index with user-defined function

Dear all,
I use PostgreSQL 6.5 and create a function tmod(bpchar) written in C.
When I try to create index using the function I have the
following problem.
------ 1-------
mydb=> CREATE INDEX vi_id ON village (vi_name bpchar_ops,vi_id
bpchar_ops);
CREATE
this works find.
------ 2 -------
mydb=> CREATE INDEX tmodvi_name ON village (TMOD(vi_name) bpchar_ops);
CREATE
this also works fine.
but when I try
------ 3 -------
mydb=> CREATE INDEX xxvi_name ON village (TMOD(vi_name) bpchar_ops,vi_id
bpchar_ops);
ERROR: parser: parse error at or near ","

What wrong with this? Can I create index using multiple rows with
user-defined function??

Choosna Makarasara