155      * @return the last element, or {@code null} if this set is empty
 156      */
 157     E pollLast();
 158 
 159     /**
 160      * Returns an iterator over the elements in this set, in ascending order.
 161      *
 162      * @return an iterator over the elements in this set, in ascending order
 163      */
 164     Iterator<E> iterator();
 165 
 166     /**
 167      * Returns a reverse order view of the elements contained in this set.
 168      * The descending set is backed by this set, so changes to the set are
 169      * reflected in the descending set, and vice-versa.  If either set is
 170      * modified while an iteration over either set is in progress (except
 171      * through the iterator's own {@code remove} operation), the results of
 172      * the iteration are undefined.
 173      *
 174      * <p>The returned set has an ordering equivalent to
 175      * <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
 176      * The expression {@code s.descendingSet().descendingSet()} returns a
 177      * view of {@code s} essentially equivalent to {@code s}.
 178      *
 179      * @return a reverse order view of this set
 180      */
 181     NavigableSet<E> descendingSet();
 182 
 183     /**
 184      * Returns an iterator over the elements in this set, in descending order.
 185      * Equivalent in effect to {@code descendingSet().iterator()}.
 186      *
 187      * @return an iterator over the elements in this set, in descending order
 188      */
 189     Iterator<E> descendingIterator();
 190 
 191     /**
 192      * Returns a view of the portion of this set whose elements range from
 193      * {@code fromElement} to {@code toElement}.  If {@code fromElement} and
 194      * {@code toElement} are equal, the returned set is empty unless {@code
 195      * fromInclusive} and {@code toInclusive} are both true.  The returned set

 155      * @return the last element, or {@code null} if this set is empty
 156      */
 157     E pollLast();
 158 
 159     /**
 160      * Returns an iterator over the elements in this set, in ascending order.
 161      *
 162      * @return an iterator over the elements in this set, in ascending order
 163      */
 164     Iterator<E> iterator();
 165 
 166     /**
 167      * Returns a reverse order view of the elements contained in this set.
 168      * The descending set is backed by this set, so changes to the set are
 169      * reflected in the descending set, and vice-versa.  If either set is
 170      * modified while an iteration over either set is in progress (except
 171      * through the iterator's own {@code remove} operation), the results of
 172      * the iteration are undefined.
 173      *
 174      * <p>The returned set has an ordering equivalent to
 175      * {@link Collections#reverseOrder(Comparator) Collections.reverseOrder(comparator())}.
 176      * The expression {@code s.descendingSet().descendingSet()} returns a
 177      * view of {@code s} essentially equivalent to {@code s}.
 178      *
 179      * @return a reverse order view of this set
 180      */
 181     NavigableSet<E> descendingSet();
 182 
 183     /**
 184      * Returns an iterator over the elements in this set, in descending order.
 185      * Equivalent in effect to {@code descendingSet().iterator()}.
 186      *
 187      * @return an iterator over the elements in this set, in descending order
 188      */
 189     Iterator<E> descendingIterator();
 190 
 191     /**
 192      * Returns a view of the portion of this set whose elements range from
 193      * {@code fromElement} to {@code toElement}.  If {@code fromElement} and
 194      * {@code toElement} are equal, the returned set is empty unless {@code
 195      * fromInclusive} and {@code toInclusive} are both true.  The returned set