title: How to know which observable triggers change in Rx.Observable.combineLatest
body: <p>I can use Rx.Observable.combineLatest so that I will notify  the changes whenever any observable changes. But how can I know which observable changes?</p>
  
  <pre><code>var s1 = someObservable1();
  var s2 = someObservable2();
  Rx.Observable.combineLatest(s1, s2).subscribe(function(){
      // How to know which Observable triggers combineLatest change
  
  });
  </code></pre>