def __init__(self, env, k):
        """Stack k last frames."""
        gym.Wrapper.__init__(self, env)
        self.k = k
        self.frames = deque([], maxlen=k)
        shp = env.observation_space.shape
        self.observation_space = spaces.Box(
            low=0,
            high=255,
            shape=(shp[0], shp[1], shp[2] * k),
            dtype=env.observation_space.dtype,
        )