调用带有XPath参数的sign方法，并且数字签名放置在文档中，则生成封内签名。
Calling the sign method with an XPath argument where the digital signature is to be placed in the document helps create an enveloped signature.
封内签名（Enveloped signature）： 元素成为被签名数据的子元素。
Enveloped signature: The element becomes a child element of the data being signed.
共支持全部三种签名模式：封外、封内以及分离模式。
All three modes of signatures — enveloping, enveloped, and detached — are supported.
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