proposition: Lemma sing_el x y : x ∈ M_sing y <-> x = y.
proof: Proof. Proof using VIEQ M_ZF. split. - now intros [H|H] % M_pair. - intros ->. apply M_pair. now left. 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)