x, v), E(y, x), E(v, y), E(w, y),
                 E(w,z), E(z, z)};
    G g(edges, edges + sizeof(edges) / sizeof(E), N);

    depth_first_search(g);
    depth_first_search(g, _root_vertex = (int)x);
}
''') -->
<!-- @test('compile') -->
<p>Note the use of the nested <cite>tag::_</cite>. This is a shortcut for:</p>
<pre class="literal-block">
value_type&lt;boost::mpl::_2, tag&gt;
</pre>
<!-- @ignore() -->
<p>Intended to be used to access preceding arguments types in the
predicates.</p>
<p>We acknowledge tha