(: Add genre to A.C.'s books :)
for $book in db:open("books")//book
return if ($book/author/text() = "A.C. Doyle")
  then insert node 
       <genre>Kriminalliteratur</genre> 
       into $book,
db:optimize("books"),

(: change value of names :)
for $book in db:open("books")//book
let $author := $book/author
return if ($author/text() = "A.C. Doyle")
 then replace value of node $author 
      with "Arthur Conan Doyle"
 else if ($author/text() = "E.A. Poe") 
  then replace value of node $author 
       with "Edgar Allan Poe"
  else (),
db:optimize("books")