sort.go
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
}
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
}
// flipLast reverses the comparison order for the last sort key.