graph.addNode(gen.nextInt(NODE_POOL_SIZE));
      }
      ArrayList<Integer> nodeList = new ArrayList<>(graph.nodes());
      while (graph.edges().size() < NUM_EDGES) {
        graph.putEdge(getRandomElement(nodeList, gen), getRandomElement(nodeList, gen));
      }
      ArrayList<EndpointPair<Integer>> edgeList = new ArrayList<>(graph.edges());

      assertThat(graph.nodes()).hasSize(NUM_NODES);

      // add each node explicitly to include isolated nodes
      transitiveClosure.addNode(node);
      for (N reachableNode : getReachableNodes(graph, node, strategy)) {
        transitiveClosure.putEdge(node, reachableNode);
      }
    }
    return transitiveClosure.build();
  }

  /**
   * Equivalent to {@code transitiveClosure(graph, ADD_SELF_LOOPS_ALWAYS)}. Callers should look at

    MutableGraph<String> graph = GraphBuilder.directed().build();
    graph.putEdge("a", "b");

    Traverser.forTree(graph); // Does not throw
  }

  @Test
  public void forTree_withUndirectedGraph_throws() throws Exception {
    MutableGraph<String> graph = GraphBuilder.undirected().build();
    graph.putEdge("a", "b");

    assertThrows(IllegalArgumentException.class, () -> Traverser.forTree(graph));

        assertThat(endpointPair.source()).isEqualTo(endpointPair.adjacentNode(node));
        assertThat(endpointPair.target()).isEqualTo(node);
      }

      for (String outEdge : network.outEdges(node)) {
        EndpointPair<Integer> endpointPair = network.incidentNodes(outEdge);
        assertThat(endpointPair.source()).isEqualTo(node);
        assertThat(endpointPair.target()).isEqualTo(endpointPair.adjacentNode(node));
      }

        }

        orderedNodeConnectionsBuilder.add(new NodeConnection.Pred<>(predecessor));
        predecessorCount++;
      } else { // incidentEdge is an outEdge
        checkArgument(incidentEdge.nodeU().equals(thisNode));

        N successor = incidentEdge.nodeV();
        V value = successorNodeToValueFn.apply(successor);