proposition: Lemma ZF_pair_el x y z : ZFeq' ⊢ (z ≡ x ∨ z ≡ y) -> ZFeq' ⊢ z ∈ {x; y}.
proof: Proof. now apply ZF_pair_el'. Qed.
imports: Undecidability.FOL.ZF Undecidability.FOL.Syntax.Facts Undecidability.FOL.Semantics.Tarski.FullFacts Undecidability.FOL.Sets.ZF Undecidability.FOL.Reductions.PCPb_to_ZF Undecidability.PCP(PCP Util.PCP_facts Reductions.PCPb_iff_dPCPb) Undecidability(Shared.ListAutomation)