−z7a−1 + 2az5−4z5a−1 + z5a−3−a3z3 + 5az3−6z3a−1 + 2z3a−3−a3z + 2az−2za−1 + za−3 + a3z−1−3az−1 + 3a−1z−1−a−3z−1 + a3z−3−3az−3 + 3a−1z−3−a−3z−3