@Override
    @CheckForNull
    public E lower(@ParametricNullness E e) {
      return delegate.lower(e);
    }

    @Override
    @CheckForNull
    public E floor(@ParametricNullness E e) {
      return delegate.floor(e);
    }

    @Override
    @CheckForNull
    public E ceiling(@ParametricNullness E e) {
      return delegate.ceiling(e);
    }

    @Override

  @CheckForNull
  public E lower(E e) {
    return Iterators.<@Nullable E>getNext(headSet(e, false).descendingIterator(), null);
  }

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E floor(E e) {
    return Iterators.<@Nullable E>getNext(headSet(e, true).descendingIterator(), null);
  }

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E ceiling(E e) {

    }

    @Override
    @CheckForNull
    public K lower(@ParametricNullness K e) {
      return map().lowerKey(e);
    }

    @Override
    @CheckForNull
    public K floor(@ParametricNullness K e) {
      return map().floorKey(e);
    }

    @Override
    @CheckForNull
    public K ceiling(@ParametricNullness K e) {
      return map().ceilingKey(e);
    }

    *   Added int16x8 support for the built-in ops `space_to_batch_nd` and `batch_to_space_nd`
    *   Added 16-bit int type support for built-in op `less`, `greater_than`, `equal`
    *   Added 8-bit and 16-bit support for `floor_div` and `floor_mod`.
    *   Added 16-bit and 32-bit int support for the built-in op `bitcast`.
    *   Added 8-bit/16-bit/32-bit int/uint support for the built-in op `bitwise_xor`