* comparator} is not serializable.
     */
    public <V0 extends @Nullable Object> SortedSetMultimapBuilder<K0, V0> treeSetValues(
        Comparator<V0> comparator) {
      checkNotNull(comparator, "comparator");
      return new SortedSetMultimapBuilder<K0, V0>() {
        @Override
        public <K extends K0, V extends V0> SortedSetMultimap<K, V> build() {