digraph {
  splines = line;

  a;
  subgraph cluster0 {
    style = "invisible";
    b0 [ label = <b<sub><font point-size="10">0</font></sub>>];
    b1 [ label = <b<sub><font point-size="10">1</font></sub>>];
    b2 [ label = <b<sub><font point-size="10">2</font></sub>>];
  }
  c;
  subgraph cluster1 {
    style = "invisible";
    d0 [ label = <d<sub><font point-size="10">0</font></sub>>];
    d1 [ label = <d<sub><font point-size="10">1</font></sub>>];
    d2 [ label = <d<sub><font point-size="10">2</font></sub>>];
  }

  a -> {b0, b1, b2, c};
  {b0, c} -> d0;
  {b1, c} -> d1;
  {b2, c} -> d2;
}
