trigger function with arguments from a sql command

hi there, i'm having a big problem. I have this table

CREATE TABLE test {

id int8 NOT NULL,
asname varchar(80) NOT NULL,
priority int2 NOT NULL,
serviceid,
...

CONSTRAIN PRIMARY KEY (id),
CONSTRAIN FOREIGN KEY (serviceid) REFERENCES service (id) MATCH FULL
}

and i need to make a trigger function that accept arguments insert in a INSERT INTO. The priority value must be unique for the same serviceid but it can be the same for different serviceid. how can i do that, pass the arguments of a command? tks in advance