code: def isbns(self, key, value):
      
      def _get_medium(value):
          def _normalize(medium):
              schema = load_schema('hep')
              valid_media = schema['properties']['isbns']['items']['properties']['medium']['enum']
  
              medium = medium.lower().replace('-', '').replace(' ', '')
              if medium in valid_media:
                  return medium
              elif medium == 'ebook':
                  return 'online'
              elif medium == 'paperback':
                  return 'softcover'
  
              return ''
  
          medium = force_single_element(value.get('b', ''))
          normalized_medium = _normalize(medium)
  
          return normalized_medium
  
      def _get_isbn(value):
          a_value = force_single_element(value.get('a', ''))
          normalized_a_value = a_value.replace('.', '')
          if normalized_a_value:
              return normalize_isbn(normalized_a_value)
  
      return {
          'medium': _get_medium(value),
          'value': _get_isbn(value),
      }
docs: Populate the ``isbns`` key.