anchor.diff:        _lastTimestamp = delta;
         clear();
         drawCell(_food, "blue");
  -      _snake.update(ctx, _keyboard);
  +      _snake.update();
         _checkForCollisions();
       }
   
  
positive.code:     return false;
    }
  
    void grow() {
      // add new head based on current direction
      _body.insert(0, head + _dir);
    }
  
    void update(CanvasRenderingContext2D ctx, Keyboard keyboard) {
      _checkInput(keyboard);
      _move();
      _draw(ctx);
    }
  }
  
  
negative.code:   }
  
    bool isPressed(int keyCode) => _keys.containsKey(keyCode);
  }