column alias and group by/having/order

Hi
I found some strange column alias behaviour:

select val1+val2 as val
from some_table
group by val;

result - OK

select val1+val2 as val
from some_table
order by val;

result - OK

select val1+val2 as val
from some_table
group by val having val1+val2>1;

result - OK

select val1+val2 as val
from some_table
group by val having val>1;
ERROR:  Attribute "val" not found

Is it a bug or a feature?

Regards,
Tomasz Myrta

----------------------------------------------------------------------------------------------------------------
PostgreSQL7.3.4 on i386-pc-linux-gnu, compiled by GCC i386-linux-gcc 
 
(GCC) 3.3.2 20031005 (Debian prerelease)