|Description: Candidate definition for
df-cleq 2603 (the need for it is exposed in
bj-ax9 32083). The similarity of the hypothesis and the
it clear that this definition merely extends to class variables
something that is true for setvar variables, hence is conservative.
This definition should be directly referenced only by bj-dfcleq 32086,
which should be used instead. The proof is irrelevant since this is a
proposal for an axiom.
Another definition, which would give finer control, is actually the pair
of definitions where each has one implication of the present
biconditional as hypothesis and conclusion. They assert that
extensionality (respectively, the left-substitution axiom for the
membership predicate) extends from setvars to classes. (Contributed by
BJ, 24-Jun-2019.) (Proof modification is discouraged.)
(New usage is discouraged.)