SetMultimap<K, V> multimap, @CheckForNull Object mutex) {
    if (multimap instanceof SynchronizedSetMultimap || multimap instanceof BaseImmutableMultimap) {
      return multimap;
    }
    return new SynchronizedSetMultimap<>(multimap, mutex);
  }

  static class SynchronizedSetMultimap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedMultimap<K, V> implements SetMultimap<K, V> {

      SetMultimap<K, V> multimap, @CheckForNull Object mutex) {
    if (multimap instanceof SynchronizedSetMultimap || multimap instanceof BaseImmutableMultimap) {
      return multimap;
    }
    return new SynchronizedSetMultimap<>(multimap, mutex);
  }

  static class SynchronizedSetMultimap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedMultimap<K, V> implements SetMultimap<K, V> {

   * {@code map} and the instances generated by {@code factory} are. Concurrent read operations will
   * work correctly. To allow concurrent update operations, wrap the multimap with a call to {@link
   * #synchronizedSetMultimap}.
   *
   * <p>Call this method only when the simpler methods {@link HashMultimap#create()}, {@link
   * LinkedHashMultimap#create()}, {@link TreeMultimap#create()}, and {@link