从上述的集合副本中删除不必要的键，是可接受的，但并不是必需的。
It is acceptable, though not required, to remove unwanted keys from copies of the sets above.
答案就是只设计完成当前迭代的客户素材所必需的东西——多一点都没有必要。
The answer is that you design just what you need to complete the customer stories for the current iteration — not a bit more.
这个根标记是必需的，这样可以保证XML文档格式良好，在解析xml时，这也是必要的。
This root element is required to make a well-formed XML document, which is a requirement when parsing XML.
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