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

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

  @Test
  public void addEdge_selfLoop_notAllowed() {

            });
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  @Test
  public void edgeConnectingOrNull_orderMismatch() {
    addEdge(N1, N2, E12);
    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class,
            () -> {
              String unused = network.edgeConnectingOrNull(ENDPOINTS_N1N2);
            });