Check that this table is complete, that is, every character of this
        table can be followed by a new character.

        :return: True if the table is complete, False otherwise.

def check(self):
        """
        Check that this table is complete, that is, every character of this
        table can be followed by a new character.

        :return: True if the table is complete, False otherwise.
        """
        for character, followers in self.items():
            for follower in followers:
                if follower not in self:
                    return False
        return True