RULES and QUALIFICATION for INSERT

Hi to everyone.
I'll try to explain my problem with an example.

-- I've got 2 tables and one view
CREATE TABLE a (name varchar(20) primary key, num integer);
CREATE TABLE b (town varchar(15), name varchar(20) references a(name));

insert into a values ('tom',5);
insert into a values ('paul',99);
insert into a values ('jack',1234);
insert into b values ('london','tom');
insert into b values ('rome','paul');

CREATE VIEW vvv AS SELECT * FROM a NATURAL JOIN b;

-- I've crated a rule in this way

CREATE RULE rrr AS ON INSERT TO vvv
WHERE NEW.name = 'tom'
DO INSTEAD
INSERT INTO a VALUES (NEW.name, NEW.num);

Trying a simple INSERT INTO vvv, I receive this message:
/*ERROR: cannot insert into a view*
*HINT: You need an unconditional ON INSERT DO INSTEAD rule.* /

If I've understood well, the qualification (*/WHERE NEW.name = 'tom'/*)
is the condition under which the rule has to be executed.
Only if the condition is met, the rule is executed.
I noticed that if I remove the qualification, the rule works, but doing
so I am not able anymore to test the condition. (I could overcame this
problem with a trigger, but I'd prefer if someone could explain me how
to do this with rules).

Best regards
Srdjan Matic