code: function(indexer, newNode, nextNode) {
          var newZIndex = indexer.getZIndex(newNode);
  
          var returnVal = 0;
          if (nextNode) {
              var nextZIndex = indexer.getZIndex(nextNode);
              returnVal = newZIndex - nextZIndex; 
          }
          
          return returnVal;
      }
docs: Method: Z_ORDER
  This compare method is used by other comparison methods.
  It can be used individually for ordering, but is not recommended,
  because it doesn't subscribe to drawing order.
  
  Parameters:
  indexer - {<OpenLayers.ElementsIndexer>}
  newNode - {DOMElement}
  nextNode - {DOMElement}
  
  Returns:
  {Integer}