let rec is_included l1 l2 =
match l1, l2 with
| [], _ -> true
| _ :: _, [] -> false
| hd1 :: tl1, hd2 :: tl2 ->
(* [l1] and [l2] are sorted in the same order,
with first the newest element. *)
(hd1:leak) == hd2 && is_included tl1 tl2