anchor.diff:     * Detect if we're outside the playing board.
      */
     bool isInsidePen(Coord coord) {
  -    return pen.isInside(coord);
  +    return snakepen.isInside(coord);
     }
   
     void stopGame() {
  
positive.code:   int x;
    int y;
  
    Coord(int x, int y) {
      this.x = x;
      this.y = y;
  
negative.code:   bool isInside(Coord c) {
      return (c.x >= 0 && c.y >= 0 && c.x <= x && c.y <= y);
    }
  
    Coord randomCoordInside() {
      return new Coord(rnd.nextInt(getX()), rnd.nextInt(getY()));
    }
  }
  
  class Snake {
    int maxSize = 3;
    List<Coord> coords = new List();
  
    Coord snakepen;
  
    int points = 5;
  
    bool running = true;
  
    Snake(int w, int h) {
  
      snakepen = new Coord(w,h);
  
      coords.add(newRandomCoord());
      assert(coords.length == 1);
    }
  
    Coord newRandomCoord() {
      return snakepen.randomCoordInside();
    }
  
    void addPoints(num delta) {
  