code: function createVersion(config) {
    if (!(config && (typeof config.root === 'string'
        || config.root === null || config.root === undefined))) {
      throw new Error('"config.root" is required and must be a string');
    }
    var root = config.root;
    if (root === null || root === undefined) {
      root = defaultRoot;
    }
  
    return createRfr(function(idFromRoot) {
      if (typeof idFromRoot !== 'string') {
        throw new Error('A string is required for the argument of ' +
        'a user created RFR version.');
      }
      return require(normalizeId(idFromRoot, this.root));
    }, root);
  }
docs: Create a new version of rfr.
  @param {{root:String}} config config of this version.
  @returns {rfr} a new rfr version.
  @private