title: avoid adding a folderish object inside another object of the same type
body: <p>We have a content type (lets call it, <strong>MyFolder</strong>) that could contain any other content type registered in the site except for <strong>MyFolder</strong> itself.</p>
  
  <p>what's the simplest way to accomplish this?</p>
  
  <ol>
  <li>registering a new workflow and setting guards?</li>
  <li>removing the <strong>Add MyFolder</strong> permision of the content type from the object with an event at creation time?</li>
  <li>filtering the content type from the list of addable content types in the object? how?</li>
  <li>none of the above? how then?</li>
  </ol>