SQL & PL/SQL

I'm trying to use a WITH clause along with a UNION and getting an error. I've simplified things for test purposes to the following (not very useful, but shows the error) query (in scott/tiger)

with e as
(
select c1.empno as e1, c2.empno as e2, c3.empno as e3 from
emp c1 join emp c2 on c1.empno = c2.empno
join emp c3 on c3.empno = c2.empno
)
select e1 from e
union
select e3 from e

The error message is

ORA-00604: error occurred at recursive SQL level 1
ORA-00904: "from$_subquery$_003"."EMPNO_8_8": invalid identifier

If I don't use the union - or if I remove the third join (with c3) then the error doesn't happen.

I'm using 9i Release 2 (9.2.0.1.0) for Windows

Is this a bug? Or am I doing something wrong?

The basic problem is that I have a complex query which returns 3 columns, all of the same type, and I want to get the concatenation of all values of all three columns into one column. Any ideas?