@Override
    public NavigableSet<K> navigableKeySet() {
      return new StandardNavigableKeySet();
    }

    @Override
    public NavigableSet<K> descendingKeySet() {
      return standardDescendingKeySet();
    }

    @Override
    public K firstKey() {
      return standardFirstKey();
    }

    @Override
    public SortedMap<K, V> headMap(K toKey) {
      return standardHeadMap(toKey);

   * descendingMap}, you may wish to override {@code descendingKeySet} to forward to this
   * implementation.
   */
  protected NavigableSet<K> standardDescendingKeySet() {
    return descendingMap().navigableKeySet();
  }

  /**
   * A sensible definition of {@link #subMap(Object, Object)} in terms of {@link #subMap(Object,

   * descendingMap}, you may wish to override {@code descendingKeySet} to forward to this
   * implementation.
   */
  protected NavigableSet<K> standardDescendingKeySet() {
    return descendingMap().navigableKeySet();
  }

  /**
   * A sensible definition of {@link #subMap(Object, Object)} in terms of {@link #subMap(Object,