anchor.diff:  
     Snake(int w, int h) {
   
  -    width = w;
  -    height = h;
  +    pen = new Coord(w,h);
   
       coords.add(newRandomCoord());
       assert(coords.length == 1);
  
positive.code: 
      newHead.add(direction);
  
      if (!isOnCanvas(newHead)) {
        collisionWithCanvas();
        return;
      }
  
negative.code:   }
  
    // Detect if we're outside the playing board.
    bool isInsidePen(Coord coord) {
      return pen.isInside(coord);
    }
  
  
  