phs are maintained in a tree data structure. The main
graph is the root, and subgraphs are either children of the root or of
other subgraphs. All of the nodes in this tree, including the root
graph, are instances of the <tt>subgraph</tt> class.  The
<tt>subgraph</tt> implementation ensures that each node in the tree is
an induced subgraph of its parent. The <tt>subgraph</tt> class
implements the BGL graph interface, so each subgraph object can be
treated as a graph.</p>

<h3>Example</h3>

The full source co