OK. I also found that WildFunction nargs are not changed to FiniteSet
anywhere. I tried following the same pattern as in FunctionClass but that
didn't work (i.e. set _nargs in new then make a nargs @property). Any ideas
on how to fix that?

>>> from sympy import *
>>> WildFunction('f', nargs=1)
f_
>>>
>>> WildFunction('f', nargs=1).nargs
(1,)