BUG #5506: Error in the grammar of de joins

The following bug has been logged online:

Bug reference:      5506
Logged by:          Fernando Cano
Email address:      fcano@uniovi.es
PostgreSQL version: 8.4
Operating system:   Ubuntu 9.04
Description:        Error in the grammar of de joins
Details:

This sentences are valid with your grammar but generate an error.

create table t1 ( id_t1 smallint, name text);
create table t2 ( id_t2 smallint, name text);

select * from natural join using (id) ;
select * from t1 natural cross join t2;
select * from natural cross join using (id) ;
select * from t1 join t2;
select t1  natural left join t2;

I have problems when I explain to my students the sintax of the joins.

I use this  sintax, but I'm not sure:

from_item  {CROSS | NATURAL join_type }  JOIN  from_item
from_item join_type JOIN from_item [ ON join_condition | USING ( join_column
[, ...] ) ]

where join_type is:
     [ INNER ] | {LEFT |RIGHT | FULL} [ OUTER ]