if (this.children.contains(possibleDescendant))
if (this.getChildren().contains(possibleDescendant))
  public boolean isAncestorOf(TreeNode possibleDescendant)  {    // If we can find the possibleDescendant in this node's subtree, then    // this node is an ancestor of that possibleDescendant.    boolean foundDescendant = false;    if (possibleDescendant == null)    {      // No one can be an ancestor of a NULL node.      throw new IllegalArgumentException(          "It is illegal to inquire whether a NULL TreeNode"              + " is the descendant of some other node.");    }    if (this.children.contains(possibleDescendant))    {      foundDescendant = true;    }    else    {      Iterator childrenIterator = this.children.iterator();      while (childrenIterator.hasNext() && (foundDescendant == false))      {        foundDescendant = ((TreeNodeImpl) (childrenIterator.next()))            .isAncestorOf(possibleDescendant);      }    }    return foundDescendant;  }