let rec leftTree n x k =
 match n with
 | -1 -> k
 | n -> leftTree (n-1) (x+1) (Node(k, x, Leaf));;

let rec rightTree n x k =
 match n with
 | -1 -> k
 | n -> rightTree (n-1) (x+1) (Node(Leaf, x, k));;