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Both "No vehicles" and "in the park" already require you to have external knowledge. What is a vehicle? What constitutes park grounds?

That is part of the point of the exercise. You can expand on this rule, but however far you go there will continue to be things that are not clear.

Correct. But you (and the experimenter) are assuming that rules exist in a vacuum, which in practice it isn't

No the experimenter doesn't assume that, they say: "please answer the question of whether the rule is violated (not whether the violation should be allowed)."

The intent of the experiment is to get your view on whether X is a vehicle or not, and whether Y counts as a place "in the park" or not.

Yes, but that is an actual situation in real world.