如何使枚举符合 Swift 中的协议

Swift文档显示，classes、structs和enums都可以符合协议，我可以得出它们都符合协议的程度.但我不能让enum表现得像class和struct的例子:

protocol ExampleProtocol {
    var simpleDescription: String { get set }
    mutating func adjust()
}

class SimpleClass: ExampleProtocol {
    var simpleDescription: String = "A very simple class."
    var anotherProperty: Int = 69105

    func adjust() {
        simpleDescription += " Now 100% adjusted."
    }
}

var a = SimpleClass()
a.adjust()
let aDescription = a.simpleDescription

struct SimpleStructure: ExampleProtocol {
    var simpleDescription: String = "A simple structure"

    mutating func adjust() {
        simpleDescription += " (adjusted)"
    }
}

var b = SimpleStructure()
b.adjust()
let bDescription = b.simpleDescription

enum SimpleEnum: ExampleProtocol {
    case Base

    var simpleDescription: String {
        get {
            return "A Simple Enum"
        }
        set {
            newValue
        }
    }

    mutating func adjust() {
        self.simpleDescription += ", adjusted"
    }
}

var c = SimpleEnum.Base
c.adjust()
let cDescription = c.simpleDescription

我还不知道怎么让打了adjust()的simpleDescription换.我的例子显然不会这样做，因为getter有一个硬编码的值，但是我如何在仍然符合ExampleProtocol的情况下为simpleDescription设置一个值呢？