Add terminals into the set.
        :param terminals: Terminals to insert.

def add(self, *terminals):
        # type: (Iterable[Any]) -> None
        """
        Add terminals into the set.
        :param terminals: Terminals to insert.
        """
        for term in terminals:
            if term in self:
                continue
            super().add(term)
            self._assign_map[term] = set()