\once \revert

[David Kastrup]

Up to now, \once \revert is not really documented nor used.  I have not
yet dug through the existing code in order to figure out what it does if
anything (most likely ignoring \once, but not sure).

In order to not have the override/revert stack get into unexpected
interactions, I want to change \once\override to be impervious to normal
reverts.

That would mean that \once\revert is an obvious candidate for reverting
a \once\override before its time.  However, I have no idea whether there
is an actual sensible use for that functionality.

\once\revert could also mean to let a current non-once override become
inactive just for the current time step.  That's likely harder to
implement, I think.  Again, I have no idea whether there is an actual
sensible use for that either, and it looks rather ad-hoc.

IF one wanted to get crazy, one could use \once\revert for one of the
two things, and \revert\once for the other.  Which one for which, and
would anybody remember that?

Since the parser permits \once\revert, I tend towards making it match
the last \once\override.  It likely does not make much sense, but then
it is sort of easy to understand.

-- 
David Kastrup