从技术上看，串行化器并不是XSLT处理器的一部分，但它在实际使用中是必不可少的。
The serializer is not technically part of the XSLT processor, but it is essential for practical use.
简单的培训能够为员工提供基本的语言和符号知识，但是并不能提供使用和交流中所必不可少的技巧。
Simple training can provide people with basic language and notation awareness, but not necessarily the skills to use and communicate with them effectively.
虽然它们是必不可少的，但是这些服务对手机来说并不是唯一的。
Although I find them indispensable, the services aren't unique to a mobilephone.
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