how to use .cancel() with bluebird
<p>document API here  <a href="http://bluebirdjs.com/docs/api/cancellation.html" rel="nofollow">http://bluebirdjs.com/docs/api/cancellation.html</a></p>

<p>i try it in my demo,but doesn't work </p>

<pre><code>var Promise = require('bluebird');
var a = require('./a');
var b = require('./b');

var cancelPromise = Promise.resolve();
cancelPromise.cancel();
cancelPromise = a.fnA()
  .then(function() {
    return b.fnB();
  })
  .then(function() {
    console.log('done');
  })
  .finally(function() {
    if (cancelPromise.isCancelled()) {
      console.log('canceled');
    }
    console.log('end');
  });
</code></pre>

<p>so how to use this method?</p>