anchor.diff:  }
   
   class UpLeftRightDownStrategy extends Strategy {
  -  const UpLeftRightDownStrategy(): super("up left right down");
  +  const UpLeftRightDownStrategy() : super("up left right down");
   
     String nextDirection(Board board) {
       Board tmp = new Board.copy(board);
  
positive.code:   int val;
  
    Cell(this.x, this.y, [this.val]);
    Cell.copy(Cell cell): this(cell.x, cell.y, cell.val);
  
    bool get empty => val == null;
  }
  
negative.code: }
  
  class EdgeStrategy extends EvaluateStrategy {
    const EdgeStrategy(int searchDepth) : super("edge $searchDepth", searchDepth, true);
  
    num _cellScore(Cell cell) {
      if (cell.empty) {
  