element toc {
  (for $s in
     fs:distinct-docorder(
       let $fs:sequence :=
          doc("book.xml")
       return
         let $fs:last := fn:count($fs:sequence)
         return
           for $fs:dot at $fs:position in $fs:sequence
           return child::book
     )
   return let $fs:v4 := $s return local:toc($fs:v4))
}


function local:toc($book-or-section){
     for $section in
       fs:distinct-docorder(
         let $fs:sequence := $book-or-section
         return
           let $fs:last := fn:count($fs:sequence)
           return
             for $fs:dot at $fs:position in $fs:sequence
             return child::section
       )
     return
       element section {
         (fs:distinct-docorder(
            let $fs:sequence := $section
            return
              let $fs:last := fn:count($fs:sequence)
              return
                for $fs:dot at $fs:position in $fs:sequence
                return attribute::id
          ),
          fs:distinct-docorder(
            let $fs:sequence := $section
            return
              let $fs:last := fn:count($fs:sequence)
              return
                for $fs:dot at $fs:position in $fs:sequence
                return child::title
          ), let $fs:v4 := $section return local:toc($fs:v4))
       }
}