EMIT,
thanks for your reply. I hope you have been able to sort out your copy/paste problems.
I think you are right that the choice to use A/THR for this approach is a necessary causal factor, and I shall add it to the graph. It will be an NCF at least for node (19), and perhaps, possibly with interims nodes, to (42) as well.
It will be very similar to (23).
Bernd