Will Robertson wrote:
> Ideally, I'd like to be able to write  \def\foo{bar}  and know whether
> or not \foo is expandable; until reading about this test I didn't think
> such a thing was even possible.
> Any comments?

This won't work, will it?  A more complex example:


is not fully expandable, but if you get the test working should give
\c_true.  So I think this might not be a good idea.  (I assume that this
test is supposed to see if something is a macro.  I do that in siunitx,
but using \meaning and looking for "macro".)