| Description: Extend wff definition to
include the membership connective between
classes.
For a general discussion of the theory of classes, see
https://us.metamath.org/mpeuni/mmset.html#class.
(The purpose of introducing     here is to allow us to
express i.e. "prove" the wel 1711 of
predicate calculus in terms of the
wceq 1642 of set theory, so that we don't
"overload" the connective
with two syntax definitions. This is done to prevent ambiguity that
would complicate some Metamath parsers. The class variables and
are
introduced temporarily for the purpose of this definition but
otherwise not used in predicate calculus. See df-clab 2340 for more
information on the set theory usage of wcel 1710.) | 
