: collection.iterator();
  }

  /** Set decorator that stays in sync with the multimap values for a key. */
  @WeakOuter
  final class WrappedSet extends WrappedCollection implements Set<V> {
    WrappedSet(@ParametricNullness K key, Set<V> delegate) {
      super(key, delegate, null);
    }

    @Override
    public boolean removeAll(Collection<?> c) {
      if (c.isEmpty()) {

      } else if (collection instanceof SortedSet) {
        return new WrappedSortedSet(key, (SortedSet<V>) collection, null);
      } else if (collection instanceof Set) {
        return new WrappedSet(key, (Set<V>) collection);
      } else {
        return new WrappedCollection(key, collection, null);
      }
    }

    // can't use Serialization writeMultimap and populateMultimap methods since