Corollary or_interval:
  forall a b, 0 <= unsigned (or a b) < two_p (Z.max (size a) (size b)).

(* Specifications: *)
Theorem size_xor: forall a b, size (xor a b) <= Z.max (size a) (size b).
Theorem size_xor: forall a b, size (xor a b) <= Z.max (size a) (size b).