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();