Arrays in pl/pgsql functions

PostgreSQL version 7.3.3

With reference to pl/pgsql functions.
I can declare a function to return an array type and I get no complaints when
defining or executing it.
I can declare a variable within the function as an array type and I don't get
any complaints defining or executing the function.
When I try to access the array I have problems.
I cannot find any documentation on accessing array types so I assumed they
would follow the pgSQL notation of for example:

CREATE OR REPLACE FUNCTION test_arrays() RETURNS int4[] AS '
DECLARE
     test int4[];
BEGIN
    test[1] := 1;
    RETURN test;
END ' LANGUAGE 'plpgsql';

If I try to execute this function I get:
WARNING:  plpgsql: ERROR during compile of test_arrays near line 5
ERROR:  syntax error at or near "["

If I comment out the line:
test[1] := 1;
then the function executes ok.

Could someone be please inform me what the correct syntax for accessing arrays
is or tell me if its not possible with pl/pgSQL functions.

Thanks in advance,
Donald Fraser.