Definition maskzero (x: aval) (mask: int) : abool :=
  match x with
  | I i => Just (Int.eq (Int.and i mask) Int.zero)
  | Uns p n => if Int.eq (Int.zero_ext n mask) Int.zero then Maybe true else Btop
  | _ => Btop
  end.

Lemma maskzero_sound: forall mask v x, vmatch v x -> cmatch (Val.maskzero_bool v mask) (maskzero x mask).