code: function(evt) {
          var layer = evt.layer;
          if(evt.property == "order" &&
                          OpenLayers.Util.indexOf(this.layers, layer) != -1) {
              this.resetRoots();
              this.collectRoots();
          }
      }
docs: Method: handleChangeLayer
  Event handler for the map's changelayer event. We need to rebuild
  this container's layer dom if order of one of its layers changes.
  This handler is added with the setMap method, and removed with the
  removeMap method.
  
  Parameters:
  evt - {Object}