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RE: Review of Direct Semantics (ACTION 314)

From: Boris Motik <boris.motik@comlab.ox.ac.uk>
Date: Mon, 6 Apr 2009 14:13:06 +0100
To: "'Michael Schneider'" <schneid@fzi.de>, "'W3C OWL Working Group'" <public-owl-wg@w3.org>
Message-ID: <64024596A339480192C7ED077685BD14@wolf>
Hello Michael,

Thanks a lot for your review. Please find my answers inline.



> -----Original Message-----
> From: public-owl-wg-request@w3.org [mailto:public-owl-wg-request@w3.org] On
> Behalf Of Michael Schneider
> Sent: 05 April 2009 23:45
> To: W3C OWL Working Group
> Subject: Review of Direct Semantics (ACTION 314)
> Hi!
> This is my review of the Direct Semantics. This document has already been in a
> good shape months ago, when I first reviewed it, and so it is now. I have only
> found smaller things, none of them critical.
> Note: I did not check against Thomas' review that arrived a bit earlier, so
> there may be overlap.
> * General: The definitions in the document, in particular those in §2.5, are
> of the form "A if B". While this is a typical convention under mathematicians,
> our documents are targeted to a broader audience. In order to avoid confusion,
> I suggest to always say "if and only if" (or "iff", and say once that this
> means "if and only if").

I've had extensive discussions about this with many people (notably Uli), and they insisted that the latter form is rather ugly. I'd prefer leaving things as they are. 

> * Abstract: Suggestion: Consider not talking about inference problems in the
> abstract, but rather move that sentence into the Introduction. It is a pretty
> technical topic, and I think the abstract should only give a concise and clear
> hint about the /basic/ purpose of the document.

I'm not sure about this: the purpose of an abstract is to summarize what the document does. In this case, it actually does define the inference problems, so I do not see why this shouldn't be mentioned in the abstract.

> * §1, 1st par: There is a reference to the Document Overview. I think this is
> redundant, since the Abstract already mentions it (in the part common to all
> documents).

Agreed -- I've thrown it out.

> * §1, 2nd par: "Since OWL 2 is an extension of OWL DL, ..." Didn't we want to
> turn away from this statement?

But this actually is true: each OWL DL ontology is an OWL 2 DL ontology, and it is therefore an OWL 2 ontology. Hence, I don't really see a problem with this statement.

> * §1, 3rd par: "The semantics is defined for an OWL 2 axioms and ontologies,
> [...]". Something is grammatically wrong with this sentence.

This was already addressed by Thomas's review.

> * §1, 4th par: "OWL 2 allows for annotations of ontologies, [...], and other
> annotations." The last part of the sentence sounds confusing (to me). I guess
> you mean "annotations of annotations". If yes, then why not simply say so?

I've rephrased this sentence as part of my answer to Thomas's review. Please let me know should you not be happy with this.

> * §2.1, before the list concerning datatype maps: The sentence ends with "with
> the following components.", i.e. with a ".". In other places, e.g. the
> following list for "vocabularies", there is a ":" instead. Consider being
> coherent.

I've changed "." into ":".

> * §2.1, list of items (and other places): I always wondered why the pairs,
> such as "< F v >" and "< LV DT >" do not contain a comma, while other pairs
> later in the document do have a comma? Consider being coherent.

Fair enough -- I've added a comma everywhere. I've also updated the notation in the Syntax document.

> * §2.1, datatype maps, last item: I find the name "facet value" somewhat
> confusing, since (a) it is actually a set and (b) I rather would expect the
> "v" in a pair "< F v >" to be called the "facet value". I wonder if there is a
> better name for this, but have to admit that I do not really have an
> alternative. If this is going to change, than other documents might need to be
> changed, either.

Fair enough. I've actually avoided giving this thing a name and have just said that the interpretation function assigns to < F v > a set (< F v >)^FS. I've also changed the Syntax document accordingly. (There, only the heading of Table 4 needed to change.)

> * §2.1, vocabularies, 3rd item: Typo: "owl:bttomDataProperty" (missing "o").


> * §2.1, vocabularies, 4th item: Are you sure that V_I contains anonymous
> individuals, which are existential variables?

Yes. This is not a problem, however: we require the anonymous individuals in Ax to be standardized apart and we have appropriate restrictions in the definition of models. Introducing yet another syntactic construct would just complicate matters.

> * §2.2.2: "An n-ary data range DR is interpreted as an n-ary relation (DR)^DT
> over Δ_D." Please be more explicit what this means, just as you are more
> explicit earlier in this paragraph on what "unary relation over DELTA_D"
> means, namely "(DT)^DT subset DELTA_D". You probably mean "(DR)^DT subset
> (DELTA_D)^n"?

Thomas had a comment about this, and I've rephrased the paragraph in response to him. Please let me know should you find the new formulation insufficient.

> * §2.5, "decidability": "Each class expression of type [...] can contain only
> object property expressions that are simple [...]". I wonder why there is such
> a detailed statement here, and why there isn't just a simple pointer to the
> Structural Spec? Isn't this duplication of information?

I'm afraid this is really needed. Note that we define simple properties w.r.t. a single ontology. Here, however, a property needs to be simple across *two* ontologies. That is, you could have a situation where an object property OP is simple in O and O' alone, but this is not the case if you look at O and O' together. For ontology entailment to be decidable, however, you need the latter condition to hold; for example, you should not use in O' a property in a number restriction if that property is transitive in O. Thus, this is something that cannot be checked by looking at a single ontology (which is what we do in the Syntax document); rather, it is a "local" property of ontology entailment.

> * §3: I'm unclear whether this section is intended to be normative or not? I
> would rather say that it is informative.

Indeed. I've changed the heading to indicate this.

> * References: The [OWL 2 Profiles] reference doesn't terminate with a ".".


> Cheers,
> Michael
> --
> Dipl.-Inform. Michael Schneider
> Research Scientist, Dept. Information Process Engineering (IPE)
> Tel  : +49-721-9654-726
> Fax  : +49-721-9654-727
> Email: michael.schneider@fzi.de
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Received on Monday, 6 April 2009 13:14:18 GMT

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