Texinfo @ifset question

[Mark Polesky]

I just realized that a 2.12 user with contributor
aspirations might start reading the 2.12 CG (or a slightly
outdated 2.13.10 CG) instead of the 2.13.11 version:

http://lilypond.org/doc/v2.12/Documentation/devel/contrib-guide/index
http://lilypond.org/doc/v2.13/Documentation/contributor/index

Both of these can be found by following links on the
http://lilypond.org/web/documentation page, and even the
`latest development' link on that page is not as current as
the one on Reinhold's site.

I thought about adding a @warning to the CG that's in the
stable/2.12 branch but for ease of maintenance I'd prefer to
also automate it with the @ifset command:

**********

@ifset stable
@warning{This is an outdated version of the Contributor's
Guide (stable, not development).  If you are interested in
contributing, please access the current version at
@uref{http://kainhofer.com/~lilypond/Documentation/web/development.html}.
If that link doesn't work, go to
@uref{http://lilypond.org/doc/latest/Documentation/} and
navigate to the Contributor's Guide (in the Development
section).}
@end ifset

**********

I'm just a little hazy on how to set up something like this.
I looked at texinfo-rules.make but the syntax is foreign to
me.

1) Is this a dumb idea?
2) If not, anyone want to help me with it?

Thanks!
- Mark