Therefore the graph interface
also includes the <a href="AdjacencyGraph.html#sec:adjacent-vertices"><tt>adjacent_vertices()</tt></a>
function of the <a href="AdjacencyGraph.html">AdjacencyGraph</a> interface which
provides direct access to the adjacent vertices. This function returns a pair of
<i>adjacency iterators</i>. Dereferencing an adjacency iterator gives a vertex
descriptor for an adjacent vertex.
<p>&nbsp;
<pre>
  template &lt;class Graph&gt; struct exercise_vertex {
    //...
    void operator()(V