(** [combine] is the opposite of [split].
      Lists given to [combine] are meant to be of same length.
      If not, [combine] stops on the shorter list *)

    Fixpoint combine (l : list A) (l' : list B) : list (A*B) :=
      match l,l' with
      | x::tl, y::tl' => (x,y)::(combine tl tl')
      | _, _ => []
      end.

Lemma combine_nth : forall (l:list A)(l':list B)(n:nat)(x:A)(y:B), length l = length l' -> nth n (combine l l') (x,y) = (nth n l x, nth n l' y).