Re: By The Dawn's Normal Light

From: Kenneth Downs <firstinit.lastname_at_lastnameplusfam.net>
Date: Sun, 24 Oct 2004 10:26:52 -0400
Message-ID: <n3eglc.qgh.ln_at_mercury.downsfam.net>


Paul wrote:

>
> But then I think the constraint to stop loops (B follows A, C follows B,
> A follows C) would require second order logic. I might be wrong there
> though, maybe there's some clever way of doing it in first-order logic.
>
> But you would need second order logic to query the list - for example to
> return all list items that were after a given list item.

If the list is uniquely ordered I think you can do everything 1st order. The inserted item must be either head, tail, or have defined before and after nodes. You should also be able "SELECT..WHERE sequence>x" to get all items after item x. Don't know if you can guarantee unique sequence #'s on inserts without an occasional resequencing, like an index reorg. This seems to be the absolute positioning case.

But in the relative positioning case I think you need 2nd order. If you are given dependencies, "A must be before C" and "X must be before Y" I don't think you can determine the ordering A,X,C,Y without recursive looping through the dependencies list to determine the sequence. At least, I have not figured out how.

-- 
Kenneth Downs
Use first initial plus last name at last name plus literal "fam.net" to
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Received on Sun Oct 24 2004 - 16:26:52 CEST

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