Map id(s) to relation(s)

        Parameters
        ----------
        xs : int
            id or a list of ids

        Returns
        -------
        str or list
            relation or a list of relations

def id2rel(self, xs):
        """Map id(s) to relation(s)

        Parameters
        ----------
        xs : int
            id or a list of ids

        Returns
        -------
        str or list
            relation or a list of relations
        """
        if isinstance(xs, list):
            return [self._id2rel[x] for x in xs]
        return self._id2rel[xs]