proposition: Lemma HF_eq_bunion T x y x' y' : HFeq <<= T -> T ⊢ x ≡ x' -> T ⊢ y ≡ y' -> T ⊢ x ∪ y ≡ x' ∪ y'.
proof: Proof. intros HT H1 H2. now apply HF_eq_union, HF_eq_pair. Qed.
imports: Undecidability.FOL.ZF Undecidability.FOL.Syntax.Facts Undecidability.FOL.Semantics.Tarski.FullFacts Undecidability.FOL.Sets.ZF Undecidability.FOL.Reductions.PCPb_to_HF Undecidability.PCP(PCP Util.PCP_facts Reductions.PCPb_iff_dPCPb) Undecidability(Shared.ListAutomation)