title: Merging two notification observers in RxSwift
body: <p>I have this piece of code:</p>
  
  <pre><code>let appActiveNotifications: [Observable&lt;NSNotification&gt;] = [
      NSNotificationCenter.defaultCenter().rx_notification(UIApplicationWillEnterForegroundNotification),
      NSNotificationCenter.defaultCenter().rx_notification(Constants.AppRuntimeCallIncomingNotification)
  ]
  
  appActiveNotifications.merge()
    .takeUntil(self.rx_deallocated)
    .subscribeNext() { [weak self] _ in
    // notification handling
  }
  .addDisposableTo(disposeBag)
  </code></pre>
  
  <p>It's supposed to listen to either of the specified notifications and handle when any of them is triggered.</p>
  
  <p>However this does not compile. I get the following error:</p>
  
  <pre><code>Value of type '[Observable&lt;NSNotification&gt;]' has no member 'merge'
  </code></pre>
  
  <p>How should I merge these two signals to one then?</p>