class_docstr: Implements stacking of `num_frames` last frames of the game.
  
  Wraps an AtariPreprocessing object.
class_docstr_tok: ["Implements","stacking","of","`","num_frames","`","last","frames","of","the","game",".","Wraps","an","AtariPreprocessing","object","."]
code: def _get_array(self):
      assert len(self.frames) == self.num_frames
      return np.concatenate(self.frames, axis=-1)
  
file_path: flax/examples/ppo/env_utils.py
imports: import collections
  import gymnasium
  import numpy
  import seed_rl_atari_preprocessing
label_desc: Use image node_id 4 for calling the FrameStack obj's underlying member method code with example usage: obj._get_array() and returns: np