他能完成所有软件安装的低级功能。
It performs all the low level functions of software installation.
第三条是低级别的需求；它确实为软件必须完成的功能提供了足够的细节，使你能写成代码。
The third is a lower-level requirement; it does provide enough detail about what the software must do to enable you to write code.
在典型的软件系统中，函数的调用流是从较高级部分到低级部分进行的；而事件是以相反的方向过滤的，如Figure 1所示。
In a typical software system, function calls flow downward from higher-level components to lower-level ones; whereas events percolate up in the opposite direction. Figure 1 illustrates this pattern.
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