anchor.diff:  
       return null;
     }
  +  
  +  String getPrevSiblingUuID(String selfUuid) {
  +    int selfIndex = _rows.indexOf(_rowsMap[selfUuid]);
  +    if(selfIndex != -1) {
  +      int selfLevel = _rows[selfIndex].level;
  +      for(int i = selfIndex -1; i >= 0; i--) {
  +        if(_rows[i].level == selfLevel) {
  +          return _rows[i].nodeUID;
  +        }
  +      }
  +    }
  +    return null;
  +  }
  +  String getNextSiblingUuID(String selfUuid) {
  +    int selfIndex = _rows.indexOf(_rowsMap[selfUuid]);
  +    if(selfIndex != -1) {
  +      int selfLevel = _rows[selfIndex].level;
  +      for(int i = selfIndex + 1; i < _rows.length; i++) {
  +        if(_rows[i].level == selfLevel) {
  +          return _rows[i].nodeUID;
  +        }
  +      }
  +    }
  +    return null;
  +  }
   
     /**
      *  Make sure the given node is selected.
  
positive.code:   TreeViewCell _currentDragOverCell;
    // Whether the user can drag a cell.
    bool draggingEnabled = false;
    // Unique identifier of the tree.
    String _uuid;
  
  
negative.code:     _listView = new ListView(element, this);
      _rows = null;
      _rowsMap = null;
      _pendingExpansionTimer = null;
      _pendingExpansionUID = null;
      reloadData();
    }
  
  