digraph G {
  subgraph cluster1 {
    label = "induction";
    517 -> 518;
    517 -> 519;
  }
  subgraph cluster2 {
    label = "base case";
    528;
  }
  subgraph cluster3 {
    label = "step case";
    537 -> 538;
    538 -> 540;
    540 -> 542;
    542 -> 544;
    544 -> 546;
    546 -> 548;
  }
  512 -> 513;
  513 -> 517;
  518 -> 528;
  519 -> 537;
}