let zip (xs, ys) =
    List.rev( snd (List.fold_left (fun (tail_of_ys, list_of_pairs) hx -> match tail_of_ys with
                                                                         | h::t -> (t, (hx, h)::list_of_pairs)
                                                                         | [] -> ([], list_of_pairs)
                                                                                                               ) (ys, []) xs));;

zip( ['a';'b';'c';'d'] ,['a';'b';'c';'d']);;                (*xs.length = ys.length*)
zip( [1;2;3] ,['a';'b';'c';'d']);;                                  (*xs.length < ys.length*)
zip( [1;2;3;4;5;6;7;8;9;10] ,['a';'b';'c';'d']);;   (*xs.length > ys.length*)
zip([], [1;2;3;45]);;                                                           (*xs.length = 0*)
zip(['a';'b';'c';'d'], [1]);                                            (*ys.length = 0*)