assertThat(graphAsMutableGraph.removeNode(N1)).isTrue();

    // The choice of the size() method to call here is arbitrary.  We assume that if any of the Set
    // methods executes the validation check, they all will, and thus we only need to test one of
    // them to ensure that the validation check happens and has the expected behavior.
    assertNodeRemovedFromGraphErrorMessage(

    assertThat(graphAsMutableGraph.removeNode(N1)).isTrue();

    // The choice of the size() method to call here is arbitrary.  We assume that if any of the Set
    // methods executes the validation check, they all will, and thus we only need to test one of
    // them to ensure that the validation check happens and has the expected behavior.
    assertNodeRemovedFromGraphErrorMessage(