// Textbook fragment 06.A.04
  /** Returns a textual representation of a given node list */
  public static <E> String toString(PositionList<E> l) {
    Iterator<E> it = l.iterator();
    String s = "[";
    while (it.hasNext()) {
      s += it.next();   // implicit cast of the next element to String
      if (it.hasNext())
        s += ", ";
      }
    s += "]";
    return s;
  }