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

Lemma app_inj {A}: forall l l' (L:Zlength l = Zlength l') (m m':list A) (M: Zlength m = Zlength m'), l ++ m = l'++m' -> l=l' /\ m = m'.