code: function() {
          var activated = OpenLayers.Strategy.prototype.activate.call(this);
          if(activated) {
              this.layer.events.on({
                  "beforefeaturesadded": this.cacheFeatures,
                  "featuresremoved": this.clearCache,
                  "moveend": this.cluster,
                  scope: this
              });
          }
          return activated;
      }
docs: Constructor: OpenLayers.Strategy.Cluster
  Create a new clustering strategy.
  
  Parameters:
  options - {Object} Optional object whose properties will be set on the
  instance.
   
  APIMethod: activate
  Activate the strategy.  Register any listeners, do appropriate setup.
  
  Returns:
  {Boolean} The strategy was successfully activated.