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