3.Probably good if this is the same type of unit as the target, though not required.
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It is acceptable, though not required, to remove unwanted keys from copies of the sets above.
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。
Implementing an EJB tier, though not required, lays the foundation for creating a highly scalable, distributed application architecture.
虽然没有要求实现ejb层，但这样做可以为建立高度可伸缩的分布式应用程序架构提供基础。
The servlet filter USES log4j to log some information during processing and it is recommended, though not required, to enable the logging.
servlet筛选器在处理期间使用log4j记录一些信息，建议启用日志记录(但不是必需的)。
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