proposition: Lemma FIB_HF3 b x : i_f b x <> None -> x <> None.
proof: Proof. destruct x as [ [s HS] |]; cbn; congruence. Qed.
imports: Lia Arith Undecidability.PCP.PCP Undecidability.PCP.Util.PCP_facts Undecidability.Synthetic.DecidabilityFacts Undecidability(FOL.Utils.FullSyntax) Undecidability(FOL.FSAT) Undecidability.Shared.ListAutomation