digraph G {
  // https://dreampuf.github.io/GraphvizOnline
  rankdir=LR;
    
  subgraph cluster_2 {
    "expression_1aB";
    "expression_2aA";
  {rank=same; "expression_1aB" -> "expression_2aA" [label="derivation"];}
    "expression_1aA";
    label = "expressions";
    color=red
  }

  "Physics" -> "topic_1";
  "topic_1" -> "subtopic_1a";
  "subtopic_1a" -> "expression_1aA";
  "subtopic_1a" -> "expression_1aB";

  "Physics" -> "topic_2";
  "topic_2" -> "subtopic_2a";
  "subtopic_2a" -> "expression_2aA";
}