proposition: Lemma FST_sig_iff T x y : FSTeq <<= T -> T ⊢ y ∈ σ x -> T ⊢ y ∈ x ∨ y ≡ x.
proof: Proof. intros HT. rewrite FST_adj; auto. intros H. apply (DE H); auto. Qed.
imports: Undecidability.FOL.Syntax.Facts Undecidability.FOL.Semantics.Tarski.FullFacts Undecidability.FOL.Deduction.FullNDFacts Undecidability.FOL.Sets.FST Undecidability.FOL.FST Undecidability.FOL.Reductions.PCPb_to_FST Undecidability.PCP(PCP Util.PCP_facts Reductions.PCPb_iff_dPCPb) Undecidability(Shared.ListAutomation)