 members <tt>x</tt> and
<tt>y</tt>. For example, if <tt>p</tt> models PositionMap and <tt>v</tt>
is a vertex in the graph, <tt>p[v].x</tt> and <tt>p[v].y</tt> are valid
expressions. The type of <tt>x</tt> and <tt>y</tt> must be implicitly 
convertable to <tt>std::size_t</tt>.
</blockquote>

IN: <tt>VertexIndexMap vm</tt>

<blockquote>
A <a href="../../property_map/doc/ReadablePropertyMap.html">Readable Property Map
</a> that maps vertices from <tt>g</tt> to distinct integers in the range 
<tt>[0, num_vertic