anchor.diff:  void main() {
     DivElement root = querySelector('#sudoku_container');
     Sudoku sudoku = new Sudoku(root);
  -  sudoku.hideCells(42);
  +  sudoku.hideCells(81);
   }
   
   class Sudoku {
  
positive.code:   int _cageSize = 3;
    TableElement _table = new TableElement();
    List<TableCellElement> _hiddenCells = new List();
    
    Sudoku(DivElement root) {
      root.children.add(_table);
      List<int> rowValues = new List<int>(9);
  
negative.code:         int index = (valueIndex + shift + cageShift) % _size;
          cell.text = rowValues[index].toString();
        }
        cageShift = cageShifts[(rowIndex + 1) % _cageSize];
      }
    }
  
    void hideCells(int count) {
      while (_hiddenCells.length < count) {
        int row_index = _random.nextInt(_size - 1);
  