anchor.diff:  }
   
   class EdgeStrategy extends EvaluateStrategy {
  -  const EdgeStrategy(int searchDepth): super("edge $searchDepth", searchDepth, true);
  +  const EdgeStrategy(int searchDepth) : super("edge $searchDepth", searchDepth, true);
   
     num _cellScore(Cell cell) {
       if (cell.empty) {
  
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 UpLeftRightDownStrategy extends Strategy {
    const UpLeftRightDownStrategy() : super("up left right down");
  
    String nextDirection(Board board) {
      Board tmp = new Board.copy(board);
  