o store properties is to create an array
indexed by vertex or edge index. In the <tt>adjacency_list</tt> with <tt>vecS</tt>
specified for the <tt>VertexList</tt> template parameter, vertices are
automatically assigned indices, which can be accessed via the property map for
the <tt>vertex_index_t</tt>. Edges are not automatically assigned indices.
However the property mechanism can be used to attach indices to the edges which
can be used to index into other externally stored properties.
<p>In the following e