}

  /** Remove each element in an iterable from a set. */
  static boolean removeAllImpl(Set<?> set, Iterator<?> iterator) {
    boolean changed = false;
    while (iterator.hasNext()) {
      changed |= set.remove(iterator.next());
    }
    return changed;
  }

  static boolean removeAllImpl(Set<?> set, Collection<?> collection) {
    checkNotNull(collection); // for GWT

    if (elements.isEmpty()) {
      return false;
    }
    elements.addTo(self);
    return true;
  }

  /** An implementation of {@link Multiset#removeAll}. */
  static boolean removeAllImpl(Multiset<?> self, Collection<?> elementsToRemove) {
    Collection<?> collection =
        (elementsToRemove instanceof Multiset)
            ? ((Multiset<?>) elementsToRemove).elementSet()
            : elementsToRemove;

      try {
        return super.removeAll(checkNotNull(c));
      } catch (UnsupportedOperationException e) {
        // if the iterators don't support remove
        return Sets.removeAllImpl(this, c.iterator());
      }
    }

    @Override
    public boolean retainAll(Collection<?> c) {
      try {
        return super.retainAll(checkNotNull(c));

      try {
        return super.removeAll(checkNotNull(c));
      } catch (UnsupportedOperationException e) {
        // if the iterators don't support remove
        return Sets.removeAllImpl(this, c.iterator());
      }
    }

    @Override
    public boolean retainAll(Collection<?> c) {
      try {
        return super.retainAll(checkNotNull(c));