/** Mark a position as visited.	*/
  protected void visit(DecorablePosition p) { p.put(STATUS, VISITED); }
  /** Mark a position as unvisited.	*/
  protected void unVisit(DecorablePosition p) { p.put(STATUS, UNVISITED); }
  /** Test if a position has been visited. */
  protected boolean isVisited(DecorablePosition p) { 
    return (p.get(STATUS) == VISITED); 
  }