Foreign Key ?? - Mailing list pgsql-sql

Good-day,
How are foreign keys implemented in PostgresSQL ?
I tried using the following statment:
CREATE TABLE orders(
    order_no    int     primary key,
    client_name    char(30),
    CONSTRAINT  NAME_FK FOREIGN KEY (client_name)
        REFERENCES client(client_name)
);
This results in FUNCTION NOT IMPLETEMENTED error.

Thanks for you time
David Jackson