// unordered endpoints not OK for directed graph (undefined behavior)
    assertThat(edges).doesNotContain(EndpointPair.unordered(N1, N2));

    assertThat(edges).doesNotContain(EndpointPair.ordered(N2, N1)); // wrong order
    assertThat(edges).doesNotContain(EndpointPair.ordered(N2, N2)); // edge not present
    assertThat(edges).doesNotContain(EndpointPair.ordered(N3, N4)); // nodes not in graph
  }

  static <T> Set<T> sanityCheckSet(Set<T> set) {
    assertThat(set).hasSize(Iterators.size(set.iterator()));
    for (Object element : set) {
      assertThat(set).contains(element);
    }
    assertThat(set).doesNotContain(new Object());
    assertThat(set).isEqualTo(ImmutableSet.copyOf(set));
    return set;
  }