PREFIX foaf:    <http://xmlns.com/foaf/0.1/>
PREFIX vcard:   <http://www.w3.org/2001/vcard-rdf/3.0#>

CONSTRUCT { ?x foaf:knows ?y }

FROM <http://pagesperso-systeme.lip6.fr/Jean-Francois.Perrot/inalco/XML/RDF/Exemples/vcardCat.rdf>
WHERE
 {         ?x  vcard:N ?va .
           ?va vcard:Family ?f .
           ?y  vcard:N ?vb .
           ?vb vcard:Family ?f .
      FILTER (?va != ?vb && STR(?x) < STR(?y) )
 }