def add_output(self, output):
        """Adds an output to a Transaction's list of outputs.

            Args:
                output (:class:`~bigchaindb.common.transaction.
                    Output`): An Output to be added to the
                    Transaction.
        """
        if not isinstance(output, Output):
            raise TypeError('`output` must be an Output instance or None')
        self.outputs.append(output)