def is_opposite(self, ns1, id1, ns2, id2):
        """Return True if two entities are in an "is_opposite" relationship

        Parameters
        ----------
        ns1 : str
            Namespace code for an entity.
        id1 : str
            URI for an entity.
        ns2 : str
            Namespace code for an entity.
        id2 : str
            URI for an entity.

        Returns
        -------
        bool
            True if t1 has an "is_opposite" relationship with t2.
        """
        u1 = self.get_uri(ns1, id1)
        u2 = self.get_uri(ns2, id2)
        t1 = rdflib.term.URIRef(u1)
        t2 = rdflib.term.URIRef(u2)

        rel = rdflib.term.URIRef(self.relations_prefix + 'is_opposite')
        to = self.graph.objects(t1, rel)
        if t2 in to:
            return True
        return False