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

Lemma Z_lxor_byte_neq b1 b2 (B:b1 <> b2): exists b, Z.lxor (Byte.unsigned b1) (Byte.unsigned b2) = Byte.unsigned b /\ b <> Byte.zero.