This is a corrected (I hope) version of Figure 5.13 from the
Heileman text. 



template<class T>
void SlList<T>::Insert(const T& elm, int pos)
{
  assert( (pos > 0) && (pos <= (length + 1) ) );
  SlNode<T>* prev_node;
  SlNode<T>* current_node;
  SlNode<T>* new_node = new SlNode<T>(elm);  // create the new node
  if (tail == NULL)  // list is empty
    {
      tail = new_node;
      tail->next = tail; 
      length++;
      return;
    }

  if (pos == 1) 
    {
      new_node->next = tail->next;   tail->next = new_node;
    }
  else 
    {
      current_node = tail->next;  // point to first node
      prev_node = current_node;
      current_node = current_node->next;
      for (int i=2; i<pos; i++)   // traverse list
        {
          prev_node = current_node;   current_node = current_node->next;
        }
      prev_node->next = new_node;  // splice the new node into the list
      new_node->next = current_node;
      if (tail->next == new_node) // new_node went on the end
        tail = tail->next;
    }
  length++;
}