proposition: Lemma incl_decidable (l l':list A) : decidable (incl l l').
proof: Proof. Proof using A dec. induction l as [|a l IH]. - left. inversion 1. - destruct (In_decidable a l') as [IN|IN]. + destruct IH as [IC|IC]. * left. destruct 1; subst; auto. * right. contradict IC. intros x H. apply IC; now right. + right. contradict IN. apply IN; now left. Qed.
imports: List Decidable