creating aggregates that work on composite types (whole tuples)

I am trying to create an aggregate function that works on whole tuples,
but the system does not find them once defined ;(

hannu=# \d users
Table "users"
Column | Type |
Modifiers
----------+---------+------------------------------------------------------
fname | text | not null
lname | text | not null
username | text |
userid | integer | not null

hannu=# create or replace function add_table_row(text,users) returns
text as
hannu-# 'state = args[0]
hannu'# user = args[1]["fname"] + ":" + args[1]["lname"]
hannu'# if state:
hannu'# return state + "\\n" + user
hannu'# else:
hannu'# return user
hannu'# '
hannu-# LANGUAGE 'plpython';
CREATE
hannu=# select add_table_row('',users) from users;
add_table_row
---------------
jane:doe
john:doe
willem:doe
rick:smith
(4 rows)
hannu=# create aggregate tabulate (
hannu(# basetype = users,
hannu(# sfunc = add_table_row,
hannu(# stype = text,
hannu(# initcond = ''
hannu(# );
CREATE
hannu=# select tabulate(users) from users;
ERROR: No such attribute or function 'tabulate'

What am I doing wrong ?

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Hannu