W3C

Protocol for Web Description Resources (POWDER): Formal Semantics

W3C Working Draft — 15 August 2008

This version
http://www.w3.org/TR/2008/WD-powder-formal-20080815/
Latest version
http://www.w3.org/TR/powder-formal/
Previous version
http://www.w3.org/TR/2008/WD-powder-formal-20080709/
Editors:
Stasinos Konstantopoulos, Institute of Informatics & Telecommunications (IIT), NCSR
Phil Archer, Family Online Safety Institute

Abstract

This document underpins the Protocol for Web Description Resources (POWDER). It describes how the relatively simple operational format of a POWDER document can be transformed through two stages: first into a more tightly constrained XML format (POWDER-BASE), and then into an RDF/OWL encoding (POWDER-S) that may be processed by Semantic Web tools. Such processing is only possible, however, if tools implement the semantic extension defined within this document. The formal semantics of POWDER are best understood after the reader is acquainted with the Description Resources [DR] and Grouping of Resources [GROUP] documents.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This is the Last Call Working Draft of this document, the Last Call period being synchronized with two other documents in the set: Description Resources and Grouping of Resources. These three document are expected to be advanced to Recommendation Status. The POWDER Working Group welcomes comments on these through to 14 September 2008. Comments are equally welcome on the other documents that are also available as working drafts, in particular, the Primer and Test Suite. Changes to this document since the previous version are recorded in the Change Log.

This document and the Description Resources [DR] document both show how POWDER can carry arbitrary RDF in the attribution and descriptorset elements. As the text and examples in Section 3.2.1 show, this is potentially problematic. POWDER works well when it transports RDF properties that have literals or RDF resources as objects (see Example 3-9), but the semantics are less clear when more complex graphs are included. We therefore flag this — that is, the support for arbitrary RDF in POWDER — as a Feature at Risk. Removing this feature would have the following effects:

First, descriptions would be limited to the two types below:

<descriptorset>
  <ex:color>red</ex:color>
  <ex:finish rdf:resource="http://example.org/vocab#shiny" />
</descriptorset>

Secondly, it would not be possible to include details of the entity that created the POWDER document within the document itself. Thus DR authors would be required to publish a separate RDF description of themselves (using FOAF or DC Terms).

Against these limitations, the benefit of removing this feature is substantial: it would be possible to build specialized software that would process POWDER just as XML, without having to process RDF. For instance: Example 2-2 can be transformed into POWDER-S and processed in a semantic environment, but it could equally be processed purely as XML if the unresolved URIrefs gave sufficient information for the application's needs.

It is worth noting that POWDER-S would be unaffected by the removal of this feature.

The Working Group is anxious to receive feedback on this trade-off between flexibility and ease of implementation.

Please send comments about this document to public-powderwg@w3.org (with public archive).

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

1 Introduction
1.1 Namespaces, Terminology and Conventions Used in This Document
2 Attribution Element Semantics
2.1 POWDER Attribution Semantics
2.2 POWDER-BASE Attribution Semantics
3 Description Resource Semantics
3.1 Multiple Description Resources in a Single POWDER Document
3.2 Descriptor Set Semantics
3.2.1 Descriptor Sets expressed as RDF Properties and Values
3.2.2 Asserting the rdf:type Relationship
3.2.3 Referring to External Descriptor Sets
3.2.4 Further Descriptors
3.3 Tag Set Semantics
4 IRI Set Semantics
4.1 White Space and List Pre-Processing
4.2 POWDER and POWDER-BASE IRI Set Semantics
4.3 POWDER-S IRI Set Semantics
4.4 Direct Descriptions
4.5 Semantics of abouthosts and aboutregex
4.6 POWDER-BASE IRI Set Semantics in OWL 2 (Informative)
5 Acknowledgements
6 References
7 Change Log
7.1 Changes since First Public Working Draft

1 Introduction

The Protocol for Web Description Resources, POWDER, offers a simple method of associating RDF data with groups of resources. Its primary 'unit of information' is the Description Resource (DR). This comprises three elements:

To some extent, this approach is in tension with the core semantics of RDF and OWL. To resolve that tension, it is necessary to extend RDF semantics as described below. In order to minimize the required extension, while at the same time preserving the relatively simple encoding of POWDER in XML which is generally readable by humans, we define a multi-layered approach. The operational semantics, i.e. the encoding of POWDER in XML, is first transformed into a more restricted XML encoding that is less easily understood by humans and depends on matching IRIs against regular expressions to determine whether or not they are within the scope of the DR. This latter encoding is, in its own turn, transformed into the extended-RDF encoding.

