code: def _get_ob(self):
      assert len(self.frames) == self.k
      return np.concatenate(self.frames, axis=2)
  
imports: from collections import deque
  import gymnasium
  from gymnasium import spaces
  import numpy
  from typing import Union
  from ray.rllib.utils.annotations import PublicAPI
  from ray.rllib.utils.images import rgb2gray, resize
label_desc: Use image node_id 4 for calling the FrameStack obj's underlying member method code with example usage: obj._get_ob() and returns: np