Description: Equivalence between two common ways to characterize elements of a class
B : the LHS says that sets are elements of B if and only if they
satisfy ph while the RHS says that classes are elements of B if
and only if they are sets and satisfy ph . Therefore, the LHS is a
characterization among sets while the RHS is a characterization among
classes. Note that the LHS is often formulated using a class variable
instead of the universe _V while this is not possible for the RHS
(apart from using B itself, which would not be very useful).
(Contributed by BJ, 26-Feb-2023)