The data model makes the attribution element mandatory for all POWDER documents. These may contain any number of Description Resources (DRs) that effectively inherit the attribution of the document as a whole. Descriptor sets may also be included independently of a specific DR, and these too inherit the attribution. This model persists throughout the layers of the POWDER model, which are as follows:

POWDER

The operational encoding, a dialect of XML, that transports the RDF data. It is expected that POWDER will typically be published and processed in this form.

POWDER's resource grouping methods are mostly geared towards URLs and Information Resources as defined in the Architecture of the World Wide Web [WEBARCH].

POWDER-BASE

This is a largely theoretical XML encoding of POWDER that reduces all means of grouping resources according to their IRI into a single grouping method, that of matching IRIs against arbitrary regular expressions.

POWDER-BASE is provided as a means of formally specifying the semantics of the various IRI grouping methods defined in POWDER. POWDER-BASE is generated automatically from POWDER by means of the GRDDL transform [GRDDL] that is associated with the POWDER namespace.

Elements not concerned with IRI set definition are identical in POWDER and POWDER-BASE.

POWDER-S (Semantic POWDER)

The Semantic encoding uses a fragment of RDF/OWL that has been extended in a way that facilitates the matching of the string representation of a resource's identifier against a regular expression.

OWL classes are used to represent sets of resources, grouped according to their IRI and according to their properties (descriptors). Resources are described by asserting that a class that defines a set of IRIs is a sub class of a descriptor-defined class-set. Attribution is provided by way of an RDF description of the RDF graph as a whole.

A small RDF vocabulary is needed to support POWDER-S. Although it is valid RDF/OWL, generic tools will only be able to process the semantics of POWDER-S if they implement the necessary extension defined in this document.

POWDER-S is generated from POWDER-BASE by means of the GRDDL transform [GRDDL] that is associated with the POWDER namespace. POWDER-S MAY be created directly, but this is generally inadvisable since, whilst a POWDER Processor MUST understand and process POWDER-BASE and SHOULD understand POWDER, it MAY NOT understand and process POWDER-S. The aim of POWDER-S is to make the data available to the broader Semantic Web via GRDDL, not to create an alternative encoding.

The conformance criteria for a POWDER Processor are given in the Description Resources document [DR].

The GRDDL transform from POWDER to POWDER-BASE to POWDER-S is achieved using multiple passes of a POWDER document through an XSLT [XSLT] instance. The Working Group has developed XSLT programs that perform the transforms described in this document and the other documents in the POWDER document suite, and are associated with the POWDER GRDDL namespaces. These transforms are consistent with the normative text in this document, but their syntactic specifics are not normative; in effect, a POWDER processor MAY use different transforms to produce syntactically different but semantically equivalent OWL/RDF for processing a POWDER document.

Description Resources are defined separately [DR] and a further document defines the creation of IRI sets [GROUP]. Readers should be familiar with those documents before proceeding with this one. The full set of POWDER documents also includes its Use Cases, Primer and Test Suite, together with the namespace documents [WDR, WDRS, WDRD] and GRDDL transform [PDR-GRDDL].

1.1 Namespaces, Terminology and Conventions Used in This Document.

The POWDER vocabulary namespace is http://www.w3.org/2007/05/powder# for which we use the prefix wdr The POWDER-S vocabulary namespace is http://www.w3.org/2007/05/powder-s# for which we use the prefix wdrs All prefixes used in this document, together with their associated namespaces, are shown in the table below.

Table 1: Prefixes and Namespaces used in this document
PrefixNamespace
wdrhttp://www.w3.org/2007/05/powder#
wdrshttp://www.w3.org/2007/05/powder-s#
rdfhttp://www.w3.org/1999/02/22-rdf-syntax-ns#
rdfshttp://www.w3.org/2000/01/rdf-schema#"
owlhttp://www.w3.org/2002/07/owl#
oxhttp://www.w3.org/ns/owl2-xml#
dctermshttp://purl.org/dc/terms/
foafhttp://xmlns.com/foaf/0.1/
xsdhttp://www.w3.org/2001/XMLSchema-datatypes#
xslhttp://www.w3.org/1999/XSL/Transform
exAn arbitrary prefix used to denote an 'example vocabulary'

Unqualified elements in this document are from the wdr namespace.

In this document, the words MUST, MUST NOT, SHOULD, SHOULD NOT and MAY are to be interpreted as described in RFC2119 [RFC2119].

For convenience and transparency, we have used the RDF/XML serialization for POWDER-S as we have throughout the document set. Other serializations, such as N3 [N3], are equally valid for POWDER-S. The GRDDL Transformation associated with the POWDER namespace, which uses XSLT to effect the transform, produces RDF/XML as its output.

Examples in this document show fragments of data and each is linked to an external file that mirrors the data in the text. However, in order to be valid documents, the external files include generic data not shown in the text that has been taken largely from examples 2-1 and 2-3 in the Description Resources document [DR].

