// CrossRefMap

   {

     typedef SmartDigraph Graph;

     DIGRAPH_TYPEDEFS(Graph);

     checkConcept<ReadWriteMap<Node, int>,

                  CrossRefMap<Graph, Node, int> >();

     Graph gr;

     typedef CrossRefMap<Graph, Node, char> CRMap;

     typedef CRMap::ValueIterator ValueIt;

     CRMap map(gr);

     Node n0 = gr.addNode();

     Node n1 = gr.addNode();

     Node n2 = gr.addNode();

     map.set(n0, 'A');

     map.set(n1, 'B');

     map.set(n2, 'C');

     map.set(n2, 'A');

     map.set(n0, 'C');

     check(map[n0] == 'C' && map[n1] == 'B' && map[n2] == 'A',

           "Wrong CrossRefMap");

     check(map('A') == n2 && map.inverse()['A'] == n2, "Wrong CrossRefMap");

     check(map('B') == n1 && map.inverse()['B'] == n1, "Wrong CrossRefMap");

     check(map('C') == n0 && map.inverse()['C'] == n0, "Wrong CrossRefMap");

     ValueIt it = map.beginValue();

     check(*it++ == 'A' && *it++ == 'B' && *it++ == 'C' &&

           it == map.endValue(), "Wrong value iterator");

   }