minor fix for __pairs
minor fix for __pairs

cloudwu/pbc,CheyiLin/pbc,boyuegame/pbc,zhuoyikang/pbc,cloudwu/pbc,xiaoqunSun/pbc,zh423328/pbc,owent-contrib/pbc,MonCoder/pbc,owent-contrib/pbc,yubobarye/pbc,DoooReyn/pbc