\once \macro

[Graham Percival]

I could have sworn that

{
  c'4
  \once \stemDown
  c'4
}

worked, but it clearly doesn't.
1)  Does anybody else remember if this worked in the past?  (ie am
I going crazy?)

2)  Would it be possible to enable it?  I don't see any pressing
need for this, except that it would make the docs a little bit
easier to write.  (although since I'm not personally writing this
stuff, I suppose it doesn't really matter to me.  :)

Cheers,
- Graham