digraph G {
  subgraph cluster0 {
    label = "induction";
    517->518;
    517->519;
  }
  subgraph cluster1 {
    label = "base case";
    528->528
  }
  subgraph cluster2 {
    label = "step case";
    537->538;
    538->540;
    540->542;
    542->544;
    544->546;
    546->548;
  }
  512->513;
  513->517;
  518->528;
  519->537;
}