Fixpoint iterShr8 u n :=
  match n with O => u
   | S n' => Int.shru (iterShr8 u n') (Int.repr 8)
  end.

Lemma ByteOr_zero b1 b2 (B: Byte.or b1 b2 = Byte.zero): b1=Byte.zero /\ b2=Byte.zero.