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Re: Existentials in backward chaining

From: naudts guido <naudts_vannoten@yahoo.com>
Date: Wed, 18 Jun 2003 03:25:49 -0700 (PDT)
Message-ID: <20030618102549.830.qmail@web40508.mail.yahoo.com>
To: Jos De_Roo <jos.deroo@agfa.com>, "Graham Klyne <gk" <gk@ninebynine.org>
Cc: naudts guido <naudts_vannoten@yahoo.com>, www-archive@w3.org

Hallo,
I agree with Jos. I add that for a query the scope is
the whole query in my system i.e. this can be more
than one triple.
Guido.

--- Jos De_Roo <jos.deroo@agfa.com> wrote:
> 
> Graham, I'm completely in a hurry but I think it is
> better to think of bnodes in a premis as of univars
> with the scope of the rule (taking care of renaming
> their labels). That follows from the internal ~P V
> C.
> This is actually a God's given as a resolution based
> backward chainer needs such rule scoped univars.
> For facts, skolem constants take over and it's just
> for bnodes in conclusions that skolems function
> terms
> are needed (function of the rule scoped univars).
> For a query it's the same story as for a premis.
> 
> 
> --
> Jos De Roo, AGFA http://www.agfa.com/w3c/jdroo/
> 
> 
> 
>                                                     
>                                                     
>          
>                     Graham Klyne                    
>                                                     
>          
>                     <gk@ninebynine       To:     Jos
> De_Roo/AMDUS/MOR/Agfa-NV/BE/BAYER@AGFA, naudts guido
>          
>                     .org>                
> <naudts_vannoten@yahoo.com>                         
>                     
>                                          cc:        
>                                                     
>          
>                     2003-06-17           Subject:   
>  Existentials in backward chaining                  
>          
>                     02:49 PM                        
>                                                     
>          
>                                                     
>                                                     
>          
>                                                     
>                                                     
>          
> 
> 
> 
> 
> Jos,
> Guido,
> 
> I'm thinking about implementing a backward chaining
> component for my
> Haskell RDF toolkit, and have noticed a potential
> problem with existentials
> 
> (bnodes).  I'm intrigued how you avoid this problem.
> 
> Here's my test case:
> 
> Suppose we have a simple rule:
> 
>       ?f ex:son ?s1 .
>       ?f ex:son ?s2 .
>     =>
>       ?s1 ex:brother ?s2 .
> 
> To backward chain this requires introducing an
> existential:  to show
>     ex:s1 ex:brother ex:s2 .
> one has to show:
>     _:f ex:son ex:s1 .
>     _:f ex:son ex:s2 .
> for some _:f.
> 
> Then:
>     _:s1 ex:sister  ex:d1 .
>     _:s1 ex:brother ex:s2 .
> if
>     _:f ex:son _:s1 .
>     _:f ex:son ex:s2 .
> and
>     _:s1 ex:sister  _:d .
> 
> but if the original goal is:
>     _:s1 ex:sister  _:f .
>     _:s1 ex:brother ex:s2 .
> one might decide this is true if:
>     _:f ex:son _:s1 .
>     _:f ex:son ex:s2 .
> and
>     _:s1 ex:sister _:f .
> which is problematic because if we do a simple union
> of the graphs we get:
>     _:f ex:son _:s1 .
>     _:f ex:son ex:s2 .
>     _:s1 ex:sister _:f .
> or if we do a full merge (with bnode renaming):
>     _:f1 ex:son _:s11 .
>     _:f1 ex:son ex:s2 .
>     _:s12 ex:sister  _:f2 .
> neither of which are the answer we seek.  What we
> really want to capture
> is:
>     _:f1 ex:son _:s1 .
>     _:f1 ex:son ex:s2 .
>     _:s1 ex:sister _:f .
> 
> The problem arises when the bnode introduced by
> backward chaining clashes
> with a bnode in some other part of the graph with
> which the antecedent will
> 
> subsequently be combined.
> 
> I have a plan to tackle this, but it feels a bit
> complicated and wondered
> if you have an easy strategy top avoid the problem.
> 
> #g
> 
> 
> -------------------
> Graham Klyne
> <GK@NineByNine.org>
> PGP: 0FAA 69FF C083 000B A2E9  A131 01B9 1C7A DBCA
> CB5E
> 
> 
> 
> 
> 


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Received on Wednesday, 18 June 2003 08:27:31 GMT

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