scene :scene_name do |sc|
    sc.on_init do
      s = sc.add :image, :name => :sprite_name, 
        :template => :sprite_template_name, position => [100, 100]
      s.event :on_step do |event|
        # 下に進み続ける
        cp = event.target.get_center_position
        event.target.set_center_position cp[0], cp[1] + 1
      end
    end
  end