func (is IdentifierSlice) Less(i, j int) bool {
	return is[i].ID() < is[j].ID()
}
func (is IdentifierSlice) Less(i, j int) bool {
	return is[i].ID() < is[j].ID()
}
// Less will return true if the ID of element at i is less than j;
// part of the sort.Interface