     - parameter onCompleted: Action to invoke upon graceful termination of the observable sequence.
     - returns: Subscription object used to unsubscribe from the observable sequence.
     */
+    @warn_unused_result
     public func subscribeCompleted(onCompleted: () -> Void)
         -> Disposable {
         let observer = AnonymousObserver<E> { e in

    - parameter on: Action to invoke for each event in the observable sequence.
    - returns: Subscription object used to unsubscribe from the observable sequence.
    */
    public func subscribe(on: (event: Event<E>) -> Void)
        -> Disposable {
        let observer = AnonymousObserver { e in

    - parameter onRrror: Action to invoke upon errored termination of the observable sequence.
    - returns: Subscription object used to unsubscribe from the observable sequence.
    */
    @warn_unused_result
    public func subscribeError(onError: (ErrorType) -> Void)
        -> Disposable {
        let observer = AnonymousObserver<E> { e in
