b.swift

func login() -> Observable<Void> {
    return twitterSession()
      .flatMap {
        self.network.request(self.twitterLogin(authToken: $0.authToken,
                                          authTokenSecret: $0.authTokenSecret,
                                          uid: self.uuid))
      }
      .filterSuccessfulStatusCodes()
      .debug()
      .mapObject(Session)
      .doOnNext { [weak self] (session: Session) in
        self?.session = session
      }
      .map(void)
  }