/**
   * 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

  Graph<N> asGraph();

  //
  // Network properties
  //

  /**
   * Returns true if the edges in this network 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 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.
   */
  @Override
  boolean isDirected();

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

  // here too.
  static final String ERROR_PARALLEL_EDGE = "connected by a different edge";
  static final String ERROR_REUSE_EDGE = "it cannot be reused to connect";
  static final String ERROR_MODIFIABLE_COLLECTION =
      "Collection returned is unexpectedly modifiable";
  static final String ERROR_SELF_LOOP = "self-loops are not allowed";

  // here too.
  static final String ERROR_PARALLEL_EDGE = "connected by a different edge";
  static final String ERROR_REUSE_EDGE = "it cannot be reused to connect";
  static final String ERROR_MODIFIABLE_COLLECTION =
      "Collection returned is unexpectedly modifiable";
  static final String ERROR_SELF_LOOP = "self-loops are not allowed";

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   *
   * @return {@code true} if the network was modified as a result of this call
   * @throws IllegalArgumentException if {@code edge} already exists in the graph and does not
   *     connect {@code nodeU} to {@code nodeV}
   * @throws IllegalArgumentException if the introduction of the edge would violate {@link
   *     #allowsParallelEdges()} or {@link #allowsSelfLoops()}
   */
  @CanIgnoreReturnValue

      "Edge %s that was used to generate this set is no longer in the graph.";
  static final String REUSING_EDGE =
      "Edge %s already exists between the following nodes: %s, "
          + "so it cannot be reused to connect the following nodes: %s.";
  static final String MULTIPLE_EDGES_CONNECTING =
      "Cannot call edgeConnecting() when parallel edges exist between %s and %s. Consider calling "
          + "edgesConnecting() instead.";