[texhax] Test for \mathopen

[Joel C. Salomon]

A few days ago on comp.text.tex (in thread "reduce spacing in
differentials") I mentioned trying to understand some code in this
TUGboat article: <http://www.tug.org/TUGboat/Articles/tb18-1/tb54becc.pdf>.

Basically, there is the line
	\futurelet\diffarg\opspace
and the definition of \opspace goes through a series of "\ifx\diffarg\{"
to determine if the following token is an opening brace.  This, of
course, only accounts for the brace types the macro author thought of.

Is there an easier way?  I'd like to reduce the code to something like
	\def\opspace{\ifismathopen\diffarg\relax\!\fi}
-- but is there an \ifismathopen test?

—Joel Salomon