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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.