referring to calculated column in sub select

Hi,

why column "acoltest" is not found by the subselect in this select:


SELECT
      acol + 100 as acoltest,
     (select max(t) from mytab where anothercol=acoltest) as col2
FROM mytab2
group by somet
???
Only columns belonging to a table can be used in a subselect??? What about "calculated" columns?
Thank you