> I think the proper way is to make it a music macro; is it OK to keep
> it a music function until music macros exist? (also, the music
> function code attempts to signal errors when \once is used in the
> wrong place, so from a user's point of view, little changes)
Why do you want to make it a macro?
It removes a rule from the parser, and it makes it possible to reuse
\once for future commands where it's relevant.
Because \once is tied to property settings,
\set #'foo=#bar
translates into somethign like
(property-operation 'set '() 'foo 'bar)
and \once \set #'foo=#bar
would naturally translates into something like
(property-operation 'set '() 'foo 'bar #t)
i.e., it's natural to view \once as operating on the syntax.
(I realise now that I can achieve the same inside parser by shuffling
around rules a bit, but I don't see any advantage)