code: def expand(self, problem):
          "List the nodes reachable in one step from this node."
          return [self.child_node(problem, action)
                  for action in problem.actions(self.state)f expand(self, problem):
          "List the nodes reachable in one step from this node."
          return [self.child_node(problem, action)
                  for action in problem.actions(self.state)]
docs: List the nodes reachable in one step from this node.