code: function watchOnce(paths, callback, options = { priority: 0 }) {
      let id;
      let onceCallback = function _watchOnceCallback(...args) {
        unwatch(id);
        callback(...args);
      };
  
      id = watch(paths, onceCallback, options);
    }
docs: Watch for changes on a given key once, and execute the provided callback
  when there are changes. Callback removes itself after the first call so
  that it is never called again.
  
  @param {String[]|String} String path or array of paths to watch
  @param {Function} callback to execute when there are changes
  @param {Object} options Options
  @param {Number} options.priority Controls the order the provided callback is called when multiple watches exist on the same key.