为何cocoa里可以直接使用.Normal 类似语法，自己却不能写？

如题，swift中可以如此使用。Normal

var button:UIButton = UIButton(frame:CGRectMake(10,10,10,10))
 button.setTitle("Button", forState:.Normal)

查看.Norma源码，则是 这样一个结构体：

 struct UIControlState : RawOptionSetType {
 init(_ rawValue: UInt)
 init(rawValue: UInt)
static var Normal: UIControlState { get }
static var Highlighted: UIControlState { get } // used when UIControl isHighlighted is set
static var Disabled: UIControlState { get }
static var Selected: UIControlState { get } // flag usable by app (see below)
static var Application: UIControlState { get } // additional flags available for application use
static var Reserved: UIControlState { get } // flags reserved for internal framework use
}

对于 .Normal 可以这样引用，一直很好奇。这是swift既定语法规则嘛？但是自己试图创建这样一个结构体，却无法成功。还是自己写错了？

我是这样创建的

struct Summation {
static var Normal:Summation? { get {
        return nil
    }
}


};

class Student{
 func setSum(forsm sum:Summation){
}


};

var stu = Student()
 stu.setSum(forsm:.Normal!) ／／此处报错