Nice one John - I miss the old 'challenge' style academic threads which focussed more on fundamental programming technique and your post reminded me of them.
FWIW, you could omit one level of nesting from your code to achieve the same result, i.e.:
Yeah, they were a lot of fun and very, very helpful.
Ugh! That's what I was trying to do in the beginning (well, at least that is how I tried to "think out the method" like I used to be able to do). Thanks!
...I had a problem and it eventually morphed into that mess (my luck, I probably just misspelled "mapcar" and could have saved a few brain cells).