You've got to place the boundary at exactly the right place, otherwise you've got a bit too much in your system or a bit too little.

你要把边界,设置在正确的位置上，否则你的系统就会要么多出一部分,要么少一部分。

麻省理工公开课 - 热力学与动力学课程节选

To be systematic, I'm going to build this tree depth-first left-first.

为了保持系统性，我将要把这棵树设置,为深度优先和左边优先。

麻省理工公开课 - 计算机科学及编程导论课程节选