digraph "model" {
  compound=true
  subgraph cluster_1 {
    label="cluster_1"
    "C1N1"
  }
  subgraph cluster_2 {
    label="cluster_2"
    "C2N1"
  }
  "C1N1" -> "C2N1" [
    lhead=cluster_2
    ltail=cluster_1
  ]
}
