code: def child_node(self, problem, action):
          "Fig. 3.10"
          next = problem.result(self.state, action)
          return Node(next, self, action,
                      problem.path_cost(self.path_cost, self.state, action, next)f child_node(self, problem, action):
          "Fig. 3.10"
          next = problem.result(self.state, action)
          return Node(next, self, action,
                      problem.path_cost(self.path_cost, self.state, action, next))
docs: Fig. 3.10