PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX data: <http://example.org/foaf/>
SELECT ?nick FROM NAMED <http://example.org/foaf/aliceFoaf> FROM NAMED <http://example.org/foaf/bobFoaf> WHERE {
  GRAPH data:bobFoaf {
    ?x foaf:mbox <mailto:bob@work.example> .
    ?x foaf:nick ?nick .
  }
}