proposition: Lemma sigma_el x y : x ∈ σ y <-> x ∈ y \/ x = y.
proof: Proof. Proof using M_ZF VIEQ. split. - intros [H|H] % binunion_el; auto. apply sing_el in H. now right. - intros [H| ->]; apply binunion_el; auto. right. now apply sing_el. Qed.
imports: Undecidability.FOL(Syntax.Facts Semantics.Tarski.FullFacts) Undecidability.FOL(Sets.ZF) Undecidability.FOL(ZF) Undecidability.FOL.TRAKHTENBROT(hfs) Undecidability.Shared.Libs.DLW.Utils(finite) Undecidability(Shared.ListAutomation) Lia