proposition: Lemma in_combine_l : forall (l:list A)(l':list B)(x:A)(y:B), In (x,y) (combine l l') -> In x l.
proof: Proof. intro l; induction l as [|a l IHl]. - simpl; auto. - intro l'; destruct l' as [|a0 l']; simpl; auto; intros x y H. + contradiction. + destruct H as [H|H]. * injection H; auto. * right; apply IHl with l' y; auto. Qed.
imports: PeanoNat