}

  final NetworkConnections<N, E> checkedConnections(N node) {
    NetworkConnections<N, E> connections = nodeConnections.get(node);
    if (connections == null) {
      checkNotNull(node);
      throw new IllegalArgumentException(String.format(NODE_NOT_IN_GRAPH, node));
    }
    return connections;
  }

  final N checkedReferenceNode(E edge) {
    N referenceNode = edgeToReferenceNode.get(edge);

    }

    GraphConnections<N, V> connectionsU = nodeConnections.get(nodeU);
    if (connectionsU == null) {
      connectionsU = addNodeInternal(nodeU);
    }
    V previousValue = connectionsU.addSuccessor(nodeV, value);
    GraphConnections<N, V> connectionsV = nodeConnections.get(nodeV);
    if (connectionsV == null) {
      connectionsV = addNodeInternal(nodeV);
    }

  // Node/edge sets are the same, but node/edge connections differ due to edge type.
  @Test
  public void equivalent_directedVsUndirected() {
    network.addEdge(N1, N2, E12);

    MutableNetwork<Integer, String> g2 = createNetwork(oppositeType(edgeType));
    g2.addEdge(N1, N2, E12);

    assertThat(network).isNotEqualTo(g2);
  }

  // Node/edge sets and node/edge connections are the same, but directedness differs.
  @Test

    }

    if (connectionsU == null) {
      connectionsU = addNodeInternal(nodeU);
    }
    connectionsU.addOutEdge(edge, nodeV);
    NetworkConnections<N, E> connectionsV = nodeConnections.get(nodeV);
    if (connectionsV == null) {
      connectionsV = addNodeInternal(nodeV);
    }
    connectionsV.addInEdge(edge, nodeU, isSelfLoop);
    edgeToReferenceNode.put(edge, nodeU);

  // Node/edge sets are the same, but node/edge connections differ due to edge type.
  @Test
  public void equivalent_directedVsUndirected() {
    network.addEdge(N1, N2, E12);

    MutableNetwork<Integer, String> g2 = createNetwork(oppositeType(edgeType));
    g2.addEdge(N1, N2, E12);

    assertThat(network).isNotEqualTo(g2);
  }

  // Node/edge sets and node/edge connections are the same, but directedness differs.
  @Test

  }

  // Node/edge sets are the same, but node/edge connections differ due to edge type.
  @Test
  public void equivalent_directedVsUndirected() {
    graph.putEdge(N1, N2);

    MutableGraph<Integer> g2 = createGraph(oppositeType(edgeType));
    g2.putEdge(N1, N2);

    assertThat(graph).isNotEqualTo(g2);
  }

  // Node/edge sets and node/edge connections are the same, but directedness differs.
  @Test

  }

  // Node/edge sets are the same, but node/edge connections differ due to edge type.
  @Test
  public void equivalent_directedVsUndirected() {
    graph.putEdge(N1, N2);

    MutableGraph<Integer> g2 = createGraph(oppositeType(edgeType));
    g2.putEdge(N1, N2);

    assertThat(graph).isNotEqualTo(g2);
  }

  // Node/edge sets and node/edge connections are the same, but directedness differs.
  @Test

    GraphConnections<N, V> connectionsU = nodeConnections.get(nodeU);
    return (connectionsU != null) && connectionsU.successors().contains(nodeV);
  }

  @CheckForNull
  private final V edgeValueOrDefaultInternal(N nodeU, N nodeV, @CheckForNull V defaultValue) {
    GraphConnections<N, V> connectionsU = nodeConnections.get(nodeU);
    V value = (connectionsU == null) ? null : connectionsU.value(nodeV);

              orderedNodeConnections.iterator(),
              (NodeConnection<N> connection) -> {
                if (connection instanceof NodeConnection.Succ) {
                  return EndpointPair.ordered(thisNode, connection.node);
                } else {
                  return EndpointPair.ordered(connection.node, thisNode);
                }
              });
    }

  Set<N> adjacentNodes();

  Set<N> predecessors();

  Set<N> successors();

  /**
   * Returns an iterator over the incident edges.
   *
   * @param thisNode The node that this all of the connections in this class are connected to.
   */
  Iterator<EndpointPair<N>> incidentEdgeIterator(N thisNode);

  /**
   * Returns the value associated with the edge connecting the origin node to {@code node}, or null