//
  // Graph properties
  //

  /**
   * Returns true if the edges in this graph are directed. Directed edges connect a {@link
   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
   * undirected edges connect a pair of nodes to each other.
   */
  boolean isDirected();

  /**
   * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting

    assertThat(toString).contains("valueD");
  }

  @Test
  public void undirectedGraph() {
    graph = ValueGraphBuilder.undirected().allowsSelfLoops(true).build();
    graph.putEdgeValue(1, 2, "valueA");
    graph.putEdgeValue(2, 1, "valueB"); // overwrites valueA in undirected case
    graph.putEdgeValue(2, 3, "valueC");
    graph.putEdgeValue(4, 4, "valueD");

    assertThat(toString).contains("valueD");
  }

  @Test
  public void undirectedGraph() {
    graph = ValueGraphBuilder.undirected().allowsSelfLoops(true).build();
    graph.putEdgeValue(1, 2, "valueA");
    graph.putEdgeValue(2, 1, "valueB"); // overwrites valueA in undirected case
    graph.putEdgeValue(2, 3, "valueC");
    graph.putEdgeValue(4, 4, "valueD");

    MutableGraph<Integer> undirectedGraph =
        GraphBuilder.undirected().allowsSelfLoops(false).build();
    undirectedGraph.putEdge(N1, N2);
    undirectedGraph.putEdge(N1, N3);
    undirectedGraph.putEdge(N2, N3);
    undirectedGraph.addNode(N4);

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

 *
 * <p>{@code Network} supports the following use cases (<a
 * href="https://github.com/google/guava/wiki/GraphsExplained#definitions">definitions of
 * terms</a>):
 *
 * <ul>
 *   <li>directed graphs
 *   <li>undirected graphs
 *   <li>graphs that do/don't allow parallel edges
 *   <li>graphs that do/don't allow self-loops
 *   <li>graphs whose nodes/edges are insertion-ordered, sorted, or unordered

 *
 * <ul>
 *   <li>Test cases related to whether the graph is directed or undirected.
 *   <li>Test cases related to the specific implementation of the {@link Graph} interface.
 * </ul>
 *
 * TODO(user): Make this class generic (using <N, E>) for all node and edge types.
 * TODO(user): Differentiate between directed and undirected edge strings.
 */
@NullUnmarked
public abstract class AbstractGraphTest {

    ImmutableNetwork<String, Integer> emptyNetwork =
        NetworkBuilder.directed()
            .allowsSelfLoops(true)
            .nodeOrder(ElementOrder.<String>natural())
            .<String, Integer>immutable()
            .build();

    assertThat(emptyNetwork.isDirected()).isTrue();
    assertThat(emptyNetwork.allowsSelfLoops()).isTrue();

  /**
   * The undirected graph in the {@link Traverser#breadthFirst(Object)} javadoc:
   *
   * {@snippet :
   * b ---- a ---- d
   * |      |
   * |      |
   * e ---- c ---- f
   * }
   */
  private static final SuccessorsFunction<Character> JAVADOC_GRAPH =
      createUndirectedGraph("ba", "ad", "be", "ac", "ec", "cf");

  /**
   * A diamond shaped directed graph (arrows going down):
   *

 *
 * <ul>
 *   <li>Test cases related to whether the graph is directed or undirected.
 *   <li>Test cases related to the specific implementation of the {@link Graph} interface.
 * </ul>
 *
 * TODO(user): Make this class generic (using <N, E>) for all node and edge types.
 * TODO(user): Differentiate between directed and undirected edge strings.
 */
@NullUnmarked
public abstract class AbstractGraphTest {

import java.util.Optional;
import java.util.Set;
import org.jspecify.annotations.NullUnmarked;
import org.junit.After;
import org.junit.Test;

/**
 * Abstract base class for testing undirected {@link Network} implementations defined in this
 * package.
 */
@NullUnmarked
public abstract class AbstractStandardUndirectedNetworkTest extends AbstractNetworkTest {