proposition: Lemma ZF_union_el x y z : ZFeq' ⊢ y ∈ x ∧ z ∈ y -> ZFeq' ⊢ z ∈ ⋃ x.
proof: Proof. now apply ZF_union_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)