de for this example is in
<tt>example/subgraph.cpp</tt>. To create a graph and subgraphs, first
create the root graph object.  Here we use <tt>adjacency_list</tt> as
the underlying graph implementation. The underlying graph type is
required to have <tt>vertex_index</tt> and <tt>edge_index</tt>
internal properties, so we add an edge index property to the adjacency
list. We do not need to add a vertex index properety because that is
built in to the <tt>adjacency_list</tt>. We will be building the graph
and su