pl/tcl trigger question

Hello everyone,

I'm working on a tiny trigger function that needs to ensure that all
values entered in a field are lowercase'd. I can't use pl/pgsql because
I have a dozen different columns (with different names) that need a
trigger that does this and pl'pgsql can't expand variable names to
fieldnames. Writing a dozen functions (one per columnname) is /way/ too
blunt so I tried pl/tcl (which I don't know):

----------------------------------------------------------------
-- first do:
--   createdb test
--   createlang pltcl test

drop function my_lowercase() cascade;
create function my_lowercase() returns trigger as '
  set NEW($1) lower(NEW($1))
  return [array get NEW]' language 'pltcl';

drop table mytab;
create table mytab (myfield varchar);

create trigger trig_mytab before insert or update on mytab
  for each row execute procedure my_lowercase('myfield');

-- let's insert a string, hope it's lowercase'd
insert into mytab (myfield) values ('TEST');
select * from mytab;

-- wrong, myfield contains 'lower(NEW(myfield))'
----------------------------------------------------------------

Can someone please tell me what I'm doing wrong? It's probably
something very simple but I don't know TCL (and I'm planning to keep
the serverside programming on pl'pgsql as much as possible).

TIA!