<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def filter_reachable_routes( routes: List[RouteState], nodeaddresses_to_networkstates: NodeNetworkStateMap, ) -> List[RouteState]: """This function makes sure we use reachable routes only."""

    reachable_routes = []

    for route in routes:
        node_network_state = nodeaddresses_to_networkstates.get(
            route.node_address,
            NODE_NETWORK_UNREACHABLE,
        )

        if node_network_state == NODE_NETWORK_REACHABLE:
            reachable_routes.append(route)

    return reachable_routes