Lily version operators documentation

[Urs Liska]

Hi,

my patch https://sourceforge.net/p/testlilyissues/issues/5067/
http://codereview.appspot.com/317270043 is currently on countdown. It
introduces the procedures

- lilypond>?
- lilypond>=?
- lilypond<?
- lilypond<=?
- lilypond=?

comparing a given version to the one currently compiling the document.
This makes it possible to write library code supporting multiple
LilyPond versions across syntax changes.

My question is: Where can I add documentation for this? Browsing through
Extending and IR doesn't seem to indicate a suitable place. It *might*
be fitting somewhere in the "General input and output" of the NR, but
I'm way from being sure about that either.

Or are these functions part of what should eventually be documented
alongside the ly:something Scheme functions?

Urs


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