anchor.diff:    int _offset = 0;
     ObjectStore _store;
     List<PackedObject> objects = [];
  -
  -  Pack(this.data, [this._store]);
  +  Cancel _cancel;
  +
  +  Pack(this.data, [this._store, this._cancel]);
   
     Uint8List _peek(int length) => data.sublist(_offset, _offset + length);
   
  
positive.code:        }
  
         Future expandDeltified(PackedObject obj) {
           return expandDeltifiedObject(obj).then((PackedObject deltifiedObj) {
             deltifiedObj.data = null;
             // TODO(grv) : add progress.
  
negative.code: 
         Future parse(_) {
  
           _cancel.check();
           PackedObject object = _matchObjectAtOffset(_offset);
           object.crc = getCrc32(data.sublist(object.offset, _offset));
  
  