possibly spurious `EXCEPT ... may not refer to other relation...'

The current CVS version of PostgreSQL gives us the error:
       ERROR:  UNION/INTERSECT/EXCEPT member statement       may not refer to other relations of same query level

when given the following test case, despite the fact that the EXCEPT
clause does not refer to any other relation involved in the same
query.  We suspect this to be a bug.  (This simple test case has been
drastically reduced from the actual code we want to run, which is a
more complicated INSERT INTO SELECT ... EXCEPT whose goal is to avoid
inserting rows that are already in the destination table; so whereas
here the except clause involves a third, dummy table, we would
actually like to check for whether the row exists in the table which
is the target of the INSERT.)

CREATE TABLE current ( number INTEGER );
CREATE TABLE former ( number INTEGER );
CREATE TABLE trash ( number INTEGER) ;

CREATE OR REPLACE RULE current_delete AS
ON DELETE TO current
DO INSERT INTO former (number)SELECT number FROM current WHERE number = OLD.numberEXCEPT SELECT number FROM trash;

-- 
Brandon Craig Rhodes                         http://www.rhodesmill.org/brandon
Georgia Tech                                            brandon@oit.gatech.edu