graphviz dot format
so that users can make use of <a href="http://www.research.att.com/sw/tools/graphviz/">AT&amp;T graphviz</a>
to draw a picture with nice layout.
<p>
The first version with two parameters will write the graph into a
<tt>std::ostream</tt> where each vertex is represented by its numerical vertex
ID. If users have either interior or exterior properties for each vertex
in the graph, the second version above provides a way to print those
properties into the graphviz format file. For example, i