code: def _get_ob(self):
      assert len(self.frames) == self.k
      return np.concatenate(
          [
              frame
              if i == self.k - 1
              else frame[:, :, self.channel : self.channel + 1]
              for (i, frame) in enumerate(self.frames)
          ],
          axis=2,
      )
  
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 4 for calling the PartialFrameStack obj's underlying member method code with example usage: obj._get_ob() and returns: np