digraph G {
  compound=true;
    subgraph cluster_0 {
    style=filled;
    color=lightgrey;
    node [style=filled,color=white];
    a0 -> a1 -> a2 -> a3;
    label = "process #1";
  }
  subgraph cluster_1 {
    node [style=filled];
    b0 -> b1 -> b2 -> b3;
    b4->b5;
    b6;
    label = "process #2";
    color=blue

  }

  start -> a0 [lhead=cluster_0] ;
  start -> b0 ;
  a1 -> b3 [ltail=cluster_0];
  b2 -> a3 [lhead=cluster_0,ltail=cluster_1];
  b1 -> a3;
  a3 -> a0;
  a3 -> end;
  b3 -> end;


  a2->b6 ;

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