* is just more than the set's size.  We augment the test by
     * assuming that sets have fast contains() performance, and other
     * collections don't.  See
     * http://code.google.com/p/guava-libraries/issues/detail?id=1013
     */
    if (collection instanceof Set && collection.size() > set.size()) {
      return Iterators.removeAll(set.iterator(), collection);
    } else {