e = assertThrows(IllegalArgumentException.class, () -> addEdge(N2, N1, E12));
    assertThat(e).hasMessageThat().contains(ERROR_REUSE_EDGE);
  }

  @Test
  public void addEdge_parallelEdge_notAllowed() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(network.allowsParallelEdges()).isFalse();

    addEdge(N1, N2, E12);
    IllegalArgumentException e =
        assertThrows(

            IllegalArgumentException.class, () -> networkAsMutableNetwork.addEdge(N4, N5, E12));
    assertThat(e).hasMessageThat().contains(ERROR_REUSE_EDGE);
  }

  @Test
  public void addEdge_parallelEdge_notAllowed() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(network.allowsParallelEdges()).isFalse();

    addEdge(N1, N2, E12);
    IllegalArgumentException e =
        assertThrows(

            IllegalArgumentException.class, () -> networkAsMutableNetwork.addEdge(N4, N5, E12));
    assertThat(e).hasMessageThat().contains(ERROR_REUSE_EDGE);
  }

  @Test
  public void addEdge_parallelEdge_notAllowed() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(network.allowsParallelEdges()).isFalse();

    addEdge(N1, N2, E12);
    IllegalArgumentException e =
        assertThrows(