用pddl语言实现一个规划问题，一个农夫带着一只狼，一只羊，一颗白菜过河，由于船太小，只能装下农夫和另 一样东西，无人看管时，狼吃羊，羊吃菜，问，怎样才能平安过河？”。

好的，这是一个典型的规划问题，可以使用PDDL（Planning Domain Definition Language）语言来描述和求解。以下是一个可能的PDDL规划问题的描述：

(define (problem river-crossing)
  (:domain river-crossing)
  (:objects farmer - person
            wolf - animal
            goat - animal
            cabbage - vegetable
            riverbank1 - riverbank
            riverbank2 - riverbank
            boat - boat)
  (:init (at farmer riverbank1)
         (at wolf riverbank1)
         (at goat riverbank1)
         (at cabbage riverbank1)
         (at boat riverbank1)
         (on-board farmer boat))
  (:goal (and (at farmer riverbank2)
              (at wolf riverbank2)
              (at goat riverbank2)
              (at cabbage riverbank2))))

  (:operators
    ;; Move the farmer from one riverbank to the other
    (:action cross-river
      :parameters (?from - riverbank ?to - riverbank)
      :precondition (and (at farmer ?from) (at boat ?from))
      :effect (and (not (at farmer ?from))
                   (not (at boat ?from))
                   (at farmer ?to)
                   (at boat ?to)))

    ;; Move an animal or vegetable from one riverbank to the other
    (:action move
      :parameters (?thing - object ?from - riverbank ?to - riverbank)
      :precondition (and (at ?thing ?from) (at boat ?from) (on-board farmer boat))
      :effect (and (not (at ?thing ?from))
                   (not (at boat ?from))
                   (at ?thing ?to)
                   (at boat ?to))))

    ;; Make the farmer and an animal or vegetable cross the river
    (:action cross
      :parameters (?thing - object ?from - riverbank ?to - riverbank)
      :precondition (and (at farmer ?from) (at ?thing ?from) (at boat ?from))
      :effect (and (not (at farmer ?from))
                   (not (at ?thing ?from))
                   (not (at boat ?from))
                   (at farmer ?to)
                   (at ?thing ?to)
                   (at boat ?to)
                   (if (and (not (at goat ?from)) (not (at cabbage ?from)))
                       (on-board farmer boat)
                       (and (not (on-board farmer boat)))))))

在这个规划问题中，我们定义了五个对象：农夫、狼、羊、白菜和船，还定义了两个河岸：riverbank1和riverbank2。初始状态下，所有的对象都在riverbank1上，船也在riverbank1上，并且农夫在船上。目标状态是所有对象都在riverbank2上。

我们定义了三个操作：cross-river，move和cross。cross-river操作是将农夫从一个河岸移到另一个河岸，move操作是将一个动物或植物从一个河岸移到另一个河岸，cross操作是将农夫和一个动物或植物一起从一个河岸移到另一个河岸。这些操作的前置条件和效果都在代码中定义。

使用PDDL求解器，我们可以使用上述问题描述来求解这个规划问题，找到一系列可行的操作来实现平安过河。