/**
     * 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
     * directed; if it is undirected, the added edge will be undirected.
     *
     * <p>If this graph is directed, {@code endpoints} must be ordered.
     *

  public boolean hasEdgeConnecting(EndpointPair<N> endpoints) {
    return delegate().hasEdgeConnecting(endpoints);
  }

  @Override
  @CheckForNull
  public V edgeValueOrDefault(N nodeU, N nodeV, @CheckForNull V defaultValue) {
    return delegate().edgeValueOrDefault(nodeU, nodeV, defaultValue);
  }

  @Override
  @CheckForNull
  public V edgeValueOrDefault(EndpointPair<N> endpoints, @CheckForNull V defaultValue) {

    /**
     * Adds {@code edge} connecting {@code endpoints}. In an undirected network, {@code edge} will
     * also connect {@code nodeV} to {@code nodeU}.
     *
     * <p>If this network is directed, {@code edge} will be directed in this network; if it is
     * undirected, {@code edge} will be undirected in this network.
     *
     * <p>If this network is directed, {@code endpoints} must be ordered.
     *

  /**
   * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if
   * any, specified by {@code endpoints}). This is equivalent to {@code
   * edges().contains(endpoints)}.
   *
   * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the
   * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for

    assume().that(graphIsMutable()).isTrue();

    EndpointPair<Integer> endpoints = EndpointPair.unordered(N1, N2);
    IllegalArgumentException e =
        assertThrows(IllegalArgumentException.class, () -> graphAsMutableGraph.putEdge(endpoints));
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  /**

    assume().that(graphIsMutable()).isTrue();

    EndpointPair<Integer> endpoints = EndpointPair.unordered(N1, N2);
    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class, () -> networkAsMutableNetwork.addEdge(endpoints, E12));
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  @Test
  public void addEdge_selfLoop_notAllowed() {

    return create(Range.closed(lower, upper), DiscreteDomain.longs());
  }

  /**
   * Returns a contiguous set containing all {@code int} values from {@code lower} (inclusive) to
   * {@code upper} (exclusive). If the endpoints are equal, an empty set is returned. (These are the
   * same values contained in {@code Range.closedOpen(lower, upper)}.)
   *
   * @throws IllegalArgumentException if {@code lower} is greater than {@code upper}

      }

      for (EndpointPair<N> endpoints : sanityCheckSet(graph.incidentEdges(node))) {
        if (graph.isDirected()) {
          assertThat(graph.hasEdgeConnecting(endpoints.source(), endpoints.target())).isTrue();
        } else {
          assertThat(graph.hasEdgeConnecting(endpoints.nodeU(), endpoints.nodeV())).isTrue();
        }
      }
    }

    assertThat(edges).contains(EndpointPair.unordered(N1, N2));
    assertThat(edges).contains(EndpointPair.unordered(N2, N1)); // equal to unordered(N1, N2)

    // ordered endpoints not compatible with undirected graph
    assertThat(edges).doesNotContain(EndpointPair.ordered(N1, N2));

    assertThat(edges).doesNotContain(EndpointPair.unordered(N2, N2)); // edge not present

    assertThat(edges).contains(EndpointPair.unordered(N1, N2));
    assertThat(edges).contains(EndpointPair.unordered(N2, N1)); // equal to unordered(N1, N2)

    // ordered endpoints not compatible with undirected graph
    assertThat(edges).doesNotContain(EndpointPair.ordered(N1, N2));

    assertThat(edges).doesNotContain(EndpointPair.unordered(N2, N2)); // edge not present