Description: Extend wff definition to include class equality.

For a general discussion of the theory of classes, see
mmset.html#class .

(The purpose of introducing wff A = B here, and not in set theory
where it belongs, is to allow us to express, i.e., "prove", the weq of
predicate calculus in terms of the wceq of set theory, so that we do
not "overload" the = connective with two syntax definitions. This
is done to prevent ambiguity that would complicate some Metamath
parsers. For example, some parsers - although not the Metamath program
- stumble on the fact that the = in x = y could be the = of
either weq or wceq , although mathematically it makes no difference.
The class variables A and B are introduced temporarily for the
purpose of this definition but otherwise not used in predicate calculus.
See df-cleq for more information on the set theory usage of
wceq .)