Converting ABC to PS/PDF with variable number of staves in different parts of
the music.

The problem is/was a piece of music summarizable as:

P: SOLO.
[V:1] [| Phrase A1 |]
%
P: DUET.
[V:1] [| Phrase B1 |]
[V:2] [| Phrase A2 |]
%
[V:1] [| Phrase A3 |]
[V:2] [| Phrase B2 |]
%
P: TRIO.
[V:1] |: Phrase A4 :|
[V:2] |: Phrase B3 :|
[V:3] |: Phrase C1 :|

This works fine  with  the  original  abc2ps  and  my  jcabc2ps,  but  failed
spectacularly with abcm2ps (and apparently some other software).  The problem
is that some software (e.g., [jc]abc2ps]  "restarts"  formatting  at  the  P:
lines, so the different parts with different numbers of staves are handled as
expected.  However, abcm2ps has  a  different  interpretation  based  on  the
ability to have [P:...] inline, and doesn't start reformatting the music. The
result is that the first occurrences  of  [V:1],  [V:2]  and  [V:3]  are  put
together  in  the  first  system  of 3 staves, and everything goes wrong from
there on.

There seems to be two solutions.  Jef Moine (abc2ps's author) suggested this:

%%score 1
P: SOLO.
[V:1] [| Phrase A1 |]
%
%%score 1 2
P: DUET.
[V:1] [| Phrase B1 |]
[V:2] [| Phrase A2 |]
%
%%score 1 2 3
P: TRIO.
[V:1] |: Phrase A4 :|
[V:2] |: Phrase B3 :|
[V:3] |: Phrase C1 :|

This seems to work, though it fails with software that doesn't implement  the
%%score  directive.   It's  probably  specific  to  abcm2ps,  unless  it gets
incorporated into a future standard.

Another solution that I stumbled across and Jef also mentions is  to  include
"empty" staff lines, such as:

%%staffnonote 0 % Suppress staffs with no notes
P: SOLO.
[V:1] [| Phrase A1 |]
[V:2] [| invisible rests up to P: DUET |]
[V:3] [| invisible rests up to P: TRIO |]
%
P: DUET.
[V:1] [| Phrase B1 |]
[V:2] [| Phrase A2 |]
...

This works with abcm2ps, but is based on the clumsy idea of  including  dummy
staff  lines  with  only  rests,  plus  a  command that suppresses them.  The
%%staffnonote directive is probably not implemented  by  much  software,  and
isn't in any "standard", so it will probably fail with a lot of ABC software.

... to be continued as I get more info ...