F# var 12

(*
Params:
   a, b, c, d - input sets
   state - variable for state saving
   elm - single element used in contains subfunction
*)
let rec megaTask a b c d state elm =
    match state with
    //Subfunction contains
    //Checks if elm is in list a
    //Non empty list mean true, empty list - false
    | 1 -> (if Seq.isEmpty a then
                []
            elif elm = (List.head a) then
                [0]
            else
                (megaTask (List.tail a) b c d 1 elm)
            )
    //Subfunction union
    //Creates union of two sets - a and b
    | 2 -> (if Seq.isEmpty b then
                a
            elif not (Seq.isEmpty (megaTask a [] [] [] 1 (List.head b))) then
                (megaTask a (List.tail b) [] [] 2 0)
            else
                (List.head b) :: (megaTask a (List.tail b) [] [] 2 0))
    //Subfunction union
    //Creates union of two sets - a and b
    | 3 -> (if Seq.isEmpty a then
                []
            elif not (Seq.isEmpty (megaTask b [] [] [] 1 (List.head a))) then
                (megaTask (List.tail a) b [] [] 3 0)
            else
                (List.head a) :: (megaTask (List.tail a) b [] [] 3 0))
    //Subfunction intersection
    //Computes intersection of two sets - a and b
    | 4 -> if Seq.isEmpty a then
                []
            elif not (Seq.isEmpty (megaTask b [] [] [] 1 (List.head a))) then
                (List.head a) :: (megaTask (List.tail a) b [] [] 4 0)
            else
                (megaTask (List.tail a) b [] [] 4 0)
    //Computes target result - (A ∩ (B υ C)) ∩ D
    | _ -> (megaTask (megaTask (megaTask b c [] [] 2 0) a [] [] 4 0) d [] [] 4 0)


printfn "%A" (megaTask [1; 2; 3; 4; 5; 6; 7] [2; 3; 4; 5; 6] [7; 8; 9; 10; 11; 12] [5; 6; 7; 8; 9] 0 0)