How to keep one jQuery UI's draggable group above the other?

[tecaul]

I have 2 classes of divs to which I apply draggable with the stack option set.How can I make one of those classes(class "A") float above the other one(class "B"), so that no matter if the object is moved in the B all the elements in A stay above all of the elements in B?Link to the fiddle: http://bit.ly/UpQTpZ