}
      }
    } else {
      // An optimization for the undirected case: for every node B reachable from node A,
      // node A and node B have the same reachability set.
      Set<N> visitedNodes = new HashSet<>();
      for (N node : graph.nodes()) {
        if (!visitedNodes.contains(node)) {
          Set<N> reachableNodes = reachableNodes(graph, node);