z8a−4−z6a−2 + 7z6a−4−z6a−6−5z4a−2 + 17z4a−4−6z4a−6−6z2a−2 + 18z2a−4−10z2a−6 + z2a−8−a−2 + 6a−4−5a−6 + a−8