Assignable protocols

// This protocol cannot be implemented by a enum.
// Because enums may not contain stored properties
protocol SomeProtocol {
    var mustBeSettable: Bool { get set }
}

struct SomeStruct: SomeProtocol {
    var mustBeSettable:Bool
}

// Compiler Error
//enum SomeEnum: SomeProtocol {
//    var mustBeSettable:Bool
//}

class SomeClass: SomeProtocol {
    var mustBeSettable:Bool = true
}

var s = SomeStruct(mustBeSettable: true)
s.mustBeSettable = false

var c = SomeClass()
c.mustBeSettable = false

// Compiler Error, SomeProtocol could be an enum
func someFunction(foo:SomeProtocol){
    foo.mustBeSettable = true
}