(u,v)</i> where an edge needs to be added to connect <tt>g</tt>. This
behavior can be overriden by providing a vistor as the <tt>AddEdgeVisitor</tt>
parameter. The only requirement for an <tt>AddEdgeVisitor</tt> is that it
define a member function with the following signature:
<pre>
template &lt;typename Graph, typename Vertex&gt;
void visit_vertex_pair(Vertex u, Vertex v, Graph& g);
</pre>
This event point can also be used as a hook to update the underlying edge
index map automatically as edges are added. 