−z8a−2−6z6a−2 + z6a−4 + z6−13z4a−2 + 4z4a−4 + 4z4−12z2a−2 + 5z2a−4 + 5z2−5a−2 + 2a−4 + 3−2a−2z−2 + a−4z−2 + z−2