code: def add_file_group(self, fileGrp):
          
          el_fileSec = self._tree.getroot().find('mets:fileSec', NS)
          if el_fileSec is None:
              el_fileSec = ET.SubElement(self._tree.getroot(), TAG_METS_FILESEC)
          el_fileGrp = el_fileSec.find('mets:fileGrp[@USE="%s"]' % fileGrp, NS)
          if el_fileGrp is None:
              el_fileGrp = ET.SubElement(el_fileSec, TAG_METS_FILEGRP)
              el_fileGrp.set('USE', fileGrp)
          return el_fileGrp
docs: Add a new ``mets:fileGrp``.
  
          Arguments:
              fileGrp (string): ``USE`` attribute of the new filegroup.