docs: function (eventExtender) {
      if (!eventExtender || !_.isObject(eventExtender)) {
        var err = new Error('Please inject a valid event extender object!');
        debug(err);
        throw err;
      }
      if (this.eventExtenders.indexOf(eventExtender) < 0) {
        eventExtender.useCollection(this);
        this.eventExtenders.push(eventExtender);
      }
    }