recreate table tdetl(id int primary key using index tdetl_pk, pid int);
recreate table tmain(id int primary key using index tmain_pk);
alter table tdetl add constraint tdetl_fk foreign key (pid) references tmain(id) using index tdetl_fk;
set term ^;
execute block as
begin
  execute statement 'drop sequence g';
  when any do begin end
end
^
set term ;^
commit;

insert into tmain(id) values(1);
insert into tdetl(id, pid) values(1, 1);
commit;

create sequence g;
commit;
alter table tdetl add constraint tdetl_pid_gz check ( gen_id(g, -3 ) > 0 and pid>0 );
commit;

set term ^;
create or alter trigger tdetl_bi for tdetl active before insert as
  declare v int;
begin
  v = gen_id(g,0);
  v = gen_id(g, :v );
end
^
create or alter trigger tdetl_ai for tdetl active after insert as
  declare v int;
begin
  v = gen_id(g,123456);
end
^
set term ;^
commit;

set echo on;

alter sequence g restart with 5;
commit;
insert into tdetl(id, pid) values(2, -3);
rollback;
show sequence g;
commit;


alter sequence g restart with 5;
commit;
insert into tdetl(id, pid) values(2, 3);
rollback;
show sequence g;
commit;