////////////////////////
// File 1
public class Lockable {

    public struct Key {
        private init() {}
    }

    public struct Secrets {
        var message = "Treasure!"
    }

    private let secrets : Secrets = Secrets()

    public func getSecrets(key: Key)->Secrets {
        return secrets
    }

    public init(_ proof: (Key)->()) {
        proof(Key())
    }
}

/////////////////////////////
// File 2

class Derived : Lockable {
    var key : Key? = nil
    init() {
        var key : Lockable.Key?
        super.init({ k in key = k })
        self.key = key!
    }

    func callSecrets() {
        print(getSecrets(key!).message)
    }
}

var derived = Derived()
derived.callSecrets()