code: def _check_r(self, r):
          
          if abs(np.dot(r[:, 0], r[:, 0]) - 1) > eps or \
              abs(np.dot(r[:, 0], r[:, 0]) - 1) > eps or \
              abs(np.dot(r[:, 0], r[:, 0]) - 1) > eps or \
              np.dot(r[:, 0], r[:, 1]) > eps or \
              np.dot(r[:, 1], r[:, 2]) > eps or \
              np.dot(r[:, 2], r[:, 0]) > eps:
              raise ValueError("The rotation matrix is significantly non-orthonormal.")
docs: the columns must orthogonal