(* Binary tree datatype. *)
datatype 'a btree = Leaf | Node of 'a btree * 'a * 'a btree

(* A reduction function. *)
(* btree_reduce : ('b * 'a * 'b -> 'b) -> 'b -> 'a tree -> 'b) *)
fun btree_reduce f b bt =
   case bt of
      Leaf => b
    | Node (l, x, r) => f (btree_reduce f b l, x, btree_reduce f b r)


(* DEFINE btree_size HERE *)
(* btree_size : 'a btree -> int *)
fun btree_size bt =
   let
      fun f l a r = 1 + l + r
   in
      btree_reduce(f 0 bt)
   end