proposition: Lemma HF_sig_el T x : HFeq <<= T -> T ⊢ x ∈ σ x.
proof: Proof. intros H. apply HF_bunion_el'; trivial. apply DI2. apply HF_sing_iff; trivial. apply HF_refl'. trivial. 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)