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我无法解决 Learn Prolog Now 练习（本页的最后一个）。简而言之：有一堆格式为“byCar (place1, place2)”、“byTrain (place1, place2)”和“byAirplane (place1, place2)”的事实，我必须编写一个 travel/3 谓词（我称之为门票）告诉您从地点 1 到地点 2 需要购买哪些门票。

例如，查询

tickets(valmont, losAngeles, X)

应该返回这样的答案：

X = goByCar(valmont, saarbruecken,
   goByTrain(saarbruecken, frankfurt,
   goByAirplane(frankfurt, bangkok,
   goByAirplane(bangkok, auckland,
   goByAirplane(auckland, losAngeles)))))

我目前的解决方案是：

ticketsDir(A, B, goByCar(A, B)) :- byCar(A, B).
ticketsDir(A, B, goByTrain(A, B)) :- byTrain(A, B).
ticketsDir(A, B, goByAirplane(A, B)) :- byAirplane(A, B).

swapGo(goByCar(A, B)) :- goByCar(B, A).
swapGo(goByTrain(A, B)) :- goByTrain(B, A).
swapGo(goByAirplane(A, B)) :- goByAirplane(B, A).

mergeGo(goByCar(A, B), Go2) :- goByCar(A, B, Go2).
mergeGo(goByTrain(A, B), Go2) :- goByTrain(A, B, Go2).
mergeGo(goByAirplane(A, B), Go2) :- goByAirplane(A, B, Go2).

tickets(A, B, Go) :- ticketsDir(A, B, Go).
tickets(A, B, swapGo(Go)) :- ticketsDir(B, A, Go).
tickets(A, B, mergeGo(Go1, Go2)) :- ticketsDir(A, Z, Go1), tickets(Z, B, Go2).
tickets(A, B, mergeGo(Go1, Go2)) :- ticketsDir(Z, A, Go1), tickets(Z, B, Go2).

但是，该查询返回以下结果：

X = mergeGo(goByCar(valmont, saarbruecken),
   mergeGo(goByTrain(saarbruecken, frankfurt),
   mergeGo(goByAirplane(frankfurt, bangkok),
   mergeGo(goByAirplane(bangkok, auckland),
   swapGo(goByAirplane(losAngeles, auckland))))))"

现在奇怪的是，查询

swapGo(goByCar(A, B))

返回：

Error: swapGo/1: Undefined procedure: goByCar/2

和查询

mergeGo(goByCar(A, B), go2)

返回：

Error: mergeGo/2: Undefined procedure: goByCar/3

似乎我错误地定义了“swapGo”和“mergeGo”，或者“tickets”。有什么建议可以解决吗？谢谢！

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