func (es EditScript) stats() (s struct{ NI, NX, NY, NM int }) {
	for _, e := range es {
		switch e {
		case Identity:
			s.NI++
		case UniqueX:
			s.NX++
		case UniqueY:
			s.NY++
		case Modified:
			s.NM++
		default:
			panic("invalid edit-type")
		}
	}
	return
}
func (es EditScript) stats() (s struct{ NI, NX, NY, NM int }) {
	for _, e := range es {
		switch e {
		case Identity:
			s.NI++
		case UniqueX:
			s.NX++
		case UniqueY:
			s.NY++
		case Modified:
			s.NM++
		default:
			panic("invalid edit-type")
		}
	}
	return
}
// stats returns a histogram of the number of each type of edit operation.