def filter_used_routes(
        transfers_pair: List[MediationPairState],
        routes: List[RouteState],
) -> List[RouteState]:
    """This function makes sure we filter routes that have already been used.

    So in a setup like this, we want to make sure that node 2, having tried to
    route the transfer through 3 will also try 5 before sending it backwards to 1

    1 -> 2 -> 3 -> 4
         v         ^
         5 -> 6 -> 7
    This function will return routes as provided in their original order.
    """
    channelid_to_route = {r.channel_identifier: r for r in routes}
    routes_order = {route.node_address: index for index, route in enumerate(routes)}

    for pair in transfers_pair:
        channelid = pair.payer_transfer.balance_proof.channel_identifier
        if channelid in channelid_to_route:
            del channelid_to_route[channelid]

        channelid = pair.payee_transfer.balance_proof.channel_identifier
        if channelid in channelid_to_route:
            del channelid_to_route[channelid]

    return sorted(
        channelid_to_route.values(),
        key=lambda route: routes_order[route.node_address],
    )
This function makes sure we filter routes that have already been used.

    So in a setup like this, we want to make sure that node 2, having tried to
    route the transfer through 3 will also try 5 before sending it backwards to 1

    1 -> 2 -> 3 -> 4
         v         ^
         5 -> 6 -> 7
    This function will return routes as provided in their original order.