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));

   * insertion order.
   */
  private void populateTShapedGraph() {
    putEdge(2, 1);
    putEdge(1, 4);
    putEdge(1, 3);
    putEdge(1, 2); // Duplicate
    putEdge(4, 5);
  }

  // Element Mutation

  @Test
  public void putEdge_existingNodes() {
    assume().that(graphIsMutable()).isTrue();

        }
      }

      for (E outEdge : sanityCheckSet(network.outEdges(node))) {
        assertThat(network.incidentEdges(node)).contains(outEdge);
        assertThat(network.inEdges(network.incidentNodes(outEdge).adjacentNode(node)))
            .contains(outEdge);
        if (network.isDirected()) {
          assertThat(network.incidentNodes(outEdge).source()).isEqualTo(node);
        }
      }
    }

        }
      }

      for (E outEdge : sanityCheckSet(network.outEdges(node))) {
        assertThat(network.incidentEdges(node)).contains(outEdge);
        assertThat(network.inEdges(network.incidentNodes(outEdge).adjacentNode(node)))
            .contains(outEdge);
        if (network.isDirected()) {
          assertThat(network.incidentNodes(outEdge).source()).isEqualTo(node);
        }
      }
    }

    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));

    undirectedGraph.putEdge(N2, N3);
    undirectedGraph.putEdge(N3, N4);

    MutableGraph<Integer> expectedClosure = GraphBuilder.undirected().allowsSelfLoops(true).build();
    expectedClosure.putEdge(N1, N1);
    expectedClosure.putEdge(N1, N2);
    expectedClosure.putEdge(N1, N3);
    expectedClosure.putEdge(N1, N4);
    expectedClosure.putEdge(N2, N2);
    expectedClosure.putEdge(N2, N3);
    expectedClosure.putEdge(N2, N4);

 * graph.putEdge("bread", "bread");
 * graph.putEdge("chocolate", "peanut butter");
 * graph.putEdge("peanut butter", "jelly");
 *
 * // Building an immutable graph
 * ImmutableGraph<String> immutableGraph =
 *     GraphBuilder.undirected()
 *         .allowsSelfLoops(true)
 *         .<String>immutable()
 *         .putEdge("bread", "bread")
 *         .putEdge("chocolate", "peanut butter")

    assume().that(graphIsMutable()).isTrue();

    putEdge(N1, N2);
    putEdge(N1, N3);
    putEdge(N1, N4);
    assertThat(graphAsMutableGraph.removeEdge(N1, N3)).isTrue();
    assertThat(graph.adjacentNodes(N1)).containsExactly(N2, N4);
  }

  @Test
  public void removeEdge_nodeNotPresent() {
    assume().that(graphIsMutable()).isTrue();

    putEdge(N1, N2);

    // will have no isolated nodes, we can skip adding nodes explicitly and let putEdge() do it.

    if (graph.isDirected()) {
      // Note: works for both directed and undirected graphs, but we only use in the directed case.
      for (N node : graph.nodes()) {
        for (N reachableNode : reachableNodes(graph, node)) {
          transitiveClosure.putEdge(node, reachableNode);
        }
      }
    } else {

        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));
      }