case insensitive unique index (part 2)

Hi,

I've trolled the archives and the FAQ and the closest I could come up with was
the following mailing list message:

  http://www.postgresql.org/mhonarc/pgsql-general/2001-01/msg01632.html

which advises that you can do something like this to create a case-insensitive
unique index:

  CREATE UNIQUE INDEX indexname ON tablename (lower(fieldname))

however, that doesn't work under 7.0.2 at least, I get:

mydb=> create unique index forward_rr on forward (lower(name));
ERROR:  DefineIndex: function 'lower(varchar)' does not exist

Futhermore, I want to create the case-insensitive unique index on a tuple of
varchar fields, ie.:

  CREATE UNIQUE INDEX indexname ON tablename (lower(field1), lower(field2), lower(field3))

however, looking at the docs at:

  http://www.postgresql.org/users-lounge/docs/7.0/user/sql-createindex.htm

it looks like I will need to instead declare a function f such that it can be used as:

  CREATE UNIQUE INDEX indexname ON tablename (f(field1, field2, field3))

and f(n1, n2, n3) returns a unique value for unique combinations of n1, n2 and
n3...

Since this sounds like something someone must have already done, or should be
a FAQ, any hints? pointers? functions? PL/SQL code?

Thanks,
Adi