Rotates self counterclockwise by angle 
        (the angle must be given in a 360 degree system)

def rotate(self, angle):
        """Rotates self counterclockwise by angle 
        (the angle must be given in a 360 degree system)
        """
        perp = Vector(-self.y, self.x)
        angle = angle * math.pi / 180.0
        c, s = math.cos(angle), math.sin(angle)
        return Vector(self.x * c + perp.x * s, self.y * c + perp.y * s)