Definition bytes_at x q i mbytes :=
match x with
  Vint _ => list_repeat (Z.to_nat i) (Byte.zero)
| _ => sublist q (q+i) mbytes
end.

(* Specifications: *)
Lemma Zlength_bytes_at x q i mbytes : 0<=q -> 0 <= i -> q + i <= Zlength mbytes -> Zlength (bytes_at x q i mbytes) = i.