inserting via "on insert" rule

Hi,

I have a table, say "person", where I store all the information of the
people I have in my files. I have a subgroup of people, say "users",
which I want to store on the same table "persons" by imposing some
restricitons on some columns.

I was thinking of defining a view "users" over "persons" which would let
me retrive the list of useres. But How would I implement the rule for
insertiung users? I tryed the following but NEW is not known where I
want to use it:

CREATE VIEW users AS
    SELECT * FROM persons WHERE is_user(person.id) = TRUE;

CREATE RULE insert_on_users AS ON INSERT
    TO users DO INSTEAD
        INSERT INTO persons SELECT * FROM NEW;

The other thing I don't know is how to enforce the aditional constraints
that qualify as user when inserting to users. Of course I could do it on
the application side, but I would like to have it on the database too.

Any suggestions will be appreciated.

Regards,

Andreas Fromm