正如前面提到的，如果类名以一个下划线开始，那么它就没有相对应的XML标记。
As noted earlier, if a class name begins with an underscore, it has no corresponding XML tag.
开始和结束标记带有下划线用于阻止意外的字符串匹配。
Start and end each token with an underscore to prevent accidental string matches.
在一个经常使用的约定中，任何以下划线开始命名的类都表示没有相应标记的结构。
In one convention that's often used, any class that's named with an initial underscore represents a structure that does not have a corresponding tag.
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