不可恢复错误：请求必须返回给客户端。
Unrecoverable errors — the request must be returned to the client.
虽然向一个日志文件发送消息相当有用，但是很多时候您可能还想在发生不可恢复的异常时显示一个用户友好的错误消息。
Although sending messages to a log file can be quite useful, there are often times when you would also like to display a user-friendly error message in the event of an unrecoverable exception.
在代码中只能选择element类型的节点，因为该方法不支持TEXT节点，并且会在Perl中产生一个不可恢复的错误。
The code must select only ELEMENT type nodes, because TEXT nodes do not support this method and produce a non-recoverable error in Perl.
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