8731374e ​

let g: OList OList Color
let p: (Int, Int)
let c1: Color
let c2: Color
let yn: Int
let xn: Int
let ps: UList (Int, Int)

let input = fill yn (fill xn c1)
  |> fold (λp. point p c2) ps
  |> (λg2. g |> grid p g2)

let output = fill yn (fill xn c1)
  |> fold (λ(y, x). rect ((1, x, -1, x), c2)) ps
  |> fold (λ(y, x). rect ((y, 1, y, -1), c2)) ps