def undo_step(self, task: Task, **kwargs: Any) -> Optional[TaskStep]:
        """
        Undo step from task.

        If this cannot be implemented, return None.

        """
        raise NotImplementedError("Undo is not yet implemented")
        # if len(task.completed_steps) == 0:
        #     return None

        # # pop last step output
        # last_step_output = task.completed_steps.pop()
        # # add step to the front of the queue
        # task.step_queue.appendleft(last_step_output.task_step)

        # # undo any `step_state` variables that have changed
        # last_step_output.step_state["n_function_calls"] -= 1

        # # TODO: we don't have memory pop capabilities yet
        # # # now pop the memory until we get to the state
        # # last_step_response = cast(AgentChatResponse, last_step_output.output)
        # # while last_step_response != task.memory.:
        # #     last_message = last_step_output.task_step.memory.pop()
        # #     if last_message == cast(AgentChatResponse, last_step_output.output).response:
        # #         break

        # # while cast(AgentChatResponse, last_step_output.output).response !=

        Undo step from task.

        If this cannot be implemented, return None.

        
llama-index-integrations/agent/llama-index-agent-openai/llama_index/agent/openai/step.py