assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> graph.adjacentNodes(N1)));
  }

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

    putEdge(N1, N2);
    assertThat(graph.successors(N1)).containsExactly(N2);
    assertThat(graph.predecessors(N2)).containsExactly(N1);

    assertThat(n2Successors).isEmpty();
    assertThat(n2IncidentEdges).isEmpty();
    assertThat(n2InEdges).isEmpty();
    assertThat(n2OutEdges).isEmpty();
  }

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

    addEdge(N1, N2, E12);
    assertTrue(networkAsMutableNetwork.removeEdge(E12));
    assertFalse(networkAsMutableNetwork.removeEdge(E12));