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Let's look at the Win95 directory searching tool, the Explorer. When at the root of the directory system (which is Desktop typically), the icon you press to go "up" the directory system is grayed out. As a result, you cannot even ask the question about going up; neither security checks nor even validity checks are required to ensure you don't accidentally go into the nowhere space that does not exist above the root. In other words, you cannot express the invalid request.