lists.gnu.org/archive/html/lilypond-devel/2010-03/msg00089.html)

While answering a question on -user, I found what looks like
an easily-solvable (?) limitation with /partcombine.  The
part-combiner.scm file is a little beyond me, but am I right
in thinking that /partcombine uses the equivalent of
/voiceOne and /voiceTwo when redistributing the notes?

If that's true, then the proposed functions would use:

partcombineUp   (voiceOne + voiceThree)
partcombineDown (voiceTwo + voiceFour)

This would allow combinations of 3 or even 4 voices to share
a common staff while taking advantage of the partcombine
functionality.  Would this be a trivial extension or is it
more complicated than that?

Thanks.
- Mark