anchor.diff:      return index;
     }
   
  -  VNode _vectorNodeFor(int index) {
  +  _VNode _vectorNodeFor(int index) {
       if (index >= _getTailOffset(this._size)) {
         return this._tail;
       }
  
positive.code:   factory PersistentVectorImpl.empty() => new PersistentVectorImpl._prototype();
    PersistentVectorImpl._prototype() : super._prototype();
  
    factory PersistentVectorImpl._make(int origin, int size, int level, VNode root, VNode tail) {
      var x = new PersistentVectorImpl._prototype();
      x._origin = origin;
      x._size = size;
  
negative.code:     return "VNode: " + _array.toString();
    }
  
    _VNode(this._array, this._ownerID);
  
    void _set(int index, value) {
      if (_array.length > index) {
  