Why is it compulsary to wear a swimming suit in public pools?

These rules are usually established by your local health department, or set by local ordnance and there is no way of knowing the rules in your area. AS A GENERAL RULE: Most public swimming pools require proper swim suits or attire.

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Since "A GENERAL RULE" HAS MULTIPLE DEFINITIONS, so does a "proper" bathing suit.

You may have to spell that out in a very particular way so there is no confusion in the definition of your Words. You know exactly what you mean, however, when you put it on paper and others read it you will find various people have different ideas as to what you really mean.