Get the recent state (with stacked history) of the environment.

        Returns:
            a list of ``hist_len-1`` elements, each of shape ``self.state_shape``

def recent_state(self):
        """
        Get the recent state (with stacked history) of the environment.

        Returns:
            a list of ``hist_len-1`` elements, each of shape ``self.state_shape``
        """
        expected_len = self.history_len - 1
        if len(self._current_episode) >= expected_len:
            return [k.state for k in self._current_episode[-expected_len:]]
        else:
            states = [np.zeros(self.state_shape, dtype=self.dtype)] * (expected_len - len(self._current_episode))
            states.extend([k.state for k in self._current_episode])
            return states