e root of the subgraph tree, then the vertex and
edge descriptors are both the local descriptors for the root graph,
and they are the global descriptors. If the graph is not the root,
then the descriptors are local descriptors for the subgraph.
The subgraph iterators are the same iterator types as the iterators of
the underlying <tt>Graph</tt> type.

<hr>

<pre>
graph_traits&lt;subgraph&gt;::vertex_descriptor
</pre>
    The type for the vertex descriptors. 
 (Required by <a href="Graph.html">Graph</a>.)

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