filter `assign` by unification

[icecream17]

A convenient way to assign a hypothesis (for example, theorem.a $e |- A = ...) in metamath.exe is

assign -0 *.a

This works for most letters (not numbers because of the pm?.? series, et al)

However, a common instance where this doesn't work is for *.x, because c.x $f class .x. $. exists. In mathboxes there is also wl-impchain-com-n.m and wl-impchain-com-1.x.

But obviously class .x. and |- T. don't match with whatever assignment I'm doing. So I would like for assign to realize that these labels don't match and choose the unique one that does (theorem.x in this case).