The tree rotation renders the inorder traversal of the binary tree invariant. Functions for traversal and retrieval are put into the abstract Iterator interface. Traversing a "m"-ary tree is very similar to binary tree traversal. During such a traversal, every traversed vertex is marked as visited. The postfix expression is formed by the basic postorder traversal of any binary tree. A basic requirement for traversal is to visit every node eventually. This expression can then be determined by doing an inorder traversal of the tree. It is based on graph traversal methods in graph theory. This will require extra methods for traversal and object retrieval. Step one is accomplished with a post-order traversal of the quadtree. The problem of graph exploration can be seen as a variant of graph traversal. The above example illustrates an in-order traversal of the binary tree. Tree traversal is a special case of graph traversal. Other improvements include support for application silos, firewall traversal and better LDAP support. Stun is a mechanism to enable this border traversal.