static <Data> GistData<Data> valid(@Nullable Data gistData,
                                       int gistStamp) {
      if (gistStamp == NULL_STAMP) {
        throw new IllegalArgumentException("gistStamp(=" + gistStamp + ") must be valid (!=" + NULL_STAMP + ")");
      }
      return new GistData<>(gistData, gistStamp, true);
    }
static <Data> GistData<Data> valid(@Nullable Data gistData,
                                       int gistStamp)