z8−a2z6−z6a−2 + 6z6−4a2z4−5z4a−2 + 13z4−5a2z2−10z2a−2 + z2a−4 + 14z2−3a2−11a−2 + 3a−4 + 11−a2z−2−5a−2z−2 + 2a−4z−2 + 4z−2