The concept of NP-completeness has provided a powerful tool for understanding the complexity of problems.
每个人都有不为世人所知的秘密悲伤；很多时候，我们称一个人冷漠，而他只是悲伤。
The natural price of labor is that price which is necessary to enable the laborers, one with another, to subsist and to perpetuate their race, without either increase or diminution.
The past is a foreign country; they do things differently there.