create table A (ID integer not null primary key, F integer not null);
create table B (ID integer not null primary key, F integer not null);

insert into A (ID, F) values (1, 101);
insert into A (ID, F) values (2, 102);
insert into A (ID, F) values (3, 103);

insert into B (ID, F) values (1, 101);
insert into B (ID, F) values (2, 102);
insert into B (ID, F) values (4, 104);

select
  A.F
from
  A
left outer join
  B on B.F = A.F
where
  B.F is null

execute block returns (f int)
as
  declare a_f int;
  declare b_f int;
  declare b_c cursor for (select F from B order by F);
  declare b_eof boolean;
begin
  open b_c;
  fetch b_c into :b_f;
  b_eof = row_count = 0;
  for select F from A order by F into :a_f do
  begin
    while ((a_f > b_f) and (not b_eof)) do
    begin
      fetch b_c into :b_f;
      b_eof = row_count = 0;
      if (b_eof) then
        leave;
    end
    if ((a_f < b_f) or b_eof) then
    begin
      f = a_f;
      suspend;
    end
  end
end

select f
from (select f, 1 vid from a
      union all
      select f, 2 vid from b)
group by 1
having max(vid) = 1