code: def deleted(self, key, value):
      
      deleted = self.get('deleted')
      book_series = self.get('book_series')
  
      if not deleted:
          normalized_a_values = [el.upper() for el in force_list(value.get('a'))]
          normalized_c_values = [el.upper() for el in force_list(value.get('c'))]
          if 'DELETED' in normalized_a_values or 'DELETED' in normalized_c_values:
              deleted = True
  
      if not book_series:
          normalized_a_values = [el.upper() for el in force_list(value.get('a'))]
          if 'BOOKSERIES' in normalized_a_values:
              book_series = True
  
      self['book_series'] = book_series
      return deleted
docs: Populate the ``deleted`` key.
  
      Also populates the ``book_series`` key through side effects.