alternate notes within a part

[David Zelinsky]

I'm engraving a part that can be played either on cello or bassoon, but
with several differennces for short sections:  e.g. a clef change for
one and not the other; a different octave for a few measures; double
stops or not.

I want to have just one version of the source, assigned to a variable
(e.g. cello-bassoon-notes = {...} ), with the differences indicated by
short tagged sections (like \tag #'cello {...} \tag #'bassoon {...} ),
so that I can produce output for each instrument seperately from the
same source.

There seem to be a couple of problems using tags like this.  First, it's
kludgy because when the notes are parsed, Lilypond includes all notes
from both tagged parts, and complains about bar check failures.  That
doesn't really matter, since when the notes are used (as say
\keepWithTag #'cello) it all comes out right.  And I can avoid the
warnings if I tag full measures only.  But as I said, it's kludgy.

Worse is that a clef change in one tagged part affects all the
subsequent music.  And similarly, in \relative mode, the tags are
ignored when Lilypond determines the octave of following notes.

Is there a better way to accomplish what I'm trying to do?  Or do I
really just need to maintain completely separate versions for the two
instruments?

-David