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