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 app_eq A m (x x': Vector.t A m) n (y y': Vector.t A n): x = x' -> y = y' -> app x y = app x' y'.