anchor.diff:      _preOrderTraverse(root, callback);
     }
   
  -  _preOrderTraverse(_TreeNode node, void callback(T key)) {
  +  _preOrderTraverse(TreeNode node, void callback(T key)) {
       if (node != null) {
         if (callback !=null) {
           callback(node.key);
  
positive.code: class BinarySearchTree<T extends Comparable> {
    _TreeNode<T> root;
  
    insert(T key) {
      var newNode = new _TreeNode<T>(key);
  
      if (root == null) {
        root = newNode;
  
negative.code:     root = _remove(root, key);
    }
  
    TreeNode _remove(TreeNode node, T searchKey) {
      if (node == null) {
        return null;
      }
  