code: def add_to_initial_stack(self, frame):
      
      if not self._setable_initial_frames:
        raise ValueError(
            "This instance does not allow to manually set initial frame stack.")
      assert_msg = "{}, {}".format(frame.shape, self._initial_frames.shape[:1])
      assert frame.shape == self._initial_frames.shape[2:], assert_msg
      initial_frames = np.roll(self._initial_frames, shift=-1, axis=1)
      initial_frames[0, -1, ...] = frame
      self._initial_frames = initial_frames
docs: Adds new frame to (initial) frame stack, removes last one.