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