anchor.diff:    final int y;
     const BoardLoc(this.x, this.y);
     BoardLoc.from(BoardLoc loc): x = loc.x, y = loc.y;
  -  
  +
     static final BoardLoc origin =
         const BoardLoc(0, 0);
   
  
positive.code:         return new BoardLoc(this.x + 1, this.y);
      }
    }
    
    int get hashCode {
      int result = 17;
      result = 37 * result + x;
  
negative.code: class Board {
    Map<BoardLoc, PlayedCard> boardMap;
    Set<BoardLoc> fringe;
  
    Board() {
      boardMap = new Map<BoardLoc, PlayedCard>();
      fringe = new Set<BoardLoc>.from([BoardLoc.origin]);
  
    }
  
    bool isClosed() => fringe.length == 0;
  
  }
  