(Q13) For each book whose price is greater than the average price, return the title of the book and the amount by which the book's price exceeds the average price.

<result>
   LET $a := avg(//book/price)
   FOR $b IN /book
   WHERE $b/price > $a
   RETURN
      <expensive_book>
         $b/title ,
         <price_difference>
            $b/price - $a
         </price_difference>
      </expensive_book>
</result>