Mapping Concat

Oracle 11.2 has added the listagg aggregate function, which does the same job as Concat:

T group by { A } add { Concat(B, C order by { A, B }) Bs }
=>
select A, listagg(B, C) within group (order by B) as Bs from T group by A

But how can I create a device mapping for it that includes the ordering?