Mayor: The law prohibiting pedestrians from crossing against red lights serves no useful purpose  After all, in order to serve a useful purpose, a law must deter the kind of behavior it prohibits
But pedestrians who invariably violate this law are clearly not dissuaded by it; and those who comply with the law do not need it, since they would never cross against red lights even if there were no law prohibiting pedestrians from crossing against red lights
uses the word law in one sense in the premises and in another sense in the conclusion