"ought to be" under what assumption? Prolog is known not to be purely declarative. If you wish for such behavior use Datalog. If you use a purely declarative

"ought to be" under what assumption? Prolog is known not to be purely declarative. If you wish for such behavior use Datalog. If you use a purely declarative language you will run into restrictions that you won't face with Prolog. it's a trade off really.