docs: function Dummy(root, file, content, folder) {
    if (
      !root    || typeof root    !== 'string' ||
      !file    || typeof file    !== 'string' ||
      !content || typeof content !== 'string' ||
      !folder  || typeof folder  !== 'string'
    ) {
      throw new Error('Dummy constructor needs four not empty strings.');
    }
    Object.defineProperties(this, {
      _root     : { value: root    },
      _file     : { value: file    },
      _content  : { value: content },
      _folder   : { value: folder  },
      _nChildren: { value: 2       }  // number of direct root children
    });
  }