type color = W | B 
type image = L of color * int 
           | N of image * image * image * image 
Tree:
N (L (W, 4), N (L (W, 1), L (B, 1), L (W, 1), L (B, 2)),
 N (L (W, 2), L (W, 2), L (W, 2), L (B, 1)), L (B, 4))
 
Tree to array:
let tree_to_array tr n =
  match tr with
  | L(a, b) -> Array.make_matrix n n (leaf_bit a)
  | N(a, b, c, d) -> quad_arr_append (tree_to_array a (n/2)) (tree_to_array b (n/2)) (tree_to_array c (n/2)) (tree_to_array d (n/2))