title: Jquery UI - Allow droppable to have blockable div
body: <p>I am unsure how to achieve what I am trying to do.  I set up an example here:</p>
  
  <p><a href="http://jsfiddle.net/zs6US/" rel="nofollow">http://jsfiddle.net/zs6US/</a></p>
  
  <pre><code>$('.draggable').draggable({
      revert: 'invalid'
  });
  $('#droppable').droppable({
      accept: '.draggable'
  });
  </code></pre>
  
  <p>The green box is a valid droppable.  The red box is not.  If the draggable is dropped on the red, even the red that is over top of the green, I want it to be invalid and revert.  In the example this is not working.</p>
  
  <p>Is this achievable? 
  I have combed over the API and through other questions and have not been able to find an answer.</p>