attribute level rules not supported?

Here's the problem:

test=> CREATE TABLE a ( refb OID, at INT2 );
test=> CREATE TABLE b ( refa OID, bt INT2 );
test=> CREATE RULE ar AS ON UPDATE TO a.at
test->   DO UPDATE b SET b.bt = NEW.at
test->   WHERE refa = OLD.oid;
ERROR: attribute level rules currently not supported

What am I doing wrong? I've read and re-read chapter 8 - rules from the
postgresql programmer's guide, but I can't see why this rule doesn't work.
Any suggestions would be appreciated.

Marc