index on aggregate function

I somewhere read that it is possible to create an index on aggregate functions
in PostgreSQL. Which syntax do I have to use for it?

test=> CREATE TABLE test ( name VARCHAR(32) );
CREATE
test=> CREATE INDEX test_idx ON test lower(name);
ERROR: parser: parse error at or near "lower"

gives me a parser error.

--

Regards,

Sascha Schumann |
Consultant | finger sas(at)schell(dot)de
| for PGP public key