 map functions
that are described in the following sections. The <tt>vertex_descriptor</tt>
type is obtained through the <tt>graph_traits</tt> class. The <tt>typename</tt>
keyword used below is necessary because the type on the left hand side of the
scope <tt>::</tt> operator (the <tt>graph_traits&lt;Graph&gt;</tt> type) is
dependent on a template parameter (the <tt>Graph</tt> type). Here is how we
define the functor's apply method:
<p>&nbsp;
<pre>
  template &lt;class Graph&gt; struct exercise_vertex {
   