proposal: window function - change_number

Hi
I tried to solve following task:

I have a table

start, reason, km
=============
 2014-01-01 08:00:00, private, 10
 2014-01-01 09:00:00, commerc, 20
 2014-01-01 10:00:00, commerc, 20
 2014-01-01 11:00:00, private, 8

and I would reduce these rows to

 2014-01-01 08:00:00, private, 10
 2014-01-01 09:00:00, commerc, 20 + 20 = 40
 2014-01-01 11:00:00, private, 8

It is relative hard to it now with SQL only. But we can simplify this task with window function that returns number of change in some column. Then this task can be solved by

select min(start), min(reason), sum(km)

  from (select start, reason, km, change_number(reason) over (order by start))

  group by change_number;

Do you think, so it has sense?

Regards

Pavel