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Re: continuing technical issues in the RDF Semantics document

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Wed, 27 Aug 2003 19:03:29 -0400 (EDT)
Message-Id: <20030827.190329.125116618.pfps@research.bell-labs.com>
To: phayes@ihmc.us
Cc: www-rdf-comments@w3.org, bwm@hplb.hpl.hp.com

From: pat hayes <phayes@ihmc.us>
Subject: Re: continuing technical issues in the RDF Semantics document
Date: Wed, 27 Aug 2003 10:08:30 -0700

> >The current version of the RDF Semantics document, titled RDF Semantics
> >Editors Draft August 21, has continuing technical issues that I discovered
> >in a quick, incomplete pass this morning.
> >
> >
> >The document does not define ``character string'' or ``language tag''.
> >These need precise definitions as the definition of simple interpretations
> depends on them.
> 
> Section 1.2:
> "Throughout this document we use the term 'character string' or 
> 'string' to refer to a sequence of Unicode characters in Normal Form 
> C, c.f. section 6.5 in [RDF-CONCEPTS]. "
> 
> Every occurrence of the term  'language tag' is linked to the 
> definition in Concepts, which refers to RFC 3066; I have added an 
> explicit piece of text to indicate this link in the visible text.

I was only looking at the place where all the other stuff is defined, and
missed the definition of character string.  The linking of language tag was
insufficient, as that is not visible when reading a hard copy, but is now
fixed.  As an editorial matter, I suggest moving both these next to the
definition pointers to RDF Concepts (for things like literal, triple,
etc.).

I note that the processing of RDF/XML may result in lexical forms that are
not in NFC.  This issue needs to be addressed, I believe.

[...]

> >There are conditions imposed on the non-core RDF vocabulary by
> >rdf-interpretations, counter to several claims in the document.
> 
> The text refers to 'significant' formal constraints. I do not believe 
> that this is likely to be misunderstood.

I am having problems thinking of insignificant formal constraints.

[...]

> >The definition of the Herbrand interpretation of a graph has all
> >well-formed XML literals in LV
> 
> No, it only requires LV to contain all well-formed XML literals *in 
> V*, where V is the vocabulary of the interpretation.

Correct, my mistake.

> >, which is permissable, but incorrectly
> >states that these are required to be in LV, and makes Herbrand
> >interpretations non-minimal.
> 
> I do not think this is correct. The use of the terminology 'minimal' 
> is only in the text, and always surrounded by scare quotes. The 
> formal proof uses the << relationship, which need not be asymmetric.

[...]

> >As well, Herbrand interpretations abide
> >by part of the RDF meaning of rdf:type, which also makes them non-minimal.
> 
> They are in the sense defined, ie that they make the smallest number 
> of triples true. Adding items to IP makes no difference to this, and 
> is convenient for the RDF entailment lemma.

I am still having severe problems getting my head around the
subinterpretation criterion.  I had some incorrect notions, but my further
examination its definition leads me to the conclusion that something is
wrong.

In particular, the definition talks about a projection mapping.  What
requirements does this place on the mapping?  I thought that a projection
had to be idempotent, but this certainly doesn't work, as it would make the
simple switching of two domain elements result in incomparable
interpretations.   I would appreciate a more detailed definition.

[...]

> >The Herbrand lemma is false as Herbrand interpretations are non-minimal.
> 
> I believe it is true (with or without the IP condition). Can you give 
> a counterexample?

Yes, if the mapping in the definition of subinterpretation really has to be
a projection, then simply switching domein elements give a counterexample.

[...]

> >The proof of the RDF entailment lemma is suspect in other ways, as the
> >rdf-interpretation constructed (H') appears to have both XML values and
> >blank nodes in the class extension of rdf:XMLLiteral.
> 
> That was not the intention: H' is an rdf-interpretation in which XML 
> literals denote XML literal values. The 'corresponding' Herbrand 
> interpretation H does have blank nodes which are of type 
> rdf:XMLLiteral, but the RDF entailment rules ensure that these blank 
> nodes play the same role in H that their corresponding literals do in 
> H'.

> >The details of XML
> >literals are sufficiently arcane that I cannot determine whether this is
> >permissable.
> 
> Think of the RDF entailment rules as having the pattern (forall 
> literal (exists a thing...)) and then skolemize. The resulting skolem 
> terms, which could be written as rdf2("well-formed-XML"), are the 
> 'markers' in the Herbrand interpretation H for the actual XML literal 
> values in the RDF interpretation H'.  The rules guarantee that H and 
> H' make the same triples true in an closure, so that H' << H. Since H 
> is minimal, so is H'.

I guess that this works out, modulo issues with <<.


> >On a minor note, the definition of proper instance means that
> >	<ex:a> <ex:b> "a" .
> >is not a proper instance of
> >	<ex:a> <ex:b> _:xx .
> >This affects the definition of lean graphs, and reduces the scope of the
> >anonymity lemma.
> 
> Whoops. I have changed the definition to cover plain literals.

Note that nothing was particulary wrong here before. :-)

> Pat

peter
Received on Wednesday, 27 August 2003 19:05:16 GMT

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