return (valueComparator == null)
        ? new ImmutableSet.Builder<V>()
        : new ImmutableSortedSet.Builder<V>(valueComparator);
  }

  /**
   * @serialData number of distinct keys, and then for each distinct key: the key, the number of
   *     values for that key, and the key's values
   */
  @GwtIncompatible // java.io.ObjectOutputStream
  @J2ktIncompatible