docs: def _nested_transactional(fn):
      # type: (Callable) -> Callable
      """In a transactional method create a nested transaction."""
      @wraps(fn)
      def wrapped(self, *args, **kwargs):
          # type: (SessionFactory) -> Any
  
          try:
              rv = fn(self, *args, **kwargs)
          except _TransactionalPolicyViolationError as e:
              getattr(self, _TX_HOLDER_ATTRIBUTE).rollback()
              rv = e.result
  
          return rv
      return wrapped