用PDDL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？

问题分析：

该问题需要通过规划算法来解决。首先，我们需要定义问题的状态和操作。问题的状态包括：农夫、狼、羊、白菜和船的位置。同时，为了满足问题的限制条件，我们需要定义操作：农夫单独过河、农夫带狼过河、农夫带羊过河、农夫带白菜过河。

PDDL实现：

我们采用PDDL语言来实现该规划问题。首先，我们定义问题的状态和操作：

(define (problem river-crossing)
(:domain river-crossing)

(:objects farmer wolf goat cabbage left right)
(:init
(at farmer left)
(at wolf left)
(at goat left)
(at cabbage left)
(at boat left)
)

(:goal (and
(at farmer right)
(at wolf right)
(at goat right)
(at cabbage right)
)
)

(:action move-farmer-only
:precondition (and
(at farmer ?from)
(at boat ?from)
)
:effect (and
(not (at farmer ?from))
(at farmer ?to)
(not (at boat ?from))
(at boat ?to)
)
)

(:action move-farmer-wolf
:precondition (and
(at farmer ?from)
(at wolf ?from)
(at boat ?from)
)
:effect (and
(not (at farmer ?from))
(at farmer ?to)
(not (at wolf ?from))
(at wolf ?to)
(not (at boat ?from))
(at boat ?to)
)
)

(:action move-farmer-goat
:precondition (and
(at farmer ?from)
(at goat ?from)
(at boat ?from)
)
:effect (and
(not (at farmer ?from))
(at farmer ?to)
(not (at goat ?from))
(at goat ?to)
(not (at boat ?from))
(at boat ?to)
)
)

(:action move-farmer-cabbage
:precondition (and
(at farmer ?from)
(at cabbage ?from)
(at boat ?from)
)
:effect (and
(not (at farmer ?from))
(at farmer ?to)
(not (at cabbage ?from))
(at cabbage ?to)
(not (at boat ?from))
(at boat ?to)
)
)
)

在上述代码中，我们定义了农夫、狼、羊、白菜和船的位置，以及农夫单独过河、农夫带狼过河、农夫带羊过河、农夫带白菜过河四个操作。其中，每个操作都有一个前提条件和效果。前提条件指操作执行前的状态，效果指操作执行后的状态。

接下来，我们需要定义问题的初始状态和目标状态：

(define (problem river-crossing-1)
(:domain river-crossing)
(:objects farmer wolf goat cabbage left right)
(:init
(at farmer left)
(at wolf left)
(at goat left)
(at cabbage left)
(at boat left)
)
(:goal (and
(at farmer right)
(at wolf right)
(at goat right)
(at cabbage right)
))
)

在上述代码中，我们定义了初始状态，即农夫、狼、羊、白菜和船都在左岸。同时，我们定义了目标状态，即农夫、狼、羊、白菜和船都在右岸。

最后，我们可以使用PDDL Planner等工具来求解该问题。