Sean:
this is the code:

// -------------------------------------------------------

CREATE OR ALTER trigger comuni_bi for comuni
active before insert position 0
as
begin
if (new.mycode is null) then
new.mycode = gen_id(gen_comuni_id,1);
end

// ----------------------------------------------------------

Martijn:
if I do:

select gen_id( gen_comuni_id,1 ) from rdb$database

Give Me GEN_ID=1