Rotate counter-clockwise by rad radians.

        Positive y goes *up,* as in traditional mathematics.

        Interestingly, you can use this in y-down computer graphics, if
        you just remember that it turns clockwise, rather than
        counter-clockwise.

        The new position is returned as a new Point.

def rotate(self, rad):
        """
        Rotate counter-clockwise by rad radians.

        Positive y goes *up,* as in traditional mathematics.

        Interestingly, you can use this in y-down computer graphics, if
        you just remember that it turns clockwise, rather than
        counter-clockwise.

        The new position is returned as a new Point.
        """
        s, c = [f(rad) for f in (math.sin, math.cos)]
        x, y = (c * self.x - s * self.y, s * self.x + c * self.y)
        return Point(x, y)