let rec btree_size t =   match t with
  |Empty->0
  |Node(_, g, d)->1+(btree_size g)+(btree_size d);;

let rec btree_height t = match t with

    |Empty-> -1
    |Node(n,g,d) -> 1 + max (btree_height g) (btree_height d);;


let rec btree_mem x t = match t with
    |Empty->false
    |Node(a,g,d)->if a=x then true else btree_mem x g || btree_mem x d ;;

let rec arity t = match t with
  |Empty-> -1
  |Node(_,Empty,Empty)-> 0
  |Node(_,a,Empty)-> 1
  |Node(_,Empty,b)-> 1
  |Node(_,a,b)-> 2;;