The Mach layer at the bottom of the kernel is visible in userspace, but not directly used by most developers outside Apple.
内核底层的Mach层对于用户空间层（userspace）来说是可见的，但是却不能被大部分非苹果公司的开发者直接运用。
This is the layer that is most visible to users and application providers.
用户和应用程序供应商看到最多的就是这一层。
My view is that Lean is several layers deep. The most visible layer is its set of practices.
我认为精益有几个层次。
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