Kuk

let rec mem value (xs : int list) =
    match xs with
    | [] -> false
    | x::xs when x = value -> true
    | x::xs -> mem value xs

let rec union list1 list2 =
    match list2 with
    | [] -> list1
    | x::xs when mem x list1 -> union list1 xs
    | x::xs -> (union list1 xs) @ [x]