<greeting from="brian">Hi!</greeting>
<greeting from="luc">Bonjour!</greeting>
</greetings>
ex:>doc("greetings.xml")/greetings
re:><greetings>
<greeting from="weiqi">Nihao!</greeting>
<greeting from="brian">Hi!</greeting>
<greeting from="luc">Bonjour!</greeting>
</greetings>
ex:>doc("greetings.xml")//greeting
re:><greeting from="weiqi">Nihao!</greeting>
<greeting from="brian">Hi!</greeting>
<greeting from="luc">Bonjour!</greeting>
说明：注意以上两个例子的区别/greetings和//greetings，
前者表示从根节点开始逐级遍历，后者表示任何级的节点都遍历
ex:>doc("greetings.xml")//greeting[@from="weiqi"]
re:><greeting from="weiqi">Nihao!</greeting>
说明：@from 用来引用节点中的属性名称
