Definition val_inject_opt (j: meminj) (v1 v2: option val) :=
  match v1, v2 with Some v1', Some v2' => val_inject j v1' v2'
  | None, None => True
  | _, _ => False
  end.

Lemma forward_nextblock: forall m m', mem_forward m m' -> (Mem.nextblock m <= Mem.nextblock m')%positive.