proposition: Lemma M_pair2 x y : y ∈ {x; y}.
proof: Proof. Proof using VIEQ M_ZF. apply M_pair. now right. Qed.
imports: Undecidability.FOL(Syntax.Facts Semantics.Tarski.FullFacts ZF) Undecidability.FOL.Sets(ZF binZF) Undecidability.FOL.Sets.Models(Aczel Aczel_CE Aczel_TD) Coq(Lia)