extension CGPoint {
    func add(p: CGPoint) -> CGPoint {
        return CGPointMake(x+p.x, y+p.y)
    }
    func subtract(p: CGPoint) -> CGPoint {
        return CGPointMake(x-p.x, y-p.y)
    }
    func divide(n: CGFloat) -> CGPoint {
        return CGPointMake(x/n, y/n)
    }
    func dot(p: CGPoint) -> CGFloat {
        return x*p.x + y*p.y
    }
    func distance(p: CGPoint) -> CGFloat {
        let dx = x - p.x
        let dy = y - p.y
        return sqrt(dx*dx + dy*dy)
    }
}