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.