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

  static final class SynchronizedListMultimap<
          K extends @Nullable Object, V extends @Nullable Object>

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

  static final class SynchronizedListMultimap<
          K extends @Nullable Object, V extends @Nullable Object>

   * {@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
   * #synchronizedListMultimap}.
   *
   * <p>Call this method only when the simpler methods {@link ArrayListMultimap#create()} and {@link
   * LinkedListMultimap#create()} won't suffice.
   *