protocol Proto: class {

    var values: [Int] {get set}

    func add(value: Int)
    func showValue() -> [Int]
}

extension Proto {
    func add(value: Int) {
        values.append(value)
    }

    func showValue() -> [Int] {
        return values
    }
}

class MyClass: Proto {
//Can we not write this line????
    var values: [Int] = []

    func foo() {
        showValue()
    }
}