(* 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