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. ![smiley :-)](https://www.theswamp.org/Smileys/black/smiley.gif)
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!
![Smiley :)](https://www.theswamp.org/Smileys/black/smiley.gif)
...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).