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

  static class SynchronizedMultimap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedObject implements Multimap<K, V> {

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

  static class SynchronizedMultimap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedObject implements Multimap<K, V> {

import static com.google.common.collect.Lists.newArrayList;
import static com.google.common.collect.Lists.transform;
import static com.google.common.collect.Maps.immutableEntry;
import static com.google.common.collect.Multimaps.synchronizedMultimap;
import static com.google.common.collect.ReflectionFreeAssertThrows.assertThrows;
import static com.google.common.collect.Sets.newHashSet;
import static com.google.common.collect.Sets.newLinkedHashSet;

import static com.google.common.collect.Multimaps.filterKeys;
import static com.google.common.collect.Multimaps.synchronizedListMultimap;
import static com.google.common.collect.Multimaps.synchronizedMultimap;
import static com.google.common.collect.Multimaps.synchronizedSetMultimap;
import static com.google.common.collect.Multimaps.synchronizedSortedSetMultimap;

   * {@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
   * #synchronizedMultimap}.
   *
   * <p>Call this method only when the simpler methods {@link ArrayListMultimap#create()}, {@link
   * HashMultimap#create()}, {@link LinkedHashMultimap#create()}, {@link