type color =
  | Red
  | Green
  | Blue
  | Black

type row = color list

type screen = row list

(* let rec count_red_row (cs : row) : int = *)
(*   match cs with *)
(*   | [] -> 0 *)

(*   (\* option 1 *\) *)
(*   | Red::cs' -> 1 + count_red_row cs' *)
(*   | _::cs' -> count_red_row cs' *)

  (* option 2 *)
  (* | c::cs' -> *)
  (*   match c with *)
  (*   | Red -> 1 + count_red_row cs' *)
  (*   | _   -> count_red_row cs' *)

let rec count_red_screen (rs : screen) : int =
  let rec count_red_row (cs : row) : int =
    match cs with
    | [] -> 0

    (* option 1 *)
    | Red::cs' -> 1 + count_red_row cs'
    | _::cs' -> count_red_row cs'
  in
  match rs with
  | [] -> 0
  | r::rs' -> (count_red_row r) + (count_red_screen rs')


let foo x y z =
  let sum_xy = x + y
  in sum_xy + z