−z7a−1 + 2az5−4z5a−1 + 2z5a−3−a3z3 + 5az3−9z3a−1 + 5z3a−3−z3a−5−a3z + 5az−9za−1 + 6za−3−za−5 + 2az−1−4a−1z−1 + 3a−3z−1−a−5z−1