Description: Proof of ax-11 from the standard axioms of predicate calculus, similar to PM's proof of alcom (PM*11.2). This proof requires that x and y be distinct. Axiom ax-11 is used in the proof only through nfal , nfsb , sbal , sb8 . See also ax11-pm . (Contributed by BJ, 15-Sep-2018) (Proof modification is discouraged.)