Best way to prevent overlapping dates

I need to disable rows with overlapping dates in 8.1+
I created the following trigger procedure for this.

Is this best way ?
Will it prevent overlapping rows in all cases ?

Andrus.

create table puhkus (reanr serial primary key,   palgus date, plopp date);

CREATE OR REPLACE FUNCTION puhkus_sequenced_pkey() RETURNS trigger AS $$

DECLARE
   OverlappingRow INTEGER;

BEGIN
  SELECT reanr
  INTO OverlappingRow
FROM puhkus
WHERE puhkus.reanr<>NEW.reanr AND doverlaps( puhkus.palgus, puhkus.plopp,
NEW.palgus, NEW.plopp );

IF found THEN
  RAISE EXCEPTION 'Changed row % overlaps with existing row %', NEW.reanr,
OverLappingRow ;
  END IF;
RETURN NEW;
END;

$$ LANGUAGE plpgsql;

CREATE TRIGGER puhkus_sequenced_trigger BEFORE INSERT OR UPDATE ON puhkus
    FOR EACH ROW EXECUTE PROCEDURE puhkus_sequenced_pkey();

CREATE OR REPLACE FUNCTION public.doverlaps(date,
        date, date, date, out bool) IMMUTABLE AS
$_$
SELECT coalesce($1, date '0001-01-01')<=coalesce($4, date '9999-12-31') AND
     coalesce($2, date '9999-12-31')>=coalesce($3, date '0001-01-01' );
$_$ language sql;

CREATE OR REPLACE FUNCTION PUBLIC.GOMONTH(DATE, INTEGER, OUT DATE) IMMUTABLE
AS
$_$
SELECT ($1 + ($2 * '1 MONTH'::INTERVAL))::DATE;
$_$ LANGUAGE SQL;



I also tried code from SNODGRASS book
"Developing Time-Oriented Database Applications" but this causes error.

create table puhkus (palgus date, plopp date);

alter table puhkus add check
(NOT EXISTS ( SELECT *
FROM puhkus AS I1
WHERE 1 < (SELECT COUNT(*)
FROM puhkus AS I2
WHERE doverlaps(i1.palgus, i1.plopp, i2.palgus, i2.plopp)
) ));

but got error

ERROR: cannot use subquery in check constraint