@Override
  public final boolean retainAll(Collection<?> elementsToRetain) {
    return Multisets.retainAllImpl(this, elementsToRetain);
  }

  @Override
  public abstract void clear();

  // Views

  @LazyInit private transient @Nullable Set<E> elementSet;

  @Override
  public Set<E> elementSet() {
    Set<E> result = elementSet;
    if (result == null) {

 * the standard collection operations {@link #add(Object)} or {@link #remove(Object)} are encouraged
 * to implement the related methods as well. Finally, two collection views are provided: {@link
 * #elementSet} contains the distinct elements of the multiset "with duplicates collapsed", and
 * {@link #entrySet} is similar but contains {@link Entry Multiset.Entry} instances, each providing

      TestSortedSetGenerator<E> delegate, Bound to, Bound from) {
    return using(new NavigableSetSubsetTestSetGenerator<E>(delegate, to, from));
  }

  /** Create a suite whose maps are descending views of other maps. */
  private TestSuite createDescendingSuite(
      FeatureSpecificTestSuiteBuilder<?, ? extends OneSizeTestContainerGenerator<Collection<E>, E>>
          parentBuilder) {
    TestSetGenerator<E> delegate =

   * correct, but not that they're actually forwarding correctly. We also rely
   * on the other tests (e.g., SynchronizedSetTest) to verify that the
   * collection views are synchronized correctly.
   */

  public void testSize() {
    int unused = create().size();
  }

  public void testIsEmpty() {
    boolean unused = create().isEmpty();
  }

   * correct, but not that they're actually forwarding correctly. We also rely
   * on the other tests (e.g., SynchronizedSetTest) to verify that the
   * collection views are synchronized correctly.
   */

  public void testSize() {
    int unused = create().size();
  }

  public void testIsEmpty() {
    boolean unused = create().isEmpty();
  }

 *
 * <p>Each of the {@code standard} methods, where appropriate, use the comparator of the map to test
 * equality for both keys and values, unlike {@code ForwardingMap}.
 *
 * <p>The {@code standard} methods and the collection views they return are not guaranteed to be
 * thread-safe, even when all of the methods that they depend on are thread-safe.
 *
 * @author Mike Bostock
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible
/*

 * secondary maps. This class provides rapid access to records by the row key alone or by both keys,
 * but not by just the column key.
 *
 * <p>The views returned by {@link #column}, {@link #columnKeySet()}, and {@link #columnMap()} have
 * iterators that don't support {@code remove()}. Otherwise, all optional operations are supported.

        Iterator<? extends Entry<? extends E>> iterator2 = multiset2.entrySet().iterator();
        // TODO(lowasser): consider making the entries live views
        return new AbstractIterator<Entry<E>>() {
          @Override
          protected @Nullable Entry<E> computeNext() {
            if (iterator1.hasNext()) {

        Iterator<? extends Entry<? extends E>> iterator2 = multiset2.entrySet().iterator();
        // TODO(lowasser): consider making the entries live views
        return new AbstractIterator<Entry<E>>() {
          @Override
          protected @Nullable Entry<E> computeNext() {
            if (iterator1.hasNext()) {

 * #entries()}, and {@link #asMap()} return collections that are views of the multimap. If the
 * multimap is modified while an iteration over any of those collections is in progress, except
 * through the iterator's methods, the results of the iteration are undefined.
 *
 * <p>Keys and values may be null. All optional multimap methods are supported, and all returned
 * views are modifiable.
 *