XQuery,获取具有相同值的元素?

XQuery, get element with same value?

xml

<books> <book> <author>Gambardella, Tim</author> </book> <book> <author>Knorr, Stefan</author> </book> <book> <author>O'Brien, Tim</author> </book> </books>

如何获取同名作者?

我尝试搜索,但没有找到用于查找相同字符串的有效示例

通常这是一个分组问题,例如

for $author in books/book/author 
  group by $prename := $author => substring-after(',') => normalize-space() 
return 
  <group prename="{$prename}">{$author}</group>