A demonically hard puzzle game demake with fantastic sound and graphics too, though it benefits from being split into 8(!) different PICO-8 carts (by far the highest I've seen so far).
My progress: completed A--K, 1--11, 13, 15--18, 20, 22, 25, 36, and 1*; my browser deleted my save, so I probably won't go back