/write-graphviz.html#concept:PropertyWriter">PropertyWriter</a>
for graphs is a functor which is called with one parameter of type
<tt>std::ostream</tt> and should print a series of graph properties. The following
code excerpt is an example of a PropertyWriter for a graph:
<pre>
  struct sample_graph_writer {
    void operator()(std::ostream&amp; out) const {
      out &lt;&lt; "graph [bgcolor=lightgrey]" &lt;&lt; std::endl;
      out &lt;&lt; "node [shape=circle color=white]" &lt;&lt; std::endl;
      out 