Can we use +nan.0?

[David Kastrup]

Hi, I am trying to come up with an alteration value that does not match
"normal" alterations but is neither positive nor negative (since
otherwise a "change" will cause naturals to be printed).

+nan.0 appears fine for that.  But can we rely on it being available on
supported platforms?  I don't find it in the sources of Lilypond, though
there is a C constant for infinity being used in one place, and there is
lots of nan? and the C equivalent in the code.

-- 
David Kastrup