def rel2id(self, xs):
if isinstance(xs, list):<EOL><INDENT>return [self._rel2id[x] for x in xs]<EOL><DEDENT>return self._rel2id[xs]<EOL>
Map relation(s) to id(s)

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

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