def built_label(self) -> str | BaseText | UnitStr:
        """Return a list of spanner labels that have been built."""
        label = self.built if self.built is not None else self.spanner_label
        if label is None:
            raise ValueError("Spanner label must be a string and not None.")
        return label
Return a list of spanner labels that have been built.
great_tables/_gt_data.py