Map relation(s) to id(s)

        Parameters
        ----------
        xs : str or list
            relation

        Returns
        -------
        int or list
            id(s) of relation

def rel2id(self, xs):
        """Map relation(s) to id(s)

        Parameters
        ----------
        xs : str or list
            relation

        Returns
        -------
        int or list
            id(s) of relation
        """
        if isinstance(xs, list):
            return [self._rel2id[x] for x in xs]
        return self._rel2id[xs]