}

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

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

    @Override
    public K lastKey() {
      return standardLastKey();
    }

    @Override
    public SortedMap<K, V> tailMap(K fromKey) {

    }

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

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

    @Override
    public K lastKey() {
      return standardLastKey();
    }

    @Override
    public SortedMap<K, V> tailMap(K fromKey) {

   * boolean)}. If you override {@code headMap(K, boolean)}, you may wish to override {@code
   * headMap} to forward to this implementation.
   */
  protected SortedMap<K, V> standardHeadMap(@ParametricNullness K toKey) {
    return headMap(toKey, false);
  }

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

   * boolean)}. If you override {@code headMap(K, boolean)}, you may wish to override {@code
   * headMap} to forward to this implementation.
   */
  protected SortedMap<K, V> standardHeadMap(@ParametricNullness K toKey) {
    return headMap(toKey, false);
  }

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