}

    @Override
    public @Nullable K ceilingKey(K key) {
      return standardCeilingKey(key);
    }

    @Override
    public @Nullable Entry<K, V> higherEntry(K key) {
      return standardHigherEntry(key);
    }

    @Override
    public @Nullable K higherKey(K key) {
      return standardHigherKey(key);
    }

    @Override
    public @Nullable Entry<K, V> firstEntry() {

    }

    @Override
    public @Nullable K ceilingKey(K key) {
      return standardCeilingKey(key);
    }

    @Override
    public @Nullable Entry<K, V> higherEntry(K key) {
      return standardHigherEntry(key);
    }

    @Override
    public @Nullable K higherKey(K key) {
      return standardHigherKey(key);
    }

    @Override
    public @Nullable Entry<K, V> firstEntry() {

   * #tailMap(Object, boolean)}. If you override {@code tailMap}, you may wish to override {@code
   * higherEntry} to forward to this implementation.
   */
  protected @Nullable Entry<K, V> standardHigherEntry(@ParametricNullness K key) {
    return tailMap(key, false).firstEntry();
  }

  @Override
  public @Nullable K higherKey(@ParametricNullness K key) {
    return delegate().higherKey(key);
  }

   * #tailMap(Object, boolean)}. If you override {@code tailMap}, you may wish to override {@code
   * higherEntry} to forward to this implementation.
   */
  protected @Nullable Entry<K, V> standardHigherEntry(@ParametricNullness K key) {
    return tailMap(key, false).firstEntry();
  }

  @Override
  public @Nullable K higherKey(@ParametricNullness K key) {
    return delegate().higherKey(key);
  }