// Textbook fragment 03.24
  /** Returns whether a given node has a previous node */
  public boolean hasPrev(DNode v) { return v != header; }
  /** Returns whether a given node has a next node */
  public boolean hasNext(DNode v) { return v != trailer; }
  /** Returns a string representation of the list */
  public String toString() {
    String s = "[";
    DNode v = header.getNext();
    while (v != trailer) {
      s += v.getElement();
      v = v.getNext();
      if (v != trailer)
        s += ",";
    }
    s += "]";
    return s;
  }
}