proposition: Lemma ZF_bunion_el1 T x y z : ZFeq' <<= T -> T ⊢ z ∈ x -> T ⊢ z ∈ x ∪ y.
proof: Proof. intros HT H. now apply ZF_bunion_el', DI1. 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)