@Override
  public int enumerate(final @Nullable Data value) throws IOException {
    final int id = wrappedEnumerator.tryEnumerate(value);
    if (id == NULL_ID) {
      throw new IllegalStateException("Unmodifiable enumerator can't .enumerate() new values, but .enumerate(" + value + ") was requested");
    }
    return id;
  }
@Override
  public int enumerate(final @Nullable Data value)