 code snippet prints the source-target pairs for each
out-edge of vertex <tt>v</tt>.
<p>&nbsp;
<pre>
  template &lt;class Graph&gt; struct exercise_vertex {
    //...
    void operator()(const Vertex&amp; v) const
    {
      typedef graph_traits&lt;Graph&gt; GraphTraits;
      typename property_map&lt;Graph, vertex_index_t&gt;::type 
        index = get(vertex_index, g);

      std::cout &lt;&lt; &quot;out-edges: &quot;;
      typename GraphTraits::out_edge_iterator out_i, out_end;
      typename GraphTrai