it was to do the " and " instead of ", " prior to the last segment of the formatted result

javascript array reduce can provide the index and the array (so you can know whether you are on the last one). this is a HACK to man-handle ruby. I didn't like it, but I am trying to learn how to do FP in ruby come hell or high water ;)

I guess I could have done the replace-the-last-comma-with-" and" HACK (but both are equally HACKs)

maybe not!

I would never have thought that xor would remove stuff from a Set

nice and direct