recreate table tmp(id int primary key, x int, y int);
recreate sequence g_tmp;
create descending index tmp_id_desc on tmp(id);
commit;
insert into tmp(id, x) values( gen_id(g_tmp,1), 100);
commit;

set term ^;
create or alter trigger tmp_bi for tmp
active before insert or update position 0 as
begin
  new.id=gen_id(g_tmp,1);
end^

create or alter trigger tmp_bii for tmp
active before insert or update position 1000 as
begin
  if (inserting) then
    new.id=gen_id(g_tmp,1);
  new.y = (select x from tmp where id < new.id order by id desc rows 1  < new.id  );
end^
set term ;^
commit;

Overflow occurred during data type conversion.
conversion error from string "BOOLEAN".
At trigger 'TMP_BII' line: 6, col: 3.