// adds all elements of the Set items to the q
  // postcondition: the Set items is not modified;
  //                q contains its original elements followed
  //                by the elements from items
  private void appendSetToQueue(Set items, Queue q)
  {
    Iterator it = items.iterator();

    while (it.hasNext())
      q.enqueue(it.next());
  }

  // returns a Set consisting of the e-mail addresses associated with
  // the parameter alias, such that all intermediate resulting aliases
  // have been expanded;
  // precondition: alias is a key in addressBook
  public Set expandAlias(String alias)
  {
    Set result = new TreeSet();

    Queue q = new ListQueue();
    q.enqueue(alias);

    while (!q.isEmpty())
    {
      String token = (String)q.dequeue();
      Set items = (Set)addressBook.get(token);
      if (items == null) // not an alias
        result.add(token);
      else
        appendSetToQueue(items, q);
    }

    return result;
  }