Description: When the class variables in definition df-clel are replaced with
setvar variables, this theorem of predicate calculus is the result.
This theorem provides part of the justification for the consistency of
that definition, which "overloads" the setvar variables in wel with
the class variables in wcel . (Contributed by NM, 28-Jan-2004)
Revised to use equsexvw in order to remove dependencies on ax-10 ,
ax-12 , ax-13 . Note that there is no disjoint variable condition
on x , y , that is, on the variables of the left-hand side, as
should be the case for definitions. (Revised by BJ, 29-Dec-2020)