我们向一个贯穿全文的例子中逐渐添加新功能，并遵循如下简单的规则：编写一个单元测试，然后让它通过。

We'll add features to a running example by iteratively following a simple principle: write a unit test, make it pass.

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另外，好的程序风格还会遵循两条额外的规则，如下。

In addition, good programming style requires two more rules, as follows.

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