−z6a−2 + z6−a2z4−6z4a−2 + z4a−4 + 6z4−3a2z2−11z2a−2 + 3z2a−4 + 11z2−2a2−6a−2 + 2a−4 + 6 + a2z−2 + a−2z−2−2z−2