* {@code edgeConnectingOrNull(nodeU, nodeV) != null}.
   *
   * <p>In an undirected network, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}.
   *
   * @since 23.0
   */
  @Override
  boolean hasEdgeConnecting(N nodeU, N nodeV);

  /**
   * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if

   *
   * <p>If {@code nodeU} and {@code nodeV} are not already present in this graph, this method will
   * silently {@link #addNode(Object) add} {@code nodeU} and {@code nodeV} to the graph.
   *
   * @return the value previously associated with the edge connecting {@code nodeU} to {@code
   *     nodeV}, or null if there was no such edge.

   */
  @CanIgnoreReturnValue
  boolean addNode(N node);

  /**
   * Adds an edge connecting {@code nodeU} to {@code nodeV} if one is not already present.
   *
   * <p>If the graph is directed, the resultant edge will be directed; otherwise, it will be
   * undirected.
   *
   * <p>If {@code nodeU} and {@code nodeV} are not already present in this graph, this method will

    assertThat(unordered.isOrdered()).isFalse();
    assertThat(unordered).containsExactly("node", "node");
    assertThat(unordered.nodeU()).isEqualTo("node");
    assertThat(unordered.nodeV()).isEqualTo("node");
    assertThat(unordered.adjacentNode("node")).isEqualTo("node");
    assertThat(unordered.toString()).isEqualTo("[node, node]");
  }

  @Test

    assertThat(unordered.isOrdered()).isFalse();
    assertThat(unordered).containsExactly("node", "node");
    assertThat(unordered.nodeU()).isEqualTo("node");
    assertThat(unordered.nodeV()).isEqualTo("node");
    assertThat(unordered.adjacentNode("node")).isEqualTo("node");
    assertThat(unordered.toString()).isEqualTo("[node, node]");
  }

  @Test

  int outDegree(N node);

  /**
   * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is
   * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}.
   *
   * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}.
   *
   * @since 23.0
   */
  @Override
  boolean hasEdgeConnecting(N nodeU, N nodeV);

  /**

   * silently {@link #addNode(Object) add} {@code nodeU} and {@code nodeV} to the graph.
   *
   * <p>If {@code edge} already connects {@code nodeU} to {@code nodeV} (in the specified order if
   * this network {@link #isDirected()}, else in any order), then this method will have no effect.
   *
   * @return {@code true} if the network was modified as a result of this call

      "Cannot add self-loop edge on node %s, as self-loops are not allowed. To construct a graph "
          + "that allows self-loops, call allowsSelfLoops(true) on the Builder.";
  static final String NOT_AVAILABLE_ON_UNDIRECTED =
      "Cannot call source()/target() on a EndpointPair from an undirected graph. Consider calling "
          + "adjacentNode(node) if you already have a node, or nodeU()/nodeV() if you don't.";

  public @Nullable V removeEdge(N nodeU, N nodeV) {
    checkNotNull(nodeU, "nodeU");
    checkNotNull(nodeV, "nodeV");

    GraphConnections<N, V> connectionsU = nodeConnections.get(nodeU);
    GraphConnections<N, V> connectionsV = nodeConnections.get(nodeV);
    if (connectionsU == null || connectionsV == null) {
      return null;
    }

    V previousValue = connectionsU.removeSuccessor(nodeV);
    if (previousValue != null) {

          PARALLEL_EDGES_NOT_ALLOWED,
          nodeU,
          nodeV);
    }
    boolean isSelfLoop = nodeU.equals(nodeV);
    if (!allowsSelfLoops()) {
      checkArgument(!isSelfLoop, SELF_LOOPS_NOT_ALLOWED, nodeU);
    }

    if (connectionsU == null) {
      connectionsU = addNodeInternal(nodeU);
    }
    connectionsU.addOutEdge(edge, nodeV);