How to go from a road block to a list of lanes or a lane to a road block?

How to go from a road block to a list of lanes or a lane to a road block? There doesn’t seem to be a pointer from road_block record to lanes or lane record to a road block

Please take a look at https://github.com/nutonomy/nuscenes-devkit/blob/master/python-sdk/nuscenes/map_expansion/map_api.py#L596