这些依赖于下面的层以及所有使用它们的应用程序，所以，理想情况下，在应用程序被建造在它们之上之前，它们十分地稳定。
These depend upon the layer below, and all applications that use them depend on them, so, ideally, they are pretty stable by the time that applications are built atop them.
这种分离让模型能够在不依赖于表现层的情况下被建立。
This separation allows the model to be built and tested independently from the visual presentation.
仔细观察一下(芯片)包，你会发现它们实际上就是由一层层塑料膜构成的，有的上面涂上了颜色，有的上面镀上金属，防止材料被腐蚀。
If you look at a (chip) packet, what you've got is a plastic film, a few layers of inks and a printed metal layer to keep the materials fresh.
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