code: func (t *PrimaryTree) SeekLast() (e *PrimaryEnumerator, err error) {
  	q := t.last
  	if q == nil {
  		return nil, io.EOF
  	}
  
  	return btEPool1.get(nil, true, q.c-1, q.d[q.c-1].k, q, t, atomic.LoadUint64(&t.ver)), nil
  }
docs: // SeekLast returns an enumerator positioned on the last KV pair in the tree,
  // if any. For an empty tree, err == io.EOF is returned and e will be nil.