docs: function (fn) {
      if (!fn || !_.isFunction(fn)) {
        var err = new Error('Please pass a valid function!');
        debug(err);
        throw err;
      }
  
  
      if (fn.length === 2) {
        this.getNewIdForThisEventExtender = fn;
        return this;
      }
  
      this.getNewIdForThisEventExtender = function (evt, callback) {
        callback(null, fn(evt));
      };
  
      return this;
    }