he library already does this when the required
<tt class="docutils literal">graph</tt> parameter is not supplied, but we're not likely to see a
depth first search that doesn't take a graph to operate on.
Suppose, instead, that we found a different depth first search
algorithm that could work on graphs that don't model
<a class="reference external" href="../../../graph/doc/IncidenceGraph.html"><span class="concept">Incidence Graph</span></a>?  If we just added a simple overload,
it would be ambiguous:</p>
<p