// Textbook fragment 06.20
  /** Returns the k most accessed elements, for a given k;  O(k) time */
  public Iterable<E> top(int k) {
    if (k < 0 || k > size())
      throw new IllegalArgumentException("Invalid argument");
    PositionList<E> T = new NodePositionList<E>(); // top-k list
    int i = 0;  // counter of the entries added to the list
    for (Entry<E> e: fList) {
      if (i++ >= k)
        break;          // all the k entries have been added
      T.addLast(e.value());     // add one entry to the list
    }
    return T;
  }
  /** String representation of the favorite list */
  public String toString() { return fList.toString(); }
  /** Helper method that extracts the value of the entry at a given position */
  protected E value(Position<Entry<E>> p) { return ( p.element()).value(); }
  /** Helper method that extracts the counter of the entry at a given position *
/
  protected int count(Position<Entry<E>> p) { return ( p.element()).count(); }