def change_pointer_control(self, accel = None, threshold = None, onerror = None):
        """To change the pointer acceleration, set accel to a tuple (num,
        denum). The pointer will then move num/denum times the normal
        speed if it moves beyond the threshold number of pixels at once.
        To change the threshold, set it to the number of pixels. -1
        restores the default."""
        if accel is None:
            do_accel = 0
            accel_num = 0
            accel_denum = 0
        else:
            do_accel = 1
            accel_num, accel_denum = accel

        if threshold is None:
            do_threshold = 0
        else:
            do_threshold = 1

        request.ChangePointerControl(display = self.display,
                                     onerror = onerror,
                                     do_accel = do_accel,
                                     do_thresh = do_threshold,
                                     accel_num = accel_num,
                                     accel_denum = accel_denum,
                                     threshold = threshold)