digraph G {
  compound=true

  subgraph clusterA {
    a1 -> a2 -> a3
  }
  subgraph clusterB {
    subgraph clusterC {
      c1 -> c2
    }
    b1 -> b2
    b1 -> b3
    b1 -> c2
  }
  a1 -> b2 [lhead=clusterB]
  c2 -> a3 [ltail=clusterC]
  b3 -> a3 [ltail=clusterB, lhead=clusterA]
}