intros.
destruct (le_exists_plus H).
subst.
assert (x0 < n) by omega.
destruct (lt_exists_plus H1).
subst.
rewrite plus_comm.
apply In_nb_nats.
Qed.

(* Specifications: *)
Lemma In_as_nb_val n (x: natBelow n) m (l: Vector.t (natBelow n) m):
List.In (nb_val x) (map nb_val l) -> List.In x l.