if (!expression) {
      throw new IllegalStateException(lenientFormat(errorMessageTemplate, p1, p2, p3, p4));
    }
  }

  /*
   * Preconditions.checkNotNull is *intended* for performing eager null checks on parameters that a
   * nullness checker can already "prove" are non-null. That means that the first parameter to
   * checkNotNull *should* be annotated to require it to be non-null.
   *

  @Override
  public Set<N> nodes() {
    return delegate().nodes();
  }

  /**
   * Defer to {@link AbstractGraph#edges()} (based on {@link #successors(Object)}) for full edges()
   * implementation.
   */
  @Override
  protected long edgeCount() {
    return delegate().edges().size();
  }

  @Override
  public boolean isDirected() {
    return delegate().isDirected();
  }

  @Override

    assertThat(graph.nodes()).containsExactly("A");
    assertThat(graph.edges()).isEmpty();
  }

  @Test
  public void immutableGraphBuilder_putEdgeFromNodes() {
    ImmutableGraph<String> graph =
        GraphBuilder.directed().<String>immutable().putEdge("A", "B").build();

    assertThat(graph.nodes()).containsExactly("A", "B");
    assertThat(graph.edges()).containsExactly(EndpointPair.ordered("A", "B"));
  }

  @Test

          } catch (CancellationException e) {
            assertTrue(merger == Merger.allMerger);
            inputs.assertHasImmediateCancel(iFuture, jFuture, e);
          } catch (ExecutionException e) {
            assertTrue(merger == Merger.allMerger);
            inputs.assertHasImmediateFailure(iFuture, jFuture, e);
          } catch (TimeoutException e) {

  }

  // Node/edge sets are the same, but node/edge connections differ due to edge type.
  @Test
  public void equivalent_directedVsUndirected() {
    graph.putEdge(N1, N2);

    MutableGraph<Integer> g2 = createGraph(oppositeType(edgeType));
    g2.putEdge(N1, N2);

    assertThat(graph).isNotEqualTo(g2);
  }

  // Node/edge sets and node/edge connections are the same, but directedness differs.

  /**
   * 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.
   *

/**
 * 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

  }

  @Test
  public void edges_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.edges()).containsExactly(E12);
  }

  @Test
  public void edges_noEdges() {
    assertThat(network.edges()).isEmpty();
    // Network with no edges, given disconnected nodes
    addNode(N1);
    addNode(N2);
    assertThat(network.edges()).isEmpty();
  }

  @Test

  }

  @Override
  @Test
  public void edges_checkReturnedSetMutability() {
    Set<String> edges = network.edges();
    UnsupportedOperationException e =
        assertThrows(UnsupportedOperationException.class, () -> edges.add(E12));
    addEdge(N1, N2, E12);
    assertThat(network.edges()).containsExactlyElementsIn(edges);
  }

  @Override
  @Test

  }

  @Test
  public void edges_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.edges()).containsExactly(E12);
  }

  @Test
  public void edges_noEdges() {
    assertThat(network.edges()).isEmpty();
    // Network with no edges, given disconnected nodes
    addNode(N1);
    addNode(N2);
    assertThat(network.edges()).isEmpty();
  }

  @Test