triggers vs "NEW" pseudorecord

okay. postgres 7.0.3 here, on debian potato/stable.

from the docs at /usr/share/doc/postgresql-doc/user/c40874340.html
here is a working trigger:

CREATE TABLE emp (
empname text,
salary int4,
last_date datetime,
last_user name);

CREATE FUNCTION emp_stamp () RETURNS OPAQUE AS ' -- <= missing quote!
BEGIN
-- Check that empname and salary are given
IF NEW.empname ISNULL THEN
RAISE EXCEPTION ''empname cannot be NULL value'';
END IF;
IF NEW.salary ISNULL THEN
RAISE EXCEPTION ''% cannot have NULL salary'', NEW.empname;
END IF;

-- Who works for us when she must pay for?
IF NEW.salary < 0 THEN
RAISE EXCEPTION ''% cannot have a negative salary'', NEW.empname;
END IF;

-- Remember who changed the payroll when
NEW.last_date := ''now'';
NEW.last_user := getpgusername();
RETURN NEW;
END;
' LANGUAGE 'plpgsql';

CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
FOR EACH ROW EXECUTE PROCEDURE emp_stamp();

so altho the docs elsewhere say NOT to rely on access to the
pseudo table NEW within a trigger function, this part does work
like it should. but when i add SELECT or UPDATE it complains of
"NEW used in non-RULE query" -- what's the distinction?

what types of operations are NOT LEGAL within such a
trigger-invoked function? (i'd like to be able to UPDATE other
tables and SELECT from various tables within the function. bad
dog?)