anchor.diff:    ItemRecord<V> _remove(ItemRecord record) => _addToRemovals(_unlink(record));
   
     ItemRecord<V> _unlink(ItemRecord record) {
  -    if (_movedItems != null) _movedItems.remove(record);
  +    if (_linkedRecords != null) _linkedRecords.remove(record);
   
       var prev = record._prev;
       var next = record._next;
  
positive.code:       prevRecord._next = record;
      }
  
      if (_movedItems == null) _movedItems = new DuplicateMap();
  
      _movedItems.put(record);
      record.currentIndex = index;
      return record;
    }
  
negative.code:       record._prev = prev;
        if (prev != null) prev._next = prev._nextPrevious = record;
  
        if (_linkedRecords == null) _linkedRecords = new DuplicateMap();
        _linkedRecords.put(record);
      }
  
      prev._next = null;
  