template <typename Data>
void DListHeader<Data>::addBefore (DListNode<Data>* beforeThis, 
                                   const Data& value)
{
  if (beforeThis == first)
    addToFront (value);
  else
    {
      // Move to front of beforeThis
      DListNode<Data>* current = beforeThis->prev;
      // Link after that node
      addAfter (current, value);
    }
}