[math-fun] Hey kids, bathroom stuff!

I'm pretty sure this is the pattern in Oren's Hummus bathrooms on Castro St; Can you find a short proof or disproof that the dark tiles form a square grid? —Bill (During the 70s, I stared at (slowly, in those days) evolving Life patterns so habitually that the AI Lab bathroom tiles would run a couple of steps when I looked down. But those were square tiles, randomly dark and light.)