(* Baseline: binary search tree with insert + in-order traversal *)
type 'a tree =
  | Leaf
  | Node of 'a * 'a tree * 'a tree
;;

let rec insert x t =
  match t with
  | Leaf -> Node (x, Leaf, Leaf)
  | Node (v, l, r) ->
    if x < v then Node (v, insert x l, r)
    else if x > v then Node (v, l, insert x r)
    else t
;;

let rec inorder t =
  match t with
  | Leaf -> []
  | Node (v, l, r) -> List.append (inorder l) (v :: inorder r)
;;

let from_list xs = List.fold_left (fun t x -> insert x t) Leaf xs ;;

let t = from_list [5; 3; 8; 1; 4; 7; 9; 2] ;;
List.fold_left (fun a b -> a + b) 0 (inorder t)