disjoint union types

Hi,

I was after opinions as to the best way to lay tables out to get the
effect of a "disjoint union" type (also known as a "tagged union").
When I have to do this at the moment, I'm creating a structure like:

CREATE TABLE circle ( id SERIAL PRIMARY KEY, radius REAL NOT NULL );
CREATE TABLE square ( id SERIAL PRIMARY KEY, sidelen REAL NOT NULL );

CREATE TABLE shapes (
id SERIAL PRIMARY KEY,

tag INTEGER NOT NULL,

circleid INTEGER REFERENCES circle
CHECK ((tag = 1) = (circleid IS NOT NULL)),
squareid INTEGER REFERENCES square
CHECK ((tag = 2) = (squareid IS NOT NULL))
);

I can then put data into this by doing:

BEGIN;
INSERT INTO circle (radius) VALUES (1);
INSERT INTO shapes (tag,circleid) VALUES (1,currval('circle_id_seq'));
COMMIT;

This works, but it's sometimes a bit of a headache turning things around
so they fit this structure. Are there standard solutions to this that
work better?

Thanks,
Sam