code: function() {
          var activated = OpenLayers.Strategy.prototype.activate.apply(this, arguments);
          if (activated) {
              this.cache = [];
              this.layer.events.on({
                  "beforefeaturesadded": this.handleAdd,
                  "beforefeaturesremoved": this.handleRemove,
                  scope: this
              });
          }
          return activated;
      }
docs: Constructor: OpenLayers.Strategy.Filter
  Create a new filter 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.
  By default, this strategy automatically activates itself when a layer
  is added to a map.
  
  Returns:
  {Boolean} True if the strategy was successfully activated or false if
  the strategy was already active.