Ltac fold_Vbyte :=
 repeat match goal with |- context [Vint (Int.repr (Byte.signed ?c))] =>
      fold (Vbyte c)
end.

Lemma Znth_map_Vubyte: forall (i : Z) (l : list byte), 0 <= i < Zlength l -> Znth i (map Vubyte l) = Vubyte (Znth i l).