第二章是预备知识，介绍了锥、锥凸集值函数、切锥与集值映射的切导数等的相关知识。

In chapter 2, we introduce the knowledge that the paper use, introducing these conception of cone, cone convex set-valued maps, tangent cones, and tangent derivatives of set-valued mapping, etc.

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提供了完整的函数集，因此可以把这些值显式地从一种类型转换为另一种类型，将这种转换作为它们在XPath 2.0中操作的一部分。

A full set of functions is provided so you can explicitly convert these values from one type to another as part of their manipulation in XPath 2.0.

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尽量尝试将您的扩展函数设计为仅使用双精度、字符串、布尔和节点集作为实参和返回值的类型。

Try to design your extension functions so that they use only doubles, strings, booleans, and node-sets as both arguments and return values.

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