triggering a sequence

Hi,

I have seen in the documentation an example of adding a trigger to a
sequence, so no duplicates will be inserted when using the sequence.

Problem is, I can't find it anymore. Can anyone hint me, where it is?

Heiko

And below is the long description:

I have the following table:
create table test (id serial, text char(5));

as long as I don't insert a id manually, everything works fine.
(insert into test (text) VALUES '1';)
...
id | text
----+-------
1 | 1
2 | 2
3 | 3

when I insert the id manually:
insert into test (id, text) VALUES (5,'4');
I get what I expect:
id | text
----+-------
1 | 1
2 | 2
3 | 3
5 | 4

But inserting two more rows:
insert into test (text) VALUES ('5');
id | text
----+-------
1 | 1
2 | 2
3 | 3
5 | 4
4 | 5
insert into test (text) VALUES ('6');
ERROR: Cannot insert a duplicate key into unique index test_id_key

I understand, the sequence will give me 5 as id and that's already
there. I would like the table to look like this:
id | text
----+-------
1 | 1
2 | 2
3 | 3
5 | 4 --- here I inserted id manually
6 | 5
7 | 6

How do I solve this?

Heiok