let set_open_wall x y wall_pos (_, _, c) =
  let (color, wall) = c.(x).(y) in
  match wall_pos with
    1 -> let (_, b, c, d) = wall in c.(x).(y) <- (color, (wall_pos, b, c, d))
  | 2 -> let (a, _, c, d) = wall in c.(x).(y) <- (color, (a, wall_pos, c, d))
  | 3 -> let (a, b, _, d) = wall in c.(x).(y) <- (color, (a, b, wall_pos, d))
  | 4 -> let (a, b, c, _) = wall in c.(x).(y) <- (color, (a, b, c, wall_pos))