declare function local:desconto($doc , $des){
    	for $book in $doc/*/*:book ,
             $descont in $des/*/*:desconto
         where $book/@id = $descont/*:id
         return (<o> desconto de {$descont/*:desc}  para {$book/*:title}   </o>)
	};
 
let $message := local:desconto(doc('http://cin.ufpe.br/~dsmp/xquery/xml/catalog.xml') , doc('http://cin.ufpe.br/~dsmp/xquery/xml/desconto.xml') )
return {
   $message
}