Description: Define the equivalence relation predicate. (Read: R is an equivalence
relation.) For sets, being an element of the class of equivalence
relations ( df-eqvrels ) is equivalent to satisfying the equivalence
relation predicate, see eleqvrelsrel . Alternate definitions are
dfeqvrel2 and dfeqvrel3 . (Contributed by Peter Mazsa, 17-Apr-2019)