code: function(file, event, callback) {
      for(var i = 0; i < watchList.length; i++) {
          if(watchList[i].file === file) {
              watchers[i].on(event, wrapCallback(callback, watchList[i].expression));
              watchList[i].callback[event] = callback;
              return;
          }
      }
      throw new Error(file + " was not being watched previously");
  }
docs: **changeTrigger**  Function change the triggered callback on a watch event  _Parameters:_  * `file`: The file whose trigger is to be changed * `event`: The event on which the trigger is to be executed. It can be one of: * `add`: Called when a file is added * `change`: Called when a watched file is changed * `unlink`: Called when a watched file is removed from disk * `error`: Called when an error is encountered * `callback`: The function to be executed when the above event happens  _Throws:_  * `Error`: If the file is not being watched already