z6a−2 + z6a−4 + 2z4a−2 + 3z4a−4−z4a−6−z4 + 4z2a−4−3z2a−6−z2−a−2 + 3a−4−4a−6 + a−8 + 1 + a−4z−2−2a−6z−2 + a−8z−2