Fix native Power with integral
Fix native Power with integral

keisuke-umezawa/chainer,hvy/chainer,tkerola/chainer,niboshi/chainer,okuta/chainer,wkentaro/chainer,chainer/chainer,wkentaro/chainer,keisuke-umezawa/chainer,hvy/chainer,niboshi/chainer,hvy/chainer,keisuke-umezawa/chainer,wkentaro/chainer,niboshi/chainer,okuta/chainer,chainer/chainer,niboshi/chainer,chainer/chainer,okuta/chainer,pfnet/chainer,wkentaro/chainer,keisuke-umezawa/chainer,hvy/chainer,okuta/chainer,chainer/chainer