siblings

Hi.

I wrote a siblings example:

father_child(tom, sally).
father_child(tom, john).
mother_child(merry, sally).

siblings(X,Y) :-
parent_child(Z,X),
parent_child(Z,Y),

parent_child(X,Y) :-
father_child(X,Y);
mother_child(X,Y).

and when I enter query:

siblings(X,Y).

I also get results:

X=sally
Y=sally

and

X=john
Y=john

How can I add to the siblings rule that
X cannot be equal to Y ?


.

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