title: Underscore indexOf within a Collection
body: <p>I've been looking already around for the answer with no luck.
  Basically I am reviewing again the Addy Osmani guide to Backbone, and it seems I can't get through this code here.</p>
  
  <pre><code>var people = new Backbone.Collection;
  
  people.comparator = function(a, b) {
      return a.get('name') &lt; b.get('name') ? -1 : 1;
  };
  
  var tom = new Backbone.Model({name: 'Tom'});
  var rob = new Backbone.Model({name: 'Rob'});
  var tim = new Backbone.Model({name: 'Tim'});
  
  people.add(tom);
  people.add(rob);
  people.add(tim);
  
  console.log(people.indexOf(rob) === 0); // true
  console.log(people.indexOf(tim) === 1); // true
  console.log(people.indexOf(tom) === 2); // true
  </code></pre>
  
  <p>I don't see how people.comparator can reorder the collection even though is not called anywhere, plus how comes that returning 1 or -1 can reorder it.
  Or is it implicitly called once the Collection is created or indexOf is called on the Collection itself?</p>