docs: 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;
      }