ther the graph class allows the insertion of
    parallel edges (edges with the same source and target), which
    depends on the underlying <tt>Graph</tt> class. The two tags are
    <tt>allow_parallel_edge_tag</tt> and
    <tt>disallow_parallel_edge_tag</tt>. 
    (Required by <a href="Graph.html">Graph</a>.)

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<pre>
graph_traits&lt;subgraph&gt;::vertices_size_type
</pre>
  The type used for dealing with the number of vertices in
  the graph.
  (Required by <a href="VertexListGraph.html">VertexListGra