proposition: Lemma sigma_el x y : x ∈' σ y <-> x ∈' y \/ x ≡' y.
proof: Proof. Proof using M_ZF. 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.ZF Undecidability.FOL.Syntax.Facts Undecidability.FOL.Semantics.Tarski.FullFacts Undecidability.FOL.Sets(ZF) Undecidability.FOL.Sets.ZF Undecidability.FOL.Reductions.PCPb_to_ZF Lia Undecidability.PCP(PCP Util.PCP_facts Reductions.PCPb_iff_dPCPb) Undecidability(Shared.ListAutomation) Morphisms