lt="Go back up one level"/></a></h2>
      
            <p>Finally, let's have a look at how one of the callbacks works: "onListSelection", the function
        that is called when an item is selected in the listbox. The other
            callbacks obey the same pattern.</p>
            <p>The basic rule of thumb is that a callback should not update any part of
        the user interface directly - it's job is to respond to user interaction by changing the "data" structure. 
        In this example, each ca