一旦确定了测试的目标是只覆盖变更的功能，那么来自于未变更代码区域的结果就可能从一次构建转移到下一次。
Once tests have been targeted to cover only changed function, any results from unchanged areas of code may then be carried forward from one build to the next.
单元测试的目标是测试应用程序底层代码的功能，保证它们执行后产生正确的结果。
The goal is to test the functionality of the underlying code of the application to ensure that the correct results are provided upon execution.
其实，这里的目标是要创建一个小型语言，这个语言能够允许用户来输入相对简单的代数表达式，然后这个代码来为它求值并产生结果。
In essence, the goal here is to create a small language that allows users to type in relatively simple algebraic expressions that this code will evaluate and produce a result.
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