Index on computed column

Hi,

Can anyone explain to me what I'm doing wrong:
(The first select is to show that the function rubriek exists, and does work).
I want to create an index on a computed column:

palga=> select rubriek(rapport, lseek, 'naamvrouw',0) from main
     where rapport = 'T098-20900';
  rubriek
-----------
 Ramrattan
(1 row)
palga=> create index nm_idx on main (rubriek(rapport, lseek, 'naamvrouw',0));
ERROR:  parser: parse error at or near "'naamvrouw'" at character 54
palga=>

This is postgresql-7.3.4-3.rhl9.

Thanks in advance,

Han Holl