A1, B1, C2 }; // for conveniently refering to vertices in G1
enum { A2, B2 };     // for conveniently refering to vertices in G2
</pre>

We can add vertices from the root graph to the subgraphs using the
<tt>add_vertex</tt> function. Since the graph implementation is
<tt>adjacency_list</tt> with <tt>VertexList=vecS</tt>, we can use the
integers (or in this case enums) in the range <i>[0,6)</i> as vertex
descriptors.

<pre>
add_vertex(C, G1); // global vertex C becomes local A1 for G1
add_vertex(E, G1); // g