(: Print distinct authors and the books they wrote, sorted alphabetically :)
for $book in db:open("books")//book
let $title := $book/title/text()
group by $author := $book/author/text()
return <author>
         <name>{$author}</name>
         <bookCount>{count($title)}</bookCount>
         {for $t in $title
         order by $t ascending
         return <wrote>{$t}</wrote>}
       </author>