anchor.diff:    _VNode _root;
     _VNode _tail;
     int _level;
  -  // cached hashCode.
  -  int _hashCode = null;
     bool __altered = false;
   
     BaseVectorImpl._prototype() {
  
positive.code:     this._size = 0;
      this._level = _SHIFT;
      this._root = this._tail = null;
      this._hashCode = null;
      this.__altered = true;
      return this;
    }
  
negative.code: 
    BaseVectorImpl<E> _set(int index, E value) {
      index = _checkIndex(index);
  
      var vector = this;
      var newTail = vector._tail;
  