// Textbook fragment 11.03
  /**
   * Merges two sorted lists, in1 and in2, into a sorted list in.
   **/
  public static <E> void merge(PositionList<E> in1, PositionList<E> in2,
         Comparator<E> c, PositionList<E> in) {
    while (!in1.isEmpty() && !in2.isEmpty())
      if (c.compare(in1.first().element(), in2.first().element()) <= 0)
        in.addLast(in1.remove(in1.first()));
      else
        in.addLast(in2.remove(in2.first()));
    while(!in1.isEmpty()) // move the remaining elements of in1
      in.addLast(in1.remove(in1.first()));
    while(!in2.isEmpty()) // move the remaining elements of in2
      in.addLast(in2.remove(in2.first()));
  }