Definition sign_ext (n: Z) (x: nval) :=
  match x with
  | Nothing => Nothing
  | I m => I (Int.or (Int.zero_ext n m) (Int.shl Int.one (Int.repr (n - 1))))
  | All => I (Int.zero_ext n Int.mone)
  end.

Lemma sign_ext_sound: forall v w x n, vagree v w (sign_ext n x) -> 0 < n < Int.zwordsize -> vagree (Val.sign_ext n v) (Val.sign_ext n w) x.