当对整个文档进行重复访问时，这点必须注意。
When the whole document will be accessed repeatedly, this is a feature.
他们希望在容易形成“心流”的网站上停留更长，并重复访问。
They tend to stay longer, and return to web sites that facilitate flow.
这使得指标可以以关系格式重复访问，而无需每次访问都解析xml文档。
This allows the metrics to be accessed repeatedly in relational format without having to parse the XML document on each access.
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