opaque function not accepted (without error message)

Hello world,

(postgresql 7.2.1-5 on a completely patched redhat 7.3 box)

In my db I have a function that will be used as a trigger when updating
records.

drop function au_col();
create function au_col()
returns opaque
as 'begin
new.mut_id = current_user;
new.mut_timestamp = CURRENT_TIMESTAMP;
return new;
end;'
language 'plpgsql';

All seems well, "CREATE" is echoed. Later I found out the function
wasn't in the database, \df in psql doesn't show it. After some
experimenting I found out that apparantly the return type opaque is the
evildoer. When I use an integer type:

drop function au_col();
create function au_col()
returns integer
as 'begin
new.mut_id = current_user;
new.mut_timestamp = CURRENT_TIMESTAMP;
return 1;
end;'
language 'plpgsql';

the function is created, and \df shows it. Is it impossible to create
an opaque function (according to my books this is quite possible)? Am I
doing something wrong?

TIA for any reactions!

--
Jules Alberts