Definition plusnb (y x: nat) (nb: natBelow x): natBelow (x + y) :=
  match nb in natBelow x' return natBelow (x' + y) with
  | mkNatBelow a b => eq_rec_r natBelow (mkNatBelow a (b + y)) (eq_S _ _ (plus_assoc_reverse a b y))
  end.

Lemma map_cons A B (f: A -> B) (h: A) n (t: Vector.t A n): map f (Vector.cons h t) = Vector.cons (f h) (map f t).