參考：https://www.jianshu.com/p/4f025476701a

//運算子函式要宣告在檔案的最外層，不能在類裡

//宣告•的precedencegroup名：PointMultiplicationPrecedence
precedencegroup PointMultiplicationPrecedence {
    associativity: left//左結合
    higherThan: AdditionPrecedence//優先順序：比加法高
    lowerThan: MultiplicationPrecedence//優先順序：比乘法低
}

infix operator •: PointMultiplicationPrecedence

 
/// Point的數量積（Option+8）
///
/// - Parameters:
///   - left: One Point
///   - right: Other Point
/// - Returns: (left.x * right.x +  left.y + right.y)
func •(left: CGPoint, right: CGPoint) -> Double {
    return Double(left.x * right.x +  left.y + right.y)
}