lobal vertex E becomes local B1 for G1
add_vertex(F, G1); // global vertex F becomes local C1 for G1

add_vertex(A, G2); // global vertex A becomes local A2 for G2
add_vertex(B, G2); // global vertex B becomes local B2 for G2
</pre>

Next we can add edges to the main graph using the usual
<tt>add_edge</tt> function.

<pre>
add_edge(A, B, G0);
add_edge(B, C, G0);
add_edge(B, D, G0);
add_edge(E, B, G0);
add_edge(E, F, G0);
add_edge(F, D, G0);
</pre>

We can also add edges to subgraphs such as <tt>G1</tt> usin