}

  @Test
  public void source_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.incidentNodes(E12).source()).isEqualTo(N1);
  }

  @Test
  public void source_edgeNotInGraph() {
    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class,
            () -> network.incidentNodes(EDGE_NOT_IN_GRAPH).source());
    assertEdgeNotInGraphErrorMessage(e);
  }

  }

  @Test
  public void source_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.incidentNodes(E12).source()).isEqualTo(N1);
  }

  @Test
  public void source_edgeNotInGraph() {
    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class,
            () -> network.incidentNodes(EDGE_NOT_IN_GRAPH).source());
    assertEdgeNotInGraphErrorMessage(e);
  }