function expression in FROM may not refer to other relations of same query level

Hello,

I'm trying to use the ROWTYPE return value of a plpgsql function in a
SELECT query. The test code is below.

The following query is accepted:

select id, usr, code, line1, line2 from tbl, get_lines(1);

id    usr      code   line1  line2
----------------------------------
1     one      1      A      B
2     two      2      A      B
3     three    1      A      B

But the same query with a parameter returns an error:

select id, usr, code, line1, line2 from tbl, get_lines(code);
--> ERROR:  function expression in FROM may not refer to other relations
of same query level

Is there another way to run this query and get:

id    usr      code   line1  line2
----------------------------------
1     one      1      A      B
2     two      2      Z      Z
3     three    1      A      B


Thanks


----------------------------------------
TEST CODE
----------------------------------------

CREATE TYPE public.lines AS
( line1 varchar(10), line2 varchar(10)
);
CREATE TABLE public.tbl
( id int4 PRIMARY KEY, usr varchar(10),  code int4
) WITHOUT OIDS;
CREATE FUNCTION public.get_lines(int4) RETURNS lines AS
'
DECLARE
  code ALIAS FOR $1;
  lines  lines%rowtype;

BEGIN  IF code = 1 THEN     lines.line1 = ''A'';      lines.line2 = ''B'';   ELSE      lines.line1 = ''Z'';
lines.line2= ''Z'';   END IF;  
  RETURN lines;

END;
' LANGUAGE 'plpgsql' VOLATILE;
INSERT INTO tbl VALUES (1, 'one', 1);
INSERT INTO tbl VALUES (2, 'two', 2);
INSERT INTO tbl VALUES (3, 'three', 1);