es.
Here is the start of the <tt>exercise_vertex</tt> functor:
<p>&nbsp;
<pre>
  template &lt;class Graph&gt; struct exercise_vertex {
    exercise_vertex(Graph&amp; g_) : g(g_) {}
    //...
    Graph&amp; g;
  };
</pre>
<p>&nbsp;
<h3>Vertex Descriptors</h3>
<p>The first thing we need to know in order to write the <tt>operator()</tt>
method of the functor is the type for the vertex objects of the graph. The
vertex type will be the parameter to the <tt>operator()</tt> method. To be
precise, we do not deal wi