Alternating time signatures

[Urs Liska]

Hi,

in my current examination of polymetrics I stumbled over a missing item 
in LilyPond: alternating time signatures.

In contemporary notation (well, actually starting quite early in the 
20th century, so "contemporary" isn't really adequate) it's a common 
concept to write sth like

6 5
8 8

to indicate a section with (irregularly) changing meters.

This can easily be achieved by overriding the TimeSignature stencil, but 
as far as I can tell there is no suitable documentation for this.

There _had_ been a snippet to achieve

6   5
  +
8   8

which is a different thing but could easily be adapted. But (I assume) 
with the inclusion of \compoundTimeSignature this seems to have been 
removed from the docs _and_ the LSR. So you only find the solution in 
some really old and therefore dubious list posts.

I think this is common notation and should be better supported.
I see two different solutions that I volunteer to do if I get 
appropriate feedback here:

a)
Create a new LSR snippet and tag it with "doc"

b)
Create a new command for LilyPond itself, as a complement to 
\compoundTimeSignature and write appropriate documentation.

I prefer solution b) but will only create a patch if I can expect not to 
get objections about the usefulness in itself.

See attached file for my initial approach, with a few comments:

- I assume the function could be implemented more efficiently
- I would allow it to accept lists of lists
  (as compoundTimeSignatures) instead of expecting a hardcoded
   number of four arguments.
- I'm not sure if it's right to expect the user to
  write \revert Score.TimeSignature.stencil in the end, or
  if it would be more consistent to add an explicit command
  for that.

naming suggestion: \alternatingTimeSignature

What do you think?

Urs