刚刚开始学习Prolog,我不知道为什么我会收到这个错误.我无法理解我能做些什么.
这是'代码':
?- assert(likes(a , beaf)).
?- assert(likes(b , nuduls)).
?- assert(likes(b , hotdog)).
?- assert(likes(c , sandwicth)).
?- assert(likes(c , bargar).
?- assert(likes(d , juice)).
?- assert(likes(d , chicken_curry)).
?- assert(likes(d , nudules)).
?- assert(likes(e , brade)).
?- assert(likes(e , butter)).
?- assert(likes(e , bargar)).
?- friend(e,d).
?- friend(d,a).
?- friend(d,b).
?- friend(d ,c).
?- highlyrecommended(X,L):-friend(X,Y),likes(Y,L).
?- recommended(X,L):-friend(X,Z),friend(Z,Y),likes(Y,L).
?- Is d is the friend of e ?
?- Y
?- Is e is the friend of c ?
?- …prolog ×1