plpgsql trigger function with arguments

I'm sure I saw something like this on the postgresql web
site but the the search function is down in the documentation
area.

I'm unable to pass a function arguments in a CREATE
TRIGGER statement.

What am I doing wrong here?

CREATE FUNCTION pregs_func (VARCHAR(15))
RETURNS trigger
LANGUAGE plpgsql
AS '
DECLARE
trigger_type ALIAS FOR $1;

BEGIN
IF trigger_type = ''insert'' THEN
...
END IF;
END;
';

CREATE TRIGGER pregs_insert_trigger
AFTER INSERT
ON pregs FOR EACH ROW
EXECUTE PROCEDURE pregs_func('insert');

CREATE TRIGGER pregs_update_trigger
AFTER UPDATE
ON pregs FOR EACH ROW
EXECUTE PROCEDURE pregs_func('update');

The CREATE TRIGGER statements return:
ERROR: CreateTrigger: function pregs_func() does not exist

I tried doing variations on:
create trigger pregs_insert_trigger after insert on pregs
for each row execute procedure
pregs_func (cast('insert' as varchar(15)));

thinking that I don't have the right function because the
datatypes don't match, but I get:
ERROR: parser: parse error at or near "cast" at character 106

PostgreSQL 7.3.

Thanks,

Karl <kop(at)meme(dot)com>
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