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Let us consider a DOS-like directory, but with a sophisticated feature: the user does not interact with the main directory management object that knows the whole directory system. Rather, the user gets his own personal directory management object that is identical to the "full power" manager, except that it has a different data set from which it navigates through the file system. In the user's limited manager, the "root" directory is the highest-level directory to which he has access. If the user uses the DOS change-directory command, "cd ..", i.e., "go up to the next higher directory level", while he is in the "docs" subdirectory, the directory manager will say, "hey, you're already at the root, there's no place else to go." No security checking per se is involved, just the legal-operation check to see if you're at the root when you request a higher level directory.