def _q_bar_matrix(self):
        """Matrix representation of quaternion for multiplication purposes.
        """
        return np.array([
            [self.q[0], -self.q[1], -self.q[2], -self.q[3]],
            [self.q[1],  self.q[0],  self.q[3], -self.q[2]],
            [self.q[2], -self.q[3],  self.q[0],  self.q[1]],
            [self.q[3],  self.q[2], -self.q[1],  self.q[0]]])
Matrix representation of quaternion for multiplication purposes.