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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _remove_rid_from_vrf_list(self, ri):
 """Remove router ID from a VRF list. This removes a router from the list of routers that's kept in a map, using a VRF ID as the key. If the VRF exists, the router is removed from the list if it's present. If the last router in the list is removed, then the driver's method to remove the VRF is called and the map entry for that VRF is deleted. """

        if ri.ex_gw_port or ri.router.get('gw_port'):
            driver = self.driver_manager.get_driver(ri.id)
            vrf_name = driver._get_vrf_name(ri)
            if self._router_ids_by_vrf.get(vrf_name) and (
                    ri.router['id'] in self._router_ids_by_vrf[vrf_name]):
                self._router_ids_by_vrf[vrf_name].remove(ri.router['id'])
                # If this is the last router in a VRF, then we can safely
                # delete the VRF from the router config (handled by the driver)
                if not self._router_ids_by_vrf.get(vrf_name):
                    LOG.debug("++ REMOVING VRF %s" % vrf_name)
                    driver._remove_vrf(ri)
                    del self._router_ids_by_vrf[vrf_name]