loop

Hi,

I need some help.

I have two sets of curves, black and green ones.

first I trace a parallel line n1 to a1 so I get the intersection with the first black one (r4) at point A.

Through this point A I want to trace a parallel line to a2. This line cuts r3 at B.

At this point B I need to trace a new parallel line to a3 that cuts r2 at C...

and so on and so forth,

red lines are auxiliary lines parallels to green ones.

I think it could be get with a loop but I have no idea to do it.

Could anyone give me a clue?

Thanks a lot!!