title: what triggered combineLatest?
body: <p>I have a few observables. And I need to know which one triggered the subscribe.</p>
  
  <pre><code>Observable.combineLatest(
        this.tournamentsService.getUpcoming(),
        this.favoriteService.getFavoriteTournaments(),
        this.teamsService.getTeamRanking(),
  (tournament, favorite, team) =&gt; {
  //what triggered combinelatest to run?
  }).subscribe()
  </code></pre>