  digraph G {
      node [shape=rect];

      subgraph cluster_0 {
          style=filled;
          color=lightgrey;
          node [style=filled,color=white];
          a0 -> a1 -> a2 -> a3;
          label = "Hello";
      }

      subgraph cluster_1 {
          node [style=filled];
          b0 -> b1 -> b2 -> b3;
          label = "World!";
          color=blue
      }

      start -> a0;
      start -> b0;
      a1 -> b3;
      b2 -> a3;
      a3 -> a0;
      a3 -> end;
      b3 -> end;

      start [s