Corollary and_interval:
  forall a b, 0 <= unsigned (and a b) < two_p (Z.min (size a) (size b)).

(* Specifications: *)
Theorem size_or: forall a b, size (or a b) = Z.max (size a) (size b).
Theorem size_or: forall a b, size (or a b) = Z.max (size a) (size b).