proposition: Lemma cumul_In {X} (L : nat -> list X) x n : In x (L n) -> In x (to_cumul L (S n)).
proof: Proof. intros H. cbn. eapply in_app_iff. eauto. Qed.
imports: Undecidability.Synthetic(DecidabilityFacts SemiDecidabilityFacts EnumerabilityFacts) Undecidability.Shared.Dec List