protocol Observable: class {
    associatedtype T
    func add(observer: AnyObject, closure: @escaping WeakObserver<T>.Closure)
    func removeObserver(observer: AnyObject, alreadyOnQueue: Bool)
    func notifyObservers(result: Result<T>)
    var observers: [WeakObserver<T>] { get set }
}

protocol SummaryProvider : Observable {
    typealias T = SomeConcreteType
    func loadSummary()
}