boost_1_45_0/libs/graph/example/graphviz_test.dot

digraph G {

  subgraph cluster0 {  //subgraph<Graph>
    node [style=filled color=white];
    style = filled;
    bgcolor = lightgrey;

    subgraph inner { //subgraph<Graph> or subgraph of subgraph
	node [color = green];
	a1 -> a2 -> a3 

    };

    a0 -> subgraph inner;

    label = "process #1";
  }

  subgraph cluster1 {
    node [style=filled color=white];
    b0 -> b1 -> b2 -> b3;
    label = "process #2";
    bgcolor = lightgrey
  }

  subgraph cluster1 -> subgraph cluster0 [style=dashed color=red]

  start -> subgraph inner[style=dotted];

  start -> a0;
  start -> b0;
  a1 -> b3;
  b2 -> a3;
  a3 -> end;
  b3 -> end;

  start [shape=Mdiamond];
  end [shape=Msquare];
}