追踪prolog递归深度的方法？

Question

在prolog中，我的数据库设置如下

male("John").
male("Bob").
male("Billy").
male("Gary").

Parent("Bob","John").
Parent("Billy","Bob").
Parent("Gary", "Billy").


ancestor(Ancestor, Descendant) :-
    parent(Ancestor, Descendant).
ancestor(Ancestor, Descendant) :-
    parent(Ancestor, CommonAncestor),
    ancestor(CommonAncestor, Descendant).

是否有可能跟踪这个祖先函数的递归有多深？例如，如果我们跑

?- ancestor("Billy", "John", X).

是否有可能有X返回2，或在情况下

?- ancestor("Bob", "John", X).

有X返回1吗？

Answer 1

哟可以写：

ancestor(Ancestor, Descendant,1) :-
    parent(Ancestor, Descendant).
ancestor(Ancestor, Descendant,X) :-
    parent(Ancestor, CommonAncestor),
    ancestor(CommonAncestor, Descendant,X1),
    X is X1+1.

下面是一些例子：

?- ancestor("Billy", "John", X).
X = 2 ;
false.

?- ancestor("Bob", "John", X).
X = 1 ;
false.