code: function (evt) {
          if (OpenLayers.Handler.prototype.activate.apply(this, arguments)) {
              //register mousewheel events specifically on the window and document
              var wheelListener = this.wheelListener;
              OpenLayers.Event.observe(window, "DOMMouseScroll", wheelListener);
              OpenLayers.Event.observe(window, "mousewheel", wheelListener);
              OpenLayers.Event.observe(document, "mousewheel", wheelListener);
              return true;
          } else {
              return false;
          }
      }
docs: Method: activate