Writeable CTEs

Hi,

Attached is a patch where I've taken into account the problems Tom 
pointed out in his review [1].  It still needs a bit cleaning up, but 
I'm not planning on any big changes.  Any feedback welcome.

One thing I only noticed now is this:

=> select * from foo;
  a
---
  0
(1 row)

=> with t as (delete from foo returning *)
-> insert into bar
-> select * from t;
INSERT 0 2

It correctly reports 2 affected rows (one deleted and one inserted), but 
is this the answer we want?  It doesn't seem all that useful to know the 
total amount of affected rows.


Regards,
Marko Tiikkaja

[1] http://archives.postgresql.org/pgsql-hackers/2009-11/msg01860.php