Hard links to directories: why not?

[Aryeh M. Weiss]

In article <9007221600.AA00202 at mindcrf.mindcraft.com> karish at mindcrf.UUCP writes:
>In article <11070 at alice.UUCP> andrew at alice.UUCP (Andrew Hume) writes:
>
>    Links to files don't `bugger' the file system's structure,
>    because they don't create loops that make tree traversal
>    ambiguous.  It's still a tree, with some leaves fused
>    together.
>

Why can't potential loops be detected when a hard link request is given?
I.e., if I request to link directory `a' to directory `b', the
kernal could look up the tree to see if `a' already occurs as a parent
to `b', and deny the request if it does (ELOOPDEELOOP).

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