entity.go
func (is IdentifierSlice) Swap(i, j int) {
	is[i], is[j] = is[j], is[i]
}
func (is IdentifierSlice) Swap(i, j int) {
	is[i], is[j] = is[j], is[i]
}
// Swap the elements at positions i and j
// part of the sort.Interface