check with select

Hi,

I have a table referancing services of a company :
CREATE TABLE Services (
id INTEGER NOT NULL PRIMARY KEY,
nom VARCHAR(30) NOT NULL -- name of the service
);

each of them can have many workers and many boss
but at least 1 boss

So i have a table referancing workers and boss :

CREATE TABLE EmployesDsServices (
service INTEGER REFERENCES Services (id)
ON DELETE CASCADE DEFERRABLE INITIALLY DEFERRED,
employe INTEGER REFERENCES Employes (id)
ON DELETE CASCADE DEFERRABLE INITIALLY DEFERRED,
niveau CHAR(1), -- R -> boss, S -> worker
CONSTRAINT niveau_valide CHECK (NIVEAU IN ('R','S')),
PRIMARY KEY (service, employe)
);

to check that at any time a service always have at least one boss,
i would like :

CREATE ASSERTION chaque_service_a_au_moins_un_responsable CHECK
(NOT EXISTS (SELECT id FROM Services LEFT OUTER JOIN
(SELECT * FROM employesdsservices WHERE niveau='R') AS resp ON
id=service GROUP BY id HAVING COUNT(niveau)<1));

the problem is that this is not valid in postgreSQL,
so i tryed to use a trigger, but it seems that it is not possible to
make a trigger deferrable (as the 2 tables are mutualy dependent).

i would like something i can use like this :

begin;
insert into services ...
insert into employesdsservices ...
end;

is there a way to do this, a check after each insert, update, delete in
both tables?

i know my constraint is anyway very slow, but this is for a scolar
project about theory and performance realy dont matters.

thank you for help

Edouard Boucher