显示路径的Prolog

我有以下简单的图表：

x y z

示例查询：

?- path(x,z,P).
P = [x, y, z] ;                   % works as expected
false.

?- path(z,x,P).                        
false.                            % unexpectedly fails!

如何使上述查询成功？

如果要处理无向图：

---

首先，我们通过定义谓词

edge/2

来记录已知事实：

edge(x,y).
edge(y,x).
edge(y,z).
edge(z,y).

然后我们使用

edge/2

与类似的方法结合使用：

下面这个非常一般的查询怎么样？基于边/2的所有可能路径是什么

?- path(edge,Path,From,To).
  From = To,         Path = [To]
; From = x , To = y, Path = [x,y]
; From = x , To = z, Path = [x,y,z]
; From = y , To = x, Path = [y,x]
; From = y , To = z, Path = [y,z]
; From = z , To = y, Path = [z,y]
; From = z , To = x, Path = [z,y,x]
; false.

---

再倒过来。但是为什么呢？如果我想知道从z到x的路径是否可行。如果我问这个问题，我什么也得不到。如果不使用reverse和vanillaprolog，我如何做上述工作？

edge(x,y).
edge(y,x).
edge(y,z).
edge(z,y).

?- path(edge,Path,x,z).              % one direction
  Path = [x,y,z]
; false.

?- path(edge,Path,z,x).              % other direction
  Path = [z,y,x]
; false.

?- path(edge,Path,From,To).
  From = To,         Path = [To]
; From = x , To = y, Path = [x,y]
; From = x , To = z, Path = [x,y,z]
; From = y , To = x, Path = [y,x]
; From = y , To = z, Path = [y,z]
; From = z , To = y, Path = [z,y]
; From = z , To = x, Path = [z,y,x]
; false.