code: def step(self, ac):
      ob, reward, done, info = self.env.step(ac)
      self.frames.append(ob)
      return self._get_ob(), reward, done, info
  
imports: from collections import deque
  import cv2
  from .atari_wrappers import WarpFrame, ClipRewardEnv, FrameStack, ScaledFloatFrame
  from .wrappers import TimeLimit
  import numpy
  import gym
label_desc: Use image node_id 3 for calling the PartialFrameStack obj's underlying member method code with example usage: obj.step(ac) and returns: self,  reward,  done,  info