// Textbook fragment 13.04
  /** Mark a position (vertex or edge) as visited. */
  protected void visit(DecorablePosition<?> p) { p.put(STATUS, VISITED); }
  /** Mark a position (vertex or edge) as unvisited. */
  protected void unVisit(DecorablePosition<?> p) { p.put(STATUS, UNVISITED); }
  /** Test if a position (vertex or edge) has been visited. */
  protected boolean isVisited(DecorablePosition<?> p) {
    return (p.get(STATUS) == VISITED);
  }