" ::= " grammar rule not accepted by Postgres

<p><font face="sans-serif" size="2">This expression is (I think) a well-formed query, but is rejected by Postgresql 8.4
(tis a table name).</font><br /><br /><font face="sans-serif" size="2"> t t1 NATURAL JOIN t t2;</font><br /><br /><font
face="sans-serif"size="2">Here is the relevant parts of grammar (with some optional elements omitted for simplicity):
</font><br/><br /><font face="sans-serif" size="2"> <query expression> ::= <query expression
body></font><br/><br /><font face="sans-serif" size="2"> <query expression body> ::= <joined
table></font><br/><br /><font face="sans-serif" size="2"> <joined table> ::= <natural join></font><br
/><br/><font face="sans-serif" size="2"> <natural join> ::= <table reference> NATURAL JOIN <table
primary></font><br/><br /><font face="sans-serif" size="2"> <table reference> ::= <table
primary></font><br/><br /><font face="sans-serif" size="2"> <table primary> ::= <table or query name> [
<correlationname> ]</font><br /><br /><font face="sans-serif" size="2"> <table or query name> ::= <table
name></font><br/><br /><font face="sans-serif" size="2">Nor is this use of <joined table> accepted in the
<queryexpression> form of a <with list element>:</font><br /><br /><font face="sans-serif" size="2"> WITH
twAS ( t t1 NATURAL JOIN t t2 ) TABLE tw; </font><br /><br /><font face="sans-serif" size="2">This leads me to believe
thatthe unrecognized grammar rule (in SQL99 standard terms) is</font><br /><br /><font face="sans-serif" size="2">
<queryexpression body> ::= <joined table></font><br /><br /><font face="sans-serif" size="2">There are, of
course,other ways to express the operation, including</font><br /><font face="sans-serif" size="2"> SELECT * FROM (t t1
NATURALJOIN t t2);</font><br /><font face="sans-serif" size="2">But I am curious why the first expression is not
accepted,besides the immediately obvious "it's not in the Postgres version of the SQL grammar."</font><br /><br /><font
face="sans-serif"size="2">Clem Dickey</font>