当云教授和她的同事检测专利局的文件，他们发现接近一半的在19世纪被美国登记的专利是单代码发明。

When Dr Youn and her colleagues examined the patent office's files they found that nearly half the patents issued by the United States during the 19th century were for single-code inventions.

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从第399行开始，添加请单7中的代码。

Add the code in Listing 7, starting at line 399.

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静态 getInstance 方法包含单例的模板代码。

The static getInstance method contains the template code for a singleton.

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