z6a−4−4z4a−2 + 4z4a−4−z4a−6−12z2a−2 + 10z2a−4−2z2a−6 + 3z2−13a−2 + 11a−4−3a−6 + 5−5a−2z−2 + 4a−4z−2−a−6z−2 + 2z−2