Answer
Prolog will conclude that Carol is her own sibling. To solve this problem, the rule needs to include the fact that X cannot be equal to Y, which in Prolog is written X \ = Y. Thus an improved version of the rule would be
sibling (X, Y) :-X \= Y, parent(Z, X), parent(Z, Y).
which says that X is Y’s sibling if X and Y are not equal and have a common parent. The following version would insist that X and Y are siblings only if they have both parents in common:
sibling (X, Y) :− X \= Y, Z \= W
parent (Z, X), parent (Z, Y),
parent (W, X), parent (W, Y).
Work Step by Step
Prolog will conclude that Carol is her own sibling. To solve this problem, the rule needs to include the fact that X cannot be equal to Y, which in Prolog is written X \ = Y. Thus an improved version of the rule would be
sibling (X, Y) :-X \= Y, parent(Z, X), parent(Z, Y).
which says that X is Y’s sibling if X and Y are not equal and have a common parent. The following version would insist that X and Y are siblings only if they have both parents in common:
sibling (X, Y) :− X \= Y, Z \= W
parent (Z, X), parent (Z, Y),
parent (W, X), parent (W, Y).
