assertThat(network.edgesConnecting(N1, N2)).containsExactly(E12);
    assertThat(network.edgesConnecting(N2, N1)).containsExactly(E12);
  }

  @Test
  public void addEdge_existingEdgeBetweenSameNodes() {
    assume().that(graphIsMutable()).isTrue();

    assertThat(networkAsMutableNetwork.addEdge(N1, N2, E12)).isTrue();
    ImmutableSet<String> edges = ImmutableSet.copyOf(network.edges());