8719f442 ​

let g: OList OList Color

let input = g

let output =
  flat (map (λi.
    hcat (map (λj. map (map (λc. black)) g
      |> if (i == 0 && g.1.j == grey) g
      |> if (i == 4 && g.3.j == grey) g
      |> if (j == 0 && g.i.1 == grey) g
      |> if (j == 4 && g.i.3 == grey) g
      |> if (g.i.j == grey) (map (map (λc. grey)) g)
    ) [0, 1, 2, 3, 4])
  ) [0, 1, 2, 3, 4])