return descendingMap().navigableKeySet();
  }

  /**
   * A sensible definition of {@link #subMap(Object, Object)} in terms of {@link #subMap(Object,
   * boolean, Object, boolean)}. If you override {@code subMap(K, boolean, K, boolean)}, you may
   * wish to override {@code subMap} to forward to this implementation.
   */
  @Override
  protected SortedMap<K, V> standardSubMap(

    return descendingMap().navigableKeySet();
  }

  /**
   * A sensible definition of {@link #subMap(Object, Object)} in terms of {@link #subMap(Object,
   * boolean, Object, boolean)}. If you override {@code subMap(K, boolean, K, boolean)}, you may
   * wish to override {@code subMap} to forward to this implementation.
   */
  @Override
  protected SortedMap<K, V> standardSubMap(