For each book with an author, return a
book with its title and authors. For
each book with an editor, return a 
reference with the book title and the
editor's affiliation. 
<bib>
 {
   for $b in doc("bib.xml")//book[author]
   return
    <book>
     { $b/title }
     { $b/author }
    </book>,
   for $b in doc("bib.xml")//book[editor]
   return
    <reference>
     { $b/title }
     <org> { $b/editor/affiliation/text() } </org>
    </reference>
 }
</bib>Adapted from XML Query Use Cases