g the
<tt>add_edge</tt> function. Each subgraph has its own vertex and edge
descriptors, which we call <i>local</i> descriptors. We refer to root
graph's vertex and edge descriptors as the <i>global</i>
descriptors. Above, we used global vertex descriptors to add vertices
to the graph. However, most <tt>subgraph</tt> functions work with
local descriptors. So in the following call to <tt>add_edge</tt> we
add the edge <tt>(A1,C1)</tt> (or numerically <tt>(0,2)</tt>) which is
the local version (for subgraph <t