(* Binary tree with leaves car­rying an integer. *)
type Tree = Leaf of int | Node of Tree * Tree

let rec existsLeaf test tree =
  match tree with
  | Leaf v -> test v
  | Node (left, right) ->
      existsLeaf test left
      || existsLeaf test right

let hasEvenLeaf tree =
  existsLeaf (fun n -> n % 2 = 0) tree