code: def _get_ob(self):
      assert len(self.frames) == self.k
      return LazyFrames(list(self.frames), stack_axis=self.stack_axis)
  
imports: from collections import deque
  import gym
  import numpy
  from gym import spaces
  from packaging import version
  import pfrl
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: LazyFrames