assertThat(graph.adjacentNodes(N4)).containsExactly(N1);
    assertThat(graph.adjacentNodes(N5)).containsExactly(N1);
  }

  @Test
  public void putEdge_doesntAllowSelfLoops() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(graph.allowsSelfLoops()).isFalse();

    IllegalArgumentException e =

    assertThat(graph.successors(N3)).isEmpty();
    assertThat(graph.successors(N4)).containsExactly(N1);
    assertThat(graph.successors(N5)).isEmpty();
  }

  @Test
  public void putEdge_doesntAllowSelfLoops() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(graph.allowsSelfLoops()).isFalse();

    IllegalArgumentException e =

    assertThat(graph.adjacentNodes(N4)).containsExactly(N1);
    assertThat(graph.adjacentNodes(N5)).containsExactly(N1);
  }

  @Test
  public void putEdge_doesntAllowSelfLoops() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(graph.allowsSelfLoops()).isFalse();

    IllegalArgumentException e =

    assertThat(graph.successors(N3)).isEmpty();
    assertThat(graph.successors(N4)).containsExactly(N1);
    assertThat(graph.successors(N5)).isEmpty();
  }

  @Test
  public void putEdge_doesntAllowSelfLoops() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(graph.allowsSelfLoops()).isFalse();

    IllegalArgumentException e =