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?