One of the two equality axioms of standard predicate calculus, called reflexivity of equality. (The other one is stdpc7 .) Axiom 6 of [Mendelson] p. 95. Mendelson doesn't say why he prepended the redundant quantifier, but it was probably to be compatible with free logic (which is valid in the empty domain). (Contributed by NM, 16-Feb-2005.)