def add_relationship(self, txn_id, predecessors):
        """Add a predecessor-successor relationship between one txn id and
        a set of predecessors.

        Args:
            txn_id (str): The transaction id of the transaction.
            predecessors (set): The transaction ids of the
                transaction's predecessors

        Returns:
            None
        """

        all_pred = set(predecessors)
        for pred in predecessors:
            all_pred.update(self._predecessors_by_id[pred])

        self._predecessors_by_id[txn_id] = all_pred