code: def get_ISBNs(self):
          
          invalid_isbns = set(self.get_invalid_ISBNs())
  
          valid_isbns = [
              self._clean_isbn(isbn)
              for isbn in self["020a"]
              if self._clean_isbn(isbn) not in invalid_isbns
          ]
  
          if valid_isbns:
              return valid_isbns
  
          # this is used sometimes in czech national library
          return [
              self._clean_isbn(isbn)
              for isbn in self["901i"]
          ]
docs: Get list of VALID ISBN.
  
          Returns:
              list: List with *valid* ISBN strings.