docs: def identifier_director(**kwargs):
      """Direct the identifier elements based on their qualifier."""
      qualifier = kwargs.get('qualifier', '')
      content = kwargs.get('content', '')
      if qualifier == 'ISBN':
          return CitationISBN(content=content)
      elif qualifier == 'ISSN':
          return CitationISSN(content=content)
      elif qualifier == 'DOI':
          return CitationDOI(content=content)
      elif qualifier == 'REP-NO':
          return CitationTechnicalReportNumber(content=content)
      else:
          return None