code: def quaternion(self):
      assert isinstance(self._quaternion, np.ndarray)
      assert self._quaternion.shape == (4,)
      assert np.isclose(
          np.linalg.norm(self._quaternion), 1.0
      ), "self._quaternion is not normalized or valid"
      return self._quaternion
  
imports: import numpy
  from geometry import transformations
label_desc: Use image node_id 6 for calling the Rotation obj's underlying member method code with example usage: obj.quaternion() and returns: self