prev_number = null;
FOR SELECT u1_case.short_number
      FROM u1_case
      WHERE u1_case.year_reg=2016
      ORDER BY u1_case.short_number
      INTO :anumber
DO BEGIN
  WHILE (anumber - prev_number > 1) DO
  BEGIN
     prev_number = prev_number  + 1;
     SUSPEND;
  END
  prev_number  = anumber;
END

set term ^;

recreate procedure sp_gen_series(astart int, aend int)
returns (avalue int)
as
begin
  avalue = astart;
  while (aend - avalue >= 0) do
  begin
    suspend;
    avalue = avalue + 1;
  end
end^

set term ;^

with t as (
  select
    coalesce(lag(id) over(order by id), 0) as prev_id,
    id
  from mytable)
select sp.avalue as id
from t join sp_gen_series(t.prev_id + 1, t.id - 1) sp on true
where t.id - t.prev_id > 1;