default values in inheritance hierarchies

Hi,

I have two tables defined for example like this:

create table a1 (id serial primary key, data text);
create table a2 (id serial primary key, data2 integer) inherits (a1);

The point is, that I want to have two tables with inheritance relation
between them, but each with its own id column (overriden in child table).

Then I execute this sequence of commands:

insert into a1(data) values('abc');
insert into a2(data2, data) values(123, 'def');
update a1 set id = default, data = 'ghi';

I need new IDs on each update, because I store old rows in another table to
keep history of changes. Problem is that update uses values from sequence
belonging to table a1 when updating rows in table a2.

So the content of tables after operations is:

select * from only a1;
id | data
----+------
2 | ghi

select * from a2;
id | data | data2
----+------+-------
3 | ghi | 123

The following sequence of commands ends up wit