Thanks for that. GraphViz is probably over kill, at least in the sense of how much work may be required getting up to speed with it, for this task.

However your analysis of an algorythm is along the lines that I'd been thrashing around and gives me enough confidence to have another go. I was reluctant to do all the backtracking and fixups that seemed to be required and spent a lot of time trying to find a more direct approach. I'll post the code when I get it sorted out.