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I see a number of potential problems with such a requirement.
1. The task is potentially endless. If there can be a demand for a sentence that explains the meaning of sentence A, then the result is a sentence B and there can be a demand for a sentence C that explains the meaning of sentence B.
2. It's one thing to demonstrate both that A implies B and that B implies A. It's another thing to claim that A is in some respect difficult or unclear. A claim needs to be supported unless one wishes to simply construct dogma.
3. If no effort is made to find people who actually don't understand sentence A, then there is no way to check that sentence B is actually understood by some people who don't understand sentence A. In that case, the concept "good explanation" is not connected to any verifiable facts. There is an unlimited opportunity for an instructor to make subjective judgments and express arbitrary, personal preferences.
4. If the purpose of sentence B is to persuade the instructor that the student understands sentence A, then there is again an unlimited opportunity for an instructor to make subjective judgments and express arbitrary, personal preferences. If understanding doesn't help students achieve any goal, then there's no reason to demand understanding. If understanding does help students achieve goals, then it should be possible to assign a goal and judge understanding based on whether or not the student achieved the goal.
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