docs: 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");
  }