How to find current row number relative to window frame

Hello,

I have a user defined aggregate function and in Sfunc I need to
reference current row number relative to window frame. Ideallly I would like to have following construct:

select my_aggregate(x,current_row_number_relative_to window) over
(order by y rows between n preceding and current row) as..... from ….


Any ideas how I can do this.  Thanks Art.