code: func (s *SortFilter) flipLast() *SortFilter {
  	last := s.cmp[len(s.cmp)-1]
  	s.cmp[len(s.cmp)-1] = func(a, b string) int { return last(b, a) }
  	return s
  }
docs: // flipLast reverses the comparison order for the last sort key.