@NullUnmarked
public abstract class AbstractNetworkTest {

  Network<Integer, String> network;

  /**
   * The same reference as {@link #network}, except as a mutable network. This field is null in case
   * {@link #createGraph()} didn't return a mutable network.
   */
  MutableNetwork<Integer, String> networkAsMutableNetwork;

  static final Integer N1 = 1;
  static final Integer N2 = 2;
  static final Integer N3 = 3;

  }

  private static SuccessorsFunction<Character> createDirectedGraph(String... edges) {
    return createGraph(/* directed= */ true, edges);
  }

  private static SuccessorsFunction<Character> createUndirectedGraph(String... edges) {
    return createGraph(/* directed= */ false, edges);
  }

  /**

  }

  private static SuccessorsFunction<Character> createDirectedGraph(String... edges) {
    return createGraph(/* directed= */ true, edges);
  }

  private static SuccessorsFunction<Character> createUndirectedGraph(String... edges) {
    return createGraph(/* directed= */ false, edges);
  }

  /**