def multi(self):
        """
        Start a transactional block of the pipeline after WATCH commands
        are issued. End the transactional block with `execute`.
        """
        if self.explicit_transaction:
            raise RedisError("Cannot issue nested calls to MULTI")
        if self.command_stack:
            raise RedisError(
                "Commands without an initial WATCH have already " "been issued"
            )
        self.explicit_transaction = True

        Start a transactional block of the pipeline after WATCH commands
        are issued. End the transactional block with `execute`.
        
aioredis/client.py