In the xquery, how do i access the counter in for-each

Eg:
for $x in doc("books.xml")/bookstore/book
where $x/price>30  => want the
loop counter.
order by $x/title
return $x/title

If
"doc("books.xml")/bookstore/book"  returns 3 elements(book1, book2, book3),
and during the looping, I want to get loop counter value. 

For the first
element, I want to prepare string using loop counter like  
"elem-1", and for
the second, I want "elem-2"


Thank you for your help.