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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.