a.swift

func login() -> Observable<Void> {
    return Observable.combineLatest(twitterSession(), Observable.just(Globals.UUID)) { ($0, $1) }
      .flatMap { (twitterSession, uuid) in
        Network.request(PhoenixAPI.TwitterLogin(authToken: twitterSession.authToken,
          authTokenSecret: twitterSession.authTokenSecret, uid: uuid))
      }
      .filterSuccessfulStatusCodes()
      .debug()
      .mapObject(Session)
      .doOnNext { [weak self] (session: Session) in
        self?.session = session
      }
      .map(void)
  }