template <class Iterator1, class Iterator2, class Iterator3>
void merge(Iterator1 first1, Iterator1 last1, 
           Iterator2 first2, Iterator2 last2,
           Iterator3 result)
{
  while (first1 != last1 && first2 != last2) 
  {
    if (*first2 < *first1) 
    {
      *result = *first2;
      ++first2;
    }
    else 
    {
      *result = *first1;
      ++first1;
    }
    ++result;
  }

  while (first1 != last1)
  {
    *result = *first1;
    ++result;
    ++first1;
  }

  while (first2 != last2)
  {
    *result = *first2;
    ++result;
    ++first2;
  }
}