containsExactlySanityCheck(edges);

    assertThrows(
        UnsupportedOperationException.class, () -> edges.add(EndpointPair.ordered(N1, N2)));
  }

  @Test
  public void endpointPair_undirected_contains() {
    MutableGraph<Integer> undirectedGraph = GraphBuilder.undirected().allowsSelfLoops(true).build();
    undirectedGraph.putEdge(N1, N1);
    undirectedGraph.putEdge(N1, N2);