elif rt_id2 is not None:

            raise ValueError("multiple rt-id links found")

        parsed = rtutil.RT.parse(rt_id)

        if parsed is None:

            raise ValueError("rt-id is not a valid RT reference")

        else:

            return parsed

    def _get_payment_method(self, posts: AccrualPostings, ticket_id: str) -> Optional[str]:

        payment_methods = posts.meta_values('payment-method')

        payment_methods.discard(None)

        if all(isinstance(s, str) for s in payment_methods):

            payment_methods = {s.strip().lower() for s in payment_methods}  # type:ignore[union-attr]

        log_prefix = f"cannot set payment-method for rt:{ticket_id}:"

        payment_method_count = len(payment_methods)

        if payment_method_count != 1:

            self.logger.warning("%s %s metadata values found",

                                log_prefix, payment_method_count)

            return None

        payment_method = payment_methods.pop()

        if not isinstance(payment_method, str):

            self.logger.warning("%s %r is not a string value",

                                log_prefix, payment_method)

            return None