beb8660c ​

let bg: (Int, Int)
let ls: OList (Int, Int, Int, Color)

let input = fill (bg.1) (fill (bg.2) black) |> fold hline ls

let output = fill (bg.1) (fill (bg.2) black) |> fold hline (ls
  |> sort (λ(y, x1, x2, c). x1 - x2)
  |> index
  |> map (λ((y, x1, x2, c), i). (-i, x1 - x2 - 1, -1, c)))