data dependent sequences?

Advice requested :-) I have a table like:

CREATE TABLE items (
id INT,
typ INT ...
PRIMAY KEY (seq,typ));

I would like 'id' to be like a SERIAL except that I
want independent sequences for each value of 'typ'.

So if 'items' is:

id typ
----+-----
1 'a'
2 'a'
3 'a'
1 'b'

then doing:

INSERT items(typ) VALUES('a');
INSERT items(typ) VALUES('b');

will result in:

id typ
----+-----
1 'a'
2 'a'
3 'a'
1 'b'
4 'a'
2 'b'

Because there can be dozens of values of 'typ'
and new ones added not infrequently, creating
a postgresql sequence for each seems awkward.

Are there other ways I could accomplish this?