anchor.diff:    addToWorld() {
       _context.game.world.addChild(this);
       if (isMovable) {
  -      _context.game.stage.juggler.add(this);
  +      _context.game.world.juggler.add(this);
       }
     }
   
  
positive.code:    * You can make the sprite move with changing the vx or vy value.
     */
    bool advanceTime(num time) {
      if ((x <= 0 || x >= _context.game.stage.sourceWidth) || 
          (y <= 0 || y >= _context.game.stage.sourceHeight)) {
        removeFromWorld();
      }
      
  
negative.code:     alive = false;
      _context.game.world.removeChild(this);
      if (isMovable) {
        _context.game.world.juggler.remove(this);
      }
    }
  
  