plpgsql: can I use a variable in a DECLARE later whithin the DECLARE?

Hi,

In plpgsql, i'm trying to use a variable initialised in a DECLARE in
a later initialisation expression whithin the same DECLARE:

  DECLARE
    x int := 23;
    y int := x + 4
  BEGIN
   ...

Experiments suggest that it works, but the docs are silent about that.
Is this a good idea, or should I be more careful and do it in a nested
block?

(For Lispers: is that a let, or a let*? ;-)

Thanks & cheers
 - tomás