let rec allsets k l =
    if k <= 0 || k > List.length l then []
    else if k = 1 then List.map (fun x->[x]) l else
        match l with
        | [] -> []
        | h :: t ->
            let rest = (allsets (k-1) t) in
            (List.map (fun x->h::x) rest) @ (allsets k t)
;;

let rec print_list = function
    | [] -> ()
    | h :: t -> print_int h; print_string " "; print_list t
;;

let rec print_list_lists = function
    | [] -> print_newline()
    | h :: t -> print_string "[ "; print_list h; print_string "]; ";
    print_list_lists t
;;

print_list_lists (allsets 1 [1;2;3]);
print_list_lists (allsets 2 [1;2;3]);
print_list_lists (allsets 3 [1;2;3]);