string.go
func (s StringGroupedNumberPostfixSorter) Swap(i, j int) {
	s[i], s[j] = s[j], s[i]
}
func (s StringGroupedNumberPostfixSorter) Swap(i, j int) {
	s[i], s[j] = s[j], s[i]
}
// Swap swaps the elements with indexes i and j.