code: def get_ISSNs(self):
          
          invalid_issns = set(self.get_invalid_ISSNs())
  
          return [
              self._clean_isbn(issn)
              for issn in self["022a"]
              if self._clean_isbn(issn) not in invalid_issns
          ]
docs: Get list of VALID ISSNs (``022a``).
  
          Returns:
              list: List with *valid* ISSN strings.