unique across two tables

I have two tables, town and townalias, the latter containing alternative
town names.
I would like to ensure that a town name is unique per
country-region across the two tables.

Can I do this with a constraint ot do I need to implement the logic via
trigger?



=========
PostgreSQl 8.4

CREATE TABLE town
(
  country_fk character varying(3) NOT NULL,
  region_fk character varying(3) NOT NULL,
  id serial NOT NULL,
  "name" character varying(50) NOT NULL
)

CREATE TABLE townalias
(
  country_fk character varying(3) NOT NULL,
  region_fk character varying(3) NOT NULL,
  town_fk integer NOT NULL,
  id serial NOT NULL,
  "name" character varying(50) NOT NULL,
  CONSTRAINT town_townalias_fk FOREIGN KEY (country_fk, region_fk,
town_fk) REFERENCES town (country_fk, region_fk, id)
)

-- 

Best Regards,
Tarlika Elisabeth Schmitz