All caps means it's a constant.

It's mathematical notation lingo. That's how you refer to matrix indexes.

Are you referring to the variable names I gave in the ‘for’ loops?
You can change them to ‘x’ and ‘y’ if that makes more sense to you.
Or did I miss something in your question... ?

I get that a lot :D

I would never use too convoluted one-liners in production; this is just to have fun with some extra challenge.

Plus, most of my Python one-liners are much, much harder to read ^^'.