func (s *Slice) String() string {
	result := ""
	for _, id := range *s {
		result += id.String()
	}
	return result
}
String returns the slice of IDs concatenated together without spaces or newlines.
package id
import "sort"
type Slice []*ID
func (s *Slice) Insert(id *ID) {
	if id == nil {
		return
	}
	i := s.Index(id)
	if i < len(*s) && (*s)[i].Cmp(id) == 0 {
		return
	}
	(*s) = append(*s, nil)
	copy((*s)[i+1:], (*s)[i:])
	(*s)[i] = id
}