UNIQUE constraints on function results

Hi,

I've been trying to add a unique constraint on a row and a function
result of a row. I.e.:

CREATE TABLE test (
id SERIAL PRIMARY KEY,
t1 TEXT NOT NULL,
t2 TEXT NOT NULL,
UNIQUE (t1, lower(t2)));

That fails with a syntax error (on 8.2beta1). While UNIQUE(t1, t2) works
like a charm, it's not exactly what I want.

I can easily create an index for my needs [1], why can I not add such a
unique constraint? Thanks for clarification.

Regards

Markus

[1]: CREATE INDEX test_idx ON test(t1, lower(t2));