def _delete(self, tx_id):
        """Delete a transaction. Read documentation about CRAB model in https://blog.bigchaindb.com/crab-create-retrieve-append-burn-b9f6d111f460.

        :param tx_id: transaction id
        :return:
        """
        txs = self.driver.instance.transactions.get(asset_id=self.get_asset_id(tx_id))
        unspent = txs[-1]
        output_index = 0
        output = unspent['outputs'][output_index]

        transfer_input = {
            'fulfillment': output['condition']['details'],
            'fulfills': {
                'output_index': output_index,
                'transaction_id': unspent['id']
            },
            'owners_before': output['public_keys']
        }

        prepared_transfer_tx = self.driver.instance.transactions.prepare(
            operation='TRANSFER',
            asset=unspent['asset'] if 'id' in unspent['asset'] else {'id': unspent['id']},
            inputs=transfer_input,
            recipients=self.BURN_ADDRESS,
            metadata={
                'namespace': 'burned',

            }
        )
        signed_tx = self.driver.instance.transactions.fulfill(
            prepared_transfer_tx,
            private_keys=self.user.private_key,
        )
        self.driver.instance.transactions.send_commit(signed_tx)
Delete a transaction. Read documentation about CRAB model in https://blog.bigchaindb.com/crab-create-retrieve-append-burn-b9f6d111f460.

        :param tx_id: transaction id
        :return: