Different query results in prolog

问题

I have a question in prolog. Here is the Knowledge Base.

loves(vincent,mia).
loves(marcellus,mia).
loves(pumpkin,honey_bunny).
loves(honey_bunny,pumpkin).

jealous(X,Y) :-
   loves(X,Z),
   loves(Y,Z).

This is from "Learn Prolog Now" book. Now if I make a query ?- jealous(marcellus,W). it returns that W = vincent but if I give the query ?- jealous(vincent,W). the return result is W=vincent So this means that vincent is jealous of himself but marcellus is not jealous of himself. Why is the answer different ? I am using swipl in Ubuntu Linux , which is in VirtualBox on winxp. I am beginner in Prolog.

Thanks

回答1:

After the answer W = vincent at the query ?- jealous(marcellus,W). type ; instead of Enter and you will get W = marcellus. You must add the fact that X \= Y in jealous/2.

来源：https://stackoverflow.com/questions/12757406/different-query-results-in-prolog