proposition: Lemma SortA_app : forall l1 l2, SortA l1 -> SortA l2 -> (forall x y, InA x l1 -> InA y l2 -> ltA x y) -> SortA (l1 ++ l2).
proof: Proof. intros l1; induction l1; intros l2; simpl in *; intuition. inv. constructor; auto. apply InfA_app; auto. destruct l2; auto. Qed.
imports: List Sorted Setoid Basics Morphisms