I did that and I have this error,

(use-modules (gnu) (gnu system nss))
(use-service-modules desktop networking dbus cups)
(use-package-modules certs mate wicd glib cups)

(services (cons* (service-cups-service-type
(cups-configuration
(web-interface? #t)
(extensions
(list cups-filters htplip))))))

(operating-system
  (host-name "x200")
  (timezone "Europe/Paris")
  (locale "fr_FR.utf8")

sudo -E guix system reconfigure /etc/config.scm
/etc/config.scm:7:0: erreur : services : variable non liée
conseil : Auriez-vous oublié un `use-modules' ?