addEdge(N1, N2, E12);
    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class,
            () -> {
              String unused = network.edgeConnectingOrNull(ENDPOINTS_N1N2);
            });
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  @Test
  public void edgesConnecting_oneEdge() {
    addEdge(N1, N2, E12);