static final String ERROR_ADDED_SELF_LOOP = "Should not be allowed to add a self-loop edge.";
  static final String ERROR_ADDED_PARALLEL_EDGE = "Should not be allowed to add a parallel edge.";
  static final String ERROR_ADDED_EXISTING_EDGE =
      "Reusing an existing edge to connect different nodes succeeded";

  /** Creates and returns an instance of the graph to be tested. */

                <mxCell id="8" style="edgeStyle=orthogonalEdgeStyle;rounded=0;orthogonalLoop=1;jettySize=auto;html=1;strokeWidth=3;exitX=0.092;exitY=1.01;exitDx=0;exitDy=0;dashed=1;exitPerimeter=0;" parent="1" edge="1">
                    <mxGeometry relative="1" as="geometry">
                        <Array as="points">
                            <mxPoint x="800" y="521"/>
                            <mxPoint x="800" y="560"/>

   *     #allowsSelfLoops()}
   */
  @CanIgnoreReturnValue
  boolean putEdge(N nodeU, N nodeV);

  /**
   * Adds an edge connecting {@code endpoints} (in the order, if any, specified by {@code
   * endpoints}) if one is not already present.
   *
   * <p>If this graph is directed, {@code endpoints} must be ordered and the added edge will be

  static final String ERROR_ADDED_SELF_LOOP = "Should not be allowed to add a self-loop edge.";
  static final String ERROR_ADDED_PARALLEL_EDGE = "Should not be allowed to add a parallel edge.";
  static final String ERROR_ADDED_EXISTING_EDGE =
      "Reusing an existing edge to connect different nodes succeeded";

  /** Creates and returns an instance of the graph to be tested. */

  /**
   * 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.
   *

  /**
   * Returns the value associated with the edge connecting the origin node to {@code node}, or null
   * if there is no such edge.
   */
  @CheckForNull
  V value(N node);

  /** Remove {@code node} from the set of predecessors. */
  void removePredecessor(N node);

  /**
   * Remove {@code node} from the set of successors. Returns the value previously associated with
   * the edge connecting the two nodes.
   */

   *     #allowsParallelEdges()} or {@link #allowsSelfLoops()}
   */
  @CanIgnoreReturnValue
  boolean addEdge(N nodeU, N nodeV, E edge);

  /**
   * Adds {@code edge} connecting {@code endpoints}. In an undirected network, {@code edge} will
   * also connect {@code nodeV} to {@code nodeU}.
   *

    }
    return connections;
  }

  final N checkedReferenceNode(E edge) {
    N referenceNode = edgeToReferenceNode.get(edge);
    if (referenceNode == null) {
      checkNotNull(edge);
      throw new IllegalArgumentException(String.format(EDGE_NOT_IN_GRAPH, edge));
    }
    return referenceNode;
  }

  final boolean containsNode(N node) {

  static final String EDGE_ALREADY_EXISTS = "Edge %s already exists in the graph.";
  static final String ENDPOINTS_MISMATCH =
      "Mismatch: endpoints' ordering is not compatible with directionality of the graph";

  /** Singleton edge value for {@link Graph} implementations backed by {@link ValueGraph}s. */
  enum Presence {
    EDGE_EXISTS
  }

/**
 * An immutable pair representing the two endpoints of an edge in a graph. The {@link EndpointPair}
 * of a directed edge is an ordered pair of nodes ({@link #source()} and {@link #target()}). The
 * {@link EndpointPair} of an undirected edge is an unordered pair of nodes ({@link #nodeU()} and
 * {@link #nodeV()}).
 *
 * <p>The edge is a self-loop if, and only if, the two endpoints are equal.
 *
 * @author James Sexton