CountingSupplier<TreeSet<Integer>> factory = new SortedSetSupplier();
    Map<Color, Collection<Integer>> map = Maps.newEnumMap(Color.class);
    SortedSetMultimap<Color, Integer> multimap = Multimaps.newSortedSetMultimap(map, factory);
    // newSortedSetMultimap calls the factory once to determine the comparator.
    assertEquals(1, factory.count);
    multimap.putAll(Color.BLUE, asList(3, 1, 4));
    assertEquals(2, factory.count);

      return new SortedSetMultimapBuilder<K0, V0>() {
        @Override
        public <K extends K0, V extends V0> SortedSetMultimap<K, V> build() {
          return Multimaps.newSortedSetMultimap(
              MultimapBuilderWithKeys.this.<K, V>createMap(), new TreeSetSupplier<V>(comparator));
        }
      };
    }

    /** Uses an {@link EnumSet} to store value collections. */