Phewf - this was a tough one! As I call out in the comments on 21.zig,
I had cracked _most_ of the shortcutting to this, but hadn't noticed
that choice of order-of-operations was not arbitrary - needed to notice
that doing a < before a ^ ends up more efficient than the other way
around, for instance.
I shudder to think what part 2 is going to be like...
For the record, my attempts at this took 1253 lines, over two files -
considering my total line count so far is 6233, that's over one-fifth.
Wild!
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