Присоединение пустого листа

 let rec union alist blist =
    match alist@blist with
        | []->[]
        | h::rest -> if memberof h rest then union rest [] else h::(union rest [])

union rest []

let union alist blist =
    let rec add a b =
        match a with
        | [] -> b
        | h::rest -> if List.contains h b then add rest b else h::(add rest b)
    add alist (add blist [])

let union alist blist =
    let add1 x a = 
        if List.contains x a then a else x::a
 
    let rec add a b =
        match a with
        | [] -> b
        | h::rest -> add1 h (add rest b)
        
    add alist (add blist [])

let union alist blist =
    let rec add a b =
        match a with
        | [] -> b
        | h::tail -> if List.contains h b then add tail b else add tail (h::b)
        
    [] |> add alist |> add blist |> List.rev

list1 @ list2 |> Seq.distinct |> Seq.toList

let notDublicated = [1;2;3;4] @ [3;4;5;6] |> Seq.distinct |> Seq.toList

add ([] |> add alist |> add blist ) []

let union alist blist =
    let rec add a b =
        match a with
        | [] -> b
        | h::tail -> if List.contains h b then add tail b else add tail (h::b)
        
    [] |> add alist |> add blist |> List.rev
    
printfn "%A" <| union [0;1] [0]