digraph {
    compound=true;
    subgraph cluster_0 {
      label="T1";
      K1 [label="K1"];
      K2 [label="K2"];
    }
    subgraph cluster_1 {
      label="T2";
      K3 [label="K3"];
    }
    subgraph cluster_2 {
      label="T3";
      K4 [label="K4"];
    }
    subgraph cluster_3 {
      label="T4";
      cb [label="callback"];
    }

    K1 -> K2;
    K2 -> K3 [ltail=cluster_0,lhead=cluster_1,minlen=2];
    K2 -> K4 [ltail=cluster_0,lhead=cluster_2,minlen=2];
    K3 -> cb [ltail=cluster_1,lhead=cluster_3,minlen=2];
    K4 -> cb [ltail=cluster_2,lhead=cluster_3,minlen=2];
}