static <K extends @Nullable Object, V extends @Nullable Object> Multimap<K, V> multimap(
      Multimap<K, V> multimap, @Nullable Object mutex) {
    if (multimap instanceof SynchronizedMultimap || multimap instanceof BaseImmutableMultimap) {
      return multimap;
    }
    return new SynchronizedMultimap<>(multimap, mutex);
  }

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

  static <K extends @Nullable Object, V extends @Nullable Object> Multimap<K, V> multimap(
      Multimap<K, V> multimap, @Nullable Object mutex) {
    if (multimap instanceof SynchronizedMultimap || multimap instanceof BaseImmutableMultimap) {
      return multimap;
    }
    return new SynchronizedMultimap<>(multimap, mutex);
  }

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

 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained">immutable collections</a>.
 *
 * @author Jared Levy
 * @since 2.0
 */
@GwtCompatible
public abstract class ImmutableMultimap<K, V> extends BaseImmutableMultimap<K, V>
    implements Serializable {

  /**
   * Returns an empty multimap.
   *
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */

 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained">immutable collections</a>.
 *
 * @author Jared Levy
 * @since 2.0
 */
@GwtCompatible
public abstract class ImmutableMultimap<K, V> extends BaseImmutableMultimap<K, V>
    implements Serializable {

  /**
   * Returns an empty multimap.
   *
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */