function merge(a, b) {
   function merge_internal (a, b, result) {
       if (a.empty && b.empty) {
         return result;
       }
       item_a = a.pop();
       item_b = b.pop();
       result.add(item_a * item_b);
       return merge_internal(a, b, result);
   }
   return merge_internal(a, b, []);
}