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