triggers, transactions and locks

Dear All,

Is there a way to use locks within a trigger? My example below gives the
error:
ERROR:  unexpected error -8 in EXECUTE of query "BEGIN"
CONTEXT:  PL/pgSQL function "insert_into_t1" line 6 at execute statement

Thanks

Colin


CREATE FUNCTION insert_into_t1() RETURNS trigger AS'
        DECLARE
                set_qry text;
        BEGIN

                set_qry:=''BEGIN WORK'';
                execute set_qry;
                LOCK t1;
                INSERT INTO t2 VALUES (11);
                COMMIT;
                RETURN NEW;
        END;
' LANGUAGE 'plpgsql' SECURITY DEFINER;

CREATE TRIGGER insert_into_t1 BEFORE INSERT
        ON t1 FOR EACH ROW EXECUTE
                PROCEDURE insert_into_t1();