In fairness, the author does recognize that he failed in giving a good test.
The thing is, those questions demonstrate a certain type of playfulness in dealing with the subject. It is not uncommon for good students to think of this type of questions when they design their first tests, precisely because those are the kinds of questions that they like to think about themselves. What they fail to recognize is that those questions are incredibly hard for others precisely because of their playfulness.
So those questions are useful for quickly recognizing really good students (students that are so good that they transcend the requirements of the course and can deal with the subject on this playful level), but they should only make up a very small portion of any test. And teachers should probably warn their students about this type of question...
