class Joiners
{
 public static <T> LispList<T> zipLists(Joiner<T> joiner,LispList<T> ls1,LispList<T> ls2)
 {
  if(ls1.isEmpty()||ls2.isEmpty())
     return LispList.empty();
  else
     {
      T h = joiner.join(ls1.head(),ls2.head());
      LispList<T> t = zipLists(joiner,ls1.tail(),ls2.tail());
      return t.cons(h);
     }
 }
}