anchor.diff:    int state;
     _TreeNode<V> _current;
   
  -  AvlTreeIterator._(AvlTreeSet<V> tree,
  +  _AvlTreeIterator._(AvlTreeSet<V> tree,
         {reversed: false, this.inclusive: true, this.anchorObject: null}) :
       this.tree = tree, this._modCountGuard = tree._modCount,
       this.reversed = reversed {
  
positive.code:  * moveNext(), then current is 20. If the first call is movePrevious; 15.
   */
  typedef bool IteratorMove();
  class AvlTreeIterator<V> implements BidirectionalIterator<V> {
  
    static const LEFT = -1;
    static const WALK = 0;
  
negative.code:    * See [IterableBase.iterator]
     */
    BidirectionalIterator<V> get iterator =>
        new _AvlTreeIterator._(this);
  
    /**
     * See [TreeSet.reverseIterator]
     */
    BidirectionalIterator<V> get reverseIterator =>
        new _AvlTreeIterator._(this, reversed: true);
  
    /**
     * See [TreeSet.fromIterator]
     */
    BidirectionalIterator<V> fromIterator(V anchor,
        {bool reversed: false, bool inclusive: true}) =>
      new _AvlTreeIterator<V>._(this, anchorObject: anchor, reversed: reversed,
          inclusive: inclusive);
  
    /**
  