How to use result column names in having cause

CREATE TEMP TABLE foo( bar integer );

SELECT 123 AS x
  FROM foo
  GROUP BY 1
  HAVING x> AVG(bar)

causes

ERROR:  column "x" does not exist

Why ? How to make this working ?

In real application I have long expression instead of 123 and do'nt want
 repeat this expression in HAVING clause.

In VFP this select works OK.

Andrus.