Description: Commutative law for class equality. Theorem 6.5 of Quine p. 41. (Contributed by NM, 26-May-1993) (Proof shortened by Wolf Lammen, 19-Nov-2019)

|- ( A = B <-> B = A )

|- ( A = B -> A = B )

|- ( A = B -> B = A )

|- ( B = A -> B = A )

|- ( B = A -> A = B )