re:>
<greeting/>
<greeting from="brian"/>
<greeting from="weiqi"/>
说明order by 有以下几个属性：
descending，ascending，empty greatest，empty least
分别测试一下
[数量操作]该操作非常有用该操作非常有用
1
some 和 every 例子
ex:>some $x in (1, 2, 3) satisfies $x >= 2
re:>true
ex:>every $x in (1, 2, 3) satisfies $x >= 2
re:>false
ex:>some $x in (1, 2, 3), $y in (3, 4, 5) satisfies $x = $y
re:>true
ex:>every $x in (1, 2, 3), $y in (3, 4, 5) satisfies $x = $y
re:>false
2
count($a)
ex:>let $a:=collection("tonybooks")//category
return count($a)
re:>返回40个1
ex:>for $a in collection("tonybooks")//category
return count($a)
