proposition: Lemma sigma_sub x : x ⊆ σ x.
proof: Proof. Proof using VIEQ M_ZF. intros y H. apply sigma_el. 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)