code: def folder_id!(fid, change_key = nil)
        attribs = {'Id' => fid}
        attribs['ChangeKey'] = change_key if change_key
        @nbuild[NS_EWS_TYPES].FolderId(attribs)
      end
docs: Build the FolderId element
   @see http://msdn.microsoft.com/en-us/library/aa579461.aspx