Using XPATH to find the Attribute value of an XML empty element

An empty element is quite confusing...so the best way for me to ask my question is to reference a book.

The book:  Learning XML 2nd edition - O'REILLY - Erik T. Ray pg 50 chapter 2     Table 2-1. Types of tags in XML

OBJECT                           PURPOSE                                                            EXAMPLE

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empty element                Represent info at specific point in document           <xref linkend="abc"/> 

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what is the XPATH expression to obtain "abc" in the example above?

Thanks, Jerry