[ddulaney]

It’s mechanically solvable but not in any practical timescale. A naive approach would be to check every square with no repeats in the rows, which requires (26!)^26 attempts, or roughly 5e691. Obviously you can improve that by exploiting symmetry, shifts, etc, but that only gets you a few orders of magnitude. There are much cleverer techniques, but when your baseline is so ludicrously impossible you need a real breakthrough to make any progress.