digraph G
{
  subgraph cluster_G1
  {
    {
      rank=same;
      A -> B;
    }

    C -> A;
  }

  subgraph cluster_G2
  {
    {
      rank=same;
      D -> E;
    }

    F -> E;
  }

  B -> D [ constraint=none ];
  B -> D [ constraint=none ];
}

