P: E is extensional & for all y. y E x <-> P(y)>So,xEX <-> ...where... is a statement quantifying over relations and predicates.> Axioms:>> [1] If E, D are primitive binary relation symbols then:>> E,D are extensional -> (For all x,y: (for all z. z E x iff z D y) ->> x=y).>> is an axiom.>So, for definiteness, letE be membershipD be initial segmentin a theory for which every limit ordinal is a mode