where to find info on ipfw programming interface?

[Alex]

Hi ppl!

I need to be able to control ipfw rules from within my program.
A year ago I spent a week to get how to do this using ipfw utility 
sources. And at last successfully did it after a lot of probes and falls :0)
But now since 5.0 FreeBSD is using another implementation (so called 
IPFW2) which is much more comprehensive and I believe robust at least as 
I got from ip_fw.h
But in any case some thing are still unclear for me.

So the question is: where can I find some kind of  formal specification 
on what order commands and actions must be placed in a rule and what 
kind of parameters are needed for every kind of op_code.

If there're anything about thsi problem I'd be very glad to have a copy 
or at last a link to :0)

Alex