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  /**
   * Returns the nodes which are the endpoints of {@code edge} in this network.
   *
   * @throws IllegalArgumentException if {@code edge} is not an element of this network
   */
  EndpointPair<N> incidentNodes(E edge);

  /**
   * Returns a live view of the edges which have an {@link #incidentNodes(Object) incident node} in
   * common with {@code edge}. An edge is not considered adjacent to itself.
   *

 * and forbids implementations or extensions with parallel edges. If you need parallel edges, use
 * {@link Network}. (You can use a positive {@code Integer} edge value as a loose representation of
 * edge multiplicity, but the {@code *degree()} and mutation methods will not reflect your
 * interpretation of the edge value as its multiplicity.)
 *
 * <h3>Building a {@code ValueGraph}</h3>
 *

   *         <li>{@code predecessors(node)}: Connecting edge insertion order
   *         <li>{@code successors(node)}: Connecting edge insertion order
   *         <li>{@code incidentEdges(node)}: Stable order
   *         <li>{@code inEdges(node)}: Edge insertion order
   *         <li>{@code outEdges(node)}: Edge insertion order
   *         <li>{@code adjacentEdges(edge)}: Stable order

   *         <li>{@code predecessors(node)}: Connecting edge insertion order
   *         <li>{@code successors(node)}: Connecting edge insertion order
   *         <li>{@code incidentEdges(node)}: Stable order
   *         <li>{@code inEdges(node)}: Edge insertion order
   *         <li>{@code outEdges(node)}: Edge insertion order
   *         <li>{@code adjacentEdges(edge)}: Stable order

            List<String> cycle = visitCycle(graph, Collections.singleton(to), new HashMap<>(), new LinkedList<>());
            if (cycle != null) {
                // remove edge which introduced cycle
                throw new CycleDetectedException(
                        "Edge between '" + from + "' and '" + to + "' introduces to cycle in the graph", cycle);
            }
        }
    }

    private enum DfsState {
        VISITING,

      Collections.shuffle(edgeList, gen);
      int numEdgesToRemove = gen.nextInt(NUM_EDGES);
      for (int i = 0; i < numEdgesToRemove; ++i) {
        Object edge = edgeList.get(i);
        assertThat(network.removeEdge(edge)).isTrue();
      }

      assertThat(network.nodes()).hasSize(NUM_NODES);
      assertThat(network.edges()).hasSize(NUM_EDGES - numEdgesToRemove);

      mutableValueGraph.addNode(node);
      return this;
    }

    /**
     * Adds an edge connecting {@code nodeU} to {@code nodeV} if one is not already present, and
     * sets a value for that edge to {@code value} (overwriting the existing value, if any).
     *
     * <p>If the graph is directed, the resultant edge will be directed; otherwise, it will be
     * undirected.
     *

    public Builder<N> addNode(N node) {
      mutableGraph.addNode(node);
      return this;
    }

    /**
     * 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.
     *

    // Passed nodes should be in the correct edge direction, first is the
    // source node and the second is the target node
    assertThat(network.edgesConnecting(N2, N1)).isEmpty();
  }

  @Test
  public void inEdges_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.inEdges(N2)).containsExactly(E12);
    // Edge direction handled correctly
    assertThat(network.inEdges(N1)).isEmpty();