code: public void checkTree() {
  		Map<TreeNode, TreeNode> nodes = this.useIdentity ? new IdentityHashMap<TreeNode, TreeNode>()
  				:new HashMap<TreeNode,TreeNode>();
  		
  		// Traverse the tree and check if each node is only used once.
  		addUniqueNodes(nodes,tree.getRoot());
  	}
docs: Check if the tree is a "valid" tree.
  <p>
  Typically you will use this method during development when you get an
  unexpected layout from your trees.
  <p>
  The following checks are performed:
  <ul>
  <li>Each node must only occur once in the tree.</li>
  </ul>