def remove_edge(self, u, v):
        """Version of remove_edge that's much like normal networkx but only
        deletes once, since the database doesn't keep separate adj and
        succ mappings

        """
        try:
            del self.succ[u][v]
        except KeyError:
            raise NetworkXError(
                "The edge {}-{} is not in the graph.".format(u, v)
            )