title: Swift Binary operator '+=' cannot be applied to operands of type 'AnyObject!' and 'String!'
body: <p>I'd like to add the current user's username at the end of the object <code>Participants</code> in Parse using Swift but I got the error in the title.</p>
  
  <pre><code>obj["Participants"] += kCurrentUser.username
  </code></pre>
  
  <p>I don't know how to fix it. I have already tried <code>.append</code> and some other methods. Do you guys have any ideas?</p>