Xquery: for $x at $position in $seq expression

A useful expression available to xquery is the "for x at $position in $seq expression" (for at in expression)

for $x at $position in $seq
 return
  if ($position = 1) then
   (: do something :)
  else if ($position = 2) then
   (: do something else :)
  else
   (: default :)