Multimap<K, V> synchronizedMultimap(Multimap<K, V> multimap) {
    return Synchronized.multimap(multimap, null);
  }

  /**
   * Returns an unmodifiable view of the specified multimap. Query operations on the returned
   * multimap "read through" to the specified multimap, and attempts to modify the returned
   * multimap, either directly or through the multimap's views, result in an {@code

          Maps.uniqueIndex(ImmutableSet.of("one", "uno"), Functions.constant(1));
      fail();
    } catch (IllegalArgumentException expected) {
      assertThat(expected.getMessage()).contains("Multimaps.index");
    }
  }

  /** Null values are not allowed. */
  public void testUniqueIndexNullValue() {
    List<@Nullable String> listWithNull = Lists.newArrayList((String) null);
    try {

          Maps.uniqueIndex(ImmutableSet.of("one", "uno"), Functions.constant(1));
      fail();
    } catch (IllegalArgumentException expected) {
      assertThat(expected.getMessage()).contains("Multimaps.index");
    }
  }

  /** Null values are not allowed. */
  public void testUniqueIndexNullValue() {
    List<@Nullable String> listWithNull = Lists.newArrayList((String) null);
    try {

  }

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