sort.go
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
}
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
}
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 }
// By adds a sort key to sort by the output of the specified less function.
By adds a sort key to sort by the output of the specified less function .