产品类型层次结构是一个严格的树，而类别层次结构则是一个有方向的非循环图表。

A product type hierarchy is a strict tree, whereas a category hierarchy can be a directed acyclic graph.

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目前为止的示例（计数器和堆栈）都是非常简单的非阻塞算法，一旦掌握了在循环中使用 CAS，就可以容易地模仿它们。

The examples so far -- counter and stack -- are very simple nonblocking algorithms and are easy to follow once you grasp the pattern of using CAS in a loop.

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在这些情况下，TDD 的简短的 “代码、测试、代码、测试” 循环将导致重构，而这将继续驱使非复杂代码的开发。

The short "code, test, code, test" cycle of TDD invites refactoring in these cases, which continually drives the development of uncomplex code.

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