* read operations will work correctly if the last write <i>happens-before</i> any reads. To allow
 * concurrent update operations, wrap your multimap with a call to {@link
 * Multimaps#synchronizedSetMultimap}.
 *
 * <p><b>Warning:</b> Do not modify either a key <i>or a value</i> of a {@code HashMultimap} in a
 * way that affects its {@link Object#equals} behavior. Undefined behavior and bugs will result.
 *

 * read operations will work correctly if the last write <i>happens-before</i> any reads. To allow
 * concurrent update operations, wrap your multimap with a call to {@link
 * Multimaps#synchronizedSetMultimap}.
 *
 * <p><b>Warning:</b> Do not modify either a key <i>or a value</i> of a {@code HashMultimap} in a
 * way that affects its {@link Object#equals} behavior. Undefined behavior and bugs will result.
 *

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

  private static class SynchronizedSetMultimap<
          K extends @Nullable Object, V extends @Nullable Object>

    checkUnmodifiableMultimap(
        synchronizedSetMultimap(HashMultimap.<@Nullable String, @Nullable Integer>create()), false);
  }

  @J2ktIncompatible
  @GwtIncompatible // SerializableTester
  public void testSerializingUnmodifiableSynchronizedHashMultimap() {
    Multimap<String, Integer> unmodifiable =
        prepareUnmodifiableTests(

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

  private static class SynchronizedSetMultimap<
          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
   * #synchronizedSetMultimap}.
   *
   * <p>Call this method only when the simpler methods {@link HashMultimap#create()}, {@link
   * LinkedHashMultimap#create()}, {@link TreeMultimap#create()}, and {@link

   * {@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