如何创建一个返回一个列表的函数，该列表是Ocaml中嵌套列表元素的并集？

let rec flatten_list lst =
  match lst with
  | [] -> []
  | hd::tl -> List.append hd (flatten_list tl)

let rec union_of_nested_lists lst =
  let flattened_list = flatten_list lst in
  let rec remove_duplicates acc lst =
    match lst with
    | [] -> acc
    | hd::tl ->
      if List.mem hd acc then remove_duplicates acc tl
      else remove_duplicates (hd::acc) tl
  in
  remove_duplicates [] flattened_list