docs: function (key, fn) {
      if (typeof key === 'function') {
        fn = key;
        key = null;
      }
      if (typeof fn !== 'function') {
        fn = this.pickOption('renameKey');
      }
      if (typeof fn !== 'function') {
        fn = utils.identity;
      }
  
      this.options.renameKey = fn;
      if (arguments.length === 2) {
        return fn(key);
      }
      if (typeof key === 'string') {
        return fn(key);
      }
      return fn;
    }