static <Data> GistData<Data> outdated(int gistStamp) {
      if (gistStamp == NULL_STAMP) {
        throw new IllegalArgumentException("gistStamp(=" + gistStamp + ") must be valid (!=" + NULL_STAMP + ")");
      }
      return new GistData<>(null, gistStamp, false);
    }
static <Data> GistData<Data> outdated(int gistStamp)