[GENERAL] triggers, transactions and locks

[C G]

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();

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