pub type Tree(a) {
  Leaf
  Node(left: Tree(a), value: a, right: Tree(a))
}

pub fn max(t: Tree(a), compare: fn(a, a) -> Order) -> Option(a) {
  case t {
    Leaf -> None
    Node(l, v, r) -> {
      let lmax = max(l, compare) |> option.unwrap(v)
      let rmax = max(r, compare) |> option.unwrap(v)
      case compare(lmax, v), compare(rmax, v) {
        Eq, _ -> Some(v)
        Gt, _ -> Some(lmax)
        Lt, _ -> Some(rmax)
      }
    }
  }
}