type Tree<'a> =
  | Node of Tree<'a> * 'a * Tree<'a>
  | Leaf

let rec fold (f : 'b -> Tree<'a> -> 'b) (m  : 'b) (t : Tree<'a>) : 'b =
  let m' = f m t
  match t with
    | Node(tl, _, tr) -> fold f (fold f m' tl) tr
    | Leav -> m'

let sum t =
  let folder m t' : int =
    match t' with
      | Node(_, v, _) -> m + v
      | Leaf -> m
  fold folder 0 t

let testTree =
  Node(Node(Node(Leaf, 9, Leaf), 2, Node(Leaf, 21, Leaf)), 12, Leaf)