code: func (s *SortFilter) By(less func(a, b string) bool) *SortFilter {
  	s.add(func(a, b string) int {
  		if less(a, b) {
  			return -1
  		}
  		if less(b, a) {
  			return +1
  		}
  		return 0
  	})
  	return s
  }
docs: // By adds a sort key to sort by the output of the specified less function.