title: Backbone : listen to add/remove event of a collection inside model in a collection.
body: <p>I have a collection <code>x</code> (one instance) of models <code>y</code>.</p>
  
  <pre><code>var y = Backbone.Model.extend({
          defaults: {
              name: null,
              otherCollection: null
          }
      });
  </code></pre>
  
  <p><code>otherCollection</code> (multiple instances) is a collection of models <code>z</code>.</p>
  
  <p>Which event do I listen to inside collection <code>x</code> so that I would get notified when any <code>z</code> is added in / removed from <code>otherCollection</code>?  </p>