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

Lemma combine_app {A B}: forall (l1:list A) (m1:list B) l2 m2 (L:Zlength l1 = Zlength m1) (M: Zlength l2 = Zlength m2), combine (l1 ++ l2) (m1 ++ m2) = combine l1 m1 ++ combine l2 m2.