XPath 学习：

XPath is a language for finding information in an XML document. XPath is used to navigate through elements and attributes in an XML document.XPath主要应用于XSLT，作为XSLT的元素，没有XSLT是很难写出XSLT的。
XPath术语：
Nodes(节点):有7种节点
1.element
2.attribute
3.text
4.namespace
5.processing-instruction
6.comment
7.document nodes(root)
Atomic values:  are nodes with no child or parent.
Items:   atomic values or nodes;

节点关系：
1.Parent
2.Children
3.Siblings
4.Ancestors
5.Descendants