CONCAT group operator

Is there any such thing as a CONCAT group operator?

What I mean is something like the SUM operator, which totals numbers over selected rows. This one would concatenate character fields over selected rows:

create table x (t varchar2(20));
insert into x values ('First');
insert into x values ('Second');

select concat(t) from x;

CONCAT(T)

FirstSecond

1 Row Selected

Or something like.

It seems such an obvious function, but I have scoured the documentation in vain for some reference.

-- 
    Mike Gahan 
    Information Systems Division
    University College London

    http://www.ucl.ac.uk/~ccaamrg/