Update: Some commenters point out that sometimes the world does offer Pareto improvements, such as the gains from trade in a Ricardian trade model, and that these are free lunches.

• 更新：有评论指出，这个世界，有时确实存在怕累托改善，比如在李嘉图贸易模型中的贸易收益，这些就是免费的午餐。