From: "MURATA Makoto" <murata@[...].jp> 
 
>  On 28 Mar 2003 21:59:11 +0000
>  ht@[...].uk (Henry S. Thompson) wrote:
>  
>  > Somewhat surprisingly, it turns out that answering the question, for an
>  > arbitrary XML DTD, "Are there any valid instances of the document type
>  > defined by this DTD?", is an NP-hard problem.
>  
>  A similar result was shown by:
>  
>  On XML Integrity Constraints in the Presence of DTDs
>  Journal of the ACM (JACM), Volume 49 , Issue 3, pp 368 - 406, May 2002.
>  Wenfei Fan and Leonid Libkin
>  
>  http://www.bell-labs.com/user/wenfei/papers/jacm.pdf

That article should be read with care: it talks of DTDs but it means grammars
in general, and it talks of integrity constraints, but these are not ID/IDREF.

Thus it seems that they share the same proviso as with Henry's analysis: 
Henry's analysis seems to apply to DTDs with fixed IDs or fixed IDREFs, 
and this paper applies to where there is an outside fixed set of constraints 
such as keys and foreign keys.  

Cheers
Rick Jelliffe


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