On top of that is the shell SWF, which contains all of the constant site elements, such as navigation, footers, etc.

核心层之上是shell SWF，这里包含了所有通用的网站元素，诸如导航栏、脚注等等。

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Gerardus Mercator's projection, published in 1569, was immediately useful because it depicts a line of constant bearing as a straight line, which is handy for Marine navigation.

福西厄斯•墨卡托的投影于1569年问世，马上就派上了用场，因为它把不变的方位描绘为直线，这对航海非常方便。

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A zipper is a way of representing a data structure with a local context, such that navigation (iteration) and modification from the current context are, for the most part, constant-time.

zippers是使用本地上下文表示数据结构的一种方法，这使当前上下文中的导航(迭代)和修改在很大程度上具有常量时间的特点。

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