}

  @Override
  public @Nullable E edgeConnectingOrNull(N nodeU, N nodeV) {
    Set<E> edgesConnecting = edgesConnecting(nodeU, nodeV);
    switch (edgesConnecting.size()) {
      case 0:
        return null;
      case 1:
        return edgesConnecting.iterator().next();
      default:
        throw new IllegalArgumentException(String.format(MULTIPLE_EDGES_CONNECTING, nodeU, nodeV));
    }

      adjacentNodes = HashMultiset.create(incidentEdgeMap.values());
      adjacentNodesReference = new SoftReference<>(adjacentNodes);
    }
    return adjacentNodes;
  }

  @Override
  public Set<E> edgesConnecting(N node) {
    return new MultiEdgesConnecting<E>(incidentEdgeMap, node) {
      @Override
      public int size() {
        return adjacentNodesMultiset().count(node);
      }
    };
  }

  @Override

  }

  @Override
  public Set<N> adjacentNodes(N node) {
    return nodeInvalidatableSet(checkedConnections(node).adjacentNodes(), node);
  }

  @Override
  public Set<E> edgesConnecting(N nodeU, N nodeV) {
    NetworkConnections<N, E> connectionsU = checkedConnections(nodeU);
    if (!allowsSelfLoops && nodeU == nodeV) { // just an optimization, only check reference equality
      return ImmutableSet.of();

  }

  @Override
  public Set<N> adjacentNodes(N node) {
    return nodeInvalidatableSet(checkedConnections(node).adjacentNodes(), node);
  }

  @Override
  public Set<E> edgesConnecting(N nodeU, N nodeV) {
    NetworkConnections<N, E> connectionsU = checkedConnections(nodeU);
    if (!allowsSelfLoops && nodeU == nodeV) { // just an optimization, only check reference equality
      return ImmutableSet.of();

/**
 * A class to represent the set of edges connecting an (implicit) origin node to a target node.
 *
 * <p>The {@link #outEdgeToNode} map allows this class to work on networks with parallel edges. See
 * {@link EdgesConnecting} for a class that is more efficient but forbids parallel edges.
 *
 * @author James Sexton
 * @param <E> Edge parameter type
 */
abstract class MultiEdgesConnecting<E> extends AbstractSet<E> {

  Set<E> outEdges();

  /**
   * Returns the set of edges connecting the origin node to {@code node}. For networks without
   * parallel edges, this set cannot be of size greater than one.
   */
  Set<E> edgesConnecting(N node);

  /**
   * Returns the node that is adjacent to the origin node along {@code edge}.
   *
   * <p>In the directed case, {@code edge} is assumed to be an outgoing edge.
   */
  N adjacentNode(E edge);

   *         <li>{@code inEdges(node)}: Edge insertion order
   *         <li>{@code outEdges(node)}: Edge insertion order
   *         <li>{@code adjacentEdges(edge)}: Stable order
   *         <li>{@code edgesConnecting(nodeU, nodeV)}: Edge insertion order
   *       </ul>
   * </ul>
   *
   * @since 29.0
   */
  public static <S> ElementOrder<S> stable() {
    return new ElementOrder<>(Type.STABLE, null);
  }

   *         <li>{@code inEdges(node)}: Edge insertion order
   *         <li>{@code outEdges(node)}: Edge insertion order
   *         <li>{@code adjacentEdges(edge)}: Stable order
   *         <li>{@code edgesConnecting(nodeU, nodeV)}: Edge insertion order
   *       </ul>
   * </ul>
   *
   * @since 29.0
   */
  public static <S> ElementOrder<S> stable() {
    return new ElementOrder<>(Type.STABLE, null);
  }