Rotate 90 degrees clockwise `shift` times.  If `shift` is negative,
        rotate counter-clockwise.

def rotate(self, shift):
        '''
        Rotate 90 degrees clockwise `shift` times.  If `shift` is negative,
        rotate counter-clockwise.
        '''
        self.child_corners.values[:] = np.roll(self.child_corners
                                                 .values, shift, axis=0)
        self.update_transform()