def decrease(self, infile):
        '''Decrease: 任意の箇所のバイト列を 削除する
        '''
        gf = infile[31:]
        try:
            index = random.randint(len(gf)-1, 31)
        except ValueError:
            return infile
        gf = gf[:index] + gf[index+1:]
        return infile[:31] + gf