以下是代码：
bigger(elephant, horse).
bigger(horse, donkey).
is_bigger(X, Y) :-
bigger(X, Y).
is_bigger(X, Y) :-
bigger(X, Z),
is_bigger(Z, Y).
它编译但后来我得到了这个
57 ?- bigger(elephant, horse).
ERROR: toplevel: Undefined procedure: bigger/2 (DWIM could not correct goal)
58 ?- is_bigger(horse, donkey).
ERROR: toplevel: Undefined procedure: is_bigger/2 (DWIM could not correct goal)
我做错了什么？