title: Observable.Using( ) cancellation
body: <p>I have a observable made by the <code>Using</code> helper:</p>
  
  <pre><code>var o = Observable.Using(
   () =&gt; { 
            return new MyResource 
         },
   res =&gt; {
            return new Observable.Create&lt;string&gt;(observer =&gt; ....);
          });
  </code></pre>
  
  <p>How can I cancel the observable? And by that make sure <code>MyResource</code> is disposed of?</p>
  
  <p>I see there are a <code>Observable.Using( )</code> that includes a <code>cancellationToken</code>, but signature is so different, that I'm not able to make it work...</p>
  
  <p><strong>Update:</strong> 
  As James points out, by disposing the observable, my resource will be disposed as well. In my case, a plain disposal is not enough. I need to call a method on the resource first. How can that be archived?</p>