declare option saxon:output "indent=yes";
<friends-by-age>
{
  for $person in //person
  where $person/@category = "friends"
  order by $person/date-of-birth
  return
  <friend>
   { $person/date-of-birth }
   { $person/lastname }
   { $person/firstname }
  </friend>
}
</friends-by-age>