−z3a13 + za13−2z4a12 + 2z2a12−2z5a11 + z3a11−2z6a10 + 3z4a10−3z2a10−z7a9 + z5a9−2z3a9 + 2za9−3z6a8 + 7z4a8−4z2a8−z7a7 + 2z5a7−za7 + a7z−1−z6a6 + 2z4a6 + z2a6−a6−z5a5 + 4z3a5−4za5 + a5z−1