760b3cac ​

let g1: OList OList Color
let g2: OList OList Color

let input = fill 6 (fill 9 black)
  |> grid (1, 4) g1
  |> grid (4, 4) g2

let output = fill 6 (fill 9 black)
  |> grid (1, 4) g1
  |> grid (4, 4) g2
  |> grid (1, if (g2.1.1 == black) 7 1) (map rev g1)