digraph foo {
  subgraph cluster_main {
    subgraph cluster_a {
      aaa -> aab
    }
    subgraph cluster_b {
      bba -> bbb -> baa -> bab
      bba -> bbc -> aaa
    }
    subgraph cluster_c {
      cba -> cbb -> caa -> cab
      cba -> ccc -> baa
    }
    subgraph cluster_d {
      dba -> dbb -> daa
      dba -> dbc -> caa
    }
    subgraph cluster_e {
      eba -> ebb -> eaa
      eba -> ebc -> caa
    }
    subgraph cluster_f {
      fba -> fbb -> faa
      fba -> fbc -> caa
    }
  }
}