−za15−z2a14−z3a13 + za13−z4a12 + 2z2a12−z5a11 + 3z3a11−za11−z6a10 + 4z4a10−3z2a10−z7a9 + 6z5a9−11z3a9 + 7za9−a9z−1−z6a8 + 5z4a8−6z2a8 + a8−z7a7 + 7z5a7−15z3a7 + 10za7−a7z−1