2 Attribution Element Semantics

The attribution element, present in all POWDER documents, provides data about the authorship, validity period, and other issues that a user or user agent can use when deciding whether or not to confer their trust on a POWDER document.

2.1 POWDER Attribution Semantics

Most attribution elements are not involved in IRI grouping, and as such are untouched during the transformation from POWDER to POWDER-BASE. The only exception is abouthosts, which sets an outer limit on the resources described by the DRs within the document. POWDER abouthosts elements are translated into POWDER-BASE aboutregex elements, as discussed in Section 4.5 below.

2.2 POWDER-S Attribution Semantics

Since the attribution element provides data about the document itself, it is transformed from POWDER (through POWDER-BASE) into POWDER-S as an owl:Ontology property for which the subject is null (i.e. the current document). This data does not receive OWL semantics, but is only meaningful to POWDER tools when deciding whether a POWDER document as a whole should be taken into account or discarded.

Child elements of the attribution element are RDF/XML statements about the document. With the explicit exception of issued and abouthosts, these are reproduced unchanged in the POWDER-S instance. This general rule applies to the POWDER elements issuedby, validfrom, validuntil, certifiedby and supportedby where the only transformation necessary is to make their (transformed) namespace explicit. In each case the same string is used as their element name and wdrs property name. The wdrs:issuedby property, however, is noteworthy as it is required for all POWDER documents and has particular semantics discussed below the following example.

As a shortcut designed to avoid the automatic need to declare the rdf namespace in all POWDER documents, where child elements of the attribution element of a POWDER document contain an external reference, denoted by the src attribute, this is transformed into rdf:resource.

Arbitrary RDF is copied verbatim. This is a Feature at Risk, see Status Section

Example 2-1 shows the generic semantics of the attribution element.

Example 2-1: The Generic Semantics of the attribution Element

POWDER [XML]


1  <attribution>
2    <ex:property1>value</ex:property1>
3    <ex:property2 rdf:resource="http://example.org/foo.rdf#frag" />
4    <ex:property3 src="http://example.com/bar.rdf#frag" />
5    <ex:property4>
6      <ex:Class>
7        <ex:property5>value_5</ex:property5>
8      </ex:Class>
9    </ex:property4>
10 </attribution>

POWDER-S [RDF/XML]

1  <rdf:Description rdf:about="">
2    <ex:property1>value</ex:property1>
3    <ex:property2 rdf:resource="http://example.org/foo.rdf#frag" />
4    <ex:property3 rdf:resource="http://example.org/foo.rdf#frag" />
5    <ex:property4>
6      <ex:Class>
7        <ex:property5>value_5</ex:property5>
8      </ex:Class>
9    </ex:property4>
10 </rdf:Description>

As noted above, some elements within the POWDER namespace that are treated differently or have noteworthy semantics:

issuedby
The issuedby element takes its semantics from both the Dublin Core [DC] and FOAF [FOAF] namespaces. wdrs:issuedby is defined as a sub property of both dcterms:creator and foaf:maker. These have a range of dcterms:Agent and foaf:Agent respectively, so that in this example triple:

<> wdrs:issuedby <http://example.org/company.rdf#me>

http://example.org/company.rdf#me SHOULD identify an instance of one of those classes (or a subclass thereof).

Alternatively, the Agent class (from either vocabulary) can be included in the POWDER document directly. Example 2-2 in the Description Resources document shows this.

issued
The semantics of the issued element are also defined in the dcterms namespace such that

<issued>2008-03-25T00:00:00</issued>

should be understood to mean

<dcterms:issued>2008-03-25T00:00:00</dcterms:issued>

abouthosts
The abouthosts element is discussed in Section 4.5 below.

The transformation of validfrom, validuntil, certifiedby and supportedby from POWDER to POWDER-S is straightforward in that, for example:

<validfrom>2008-01-01T00:00:00</validfrom>
<validuntil>2008-12-31T00:00:00</validuntil>

is transformed into

<wdrs:validfrom>2008-01-01T00:00:00</wdrs:validfrom>
<wdrs:validuntil>2008-12-31T00:00:00</wdrs:validuntil>

The data types for these elements and their wdrs properties are:

Example 2-2 below shows all the POWDER-specific attribution elements. Note that there is no abouthosts element in the example, as it will be discussed in Section 4.3 below.

Example 2-2: The Semantics of the POWDER-Specific Child Elements of the attribution Element, except for abouthosts.

POWDER [XML]

1  <attribution>
2    <issuedby src="http://example.org/company.rdf#me" />
3    <issued>2007-12-23T00:00:00</issued>
4    <validfrom>2008-01-01T00:00:00</validfrom>
5    <validuntil>2008-12-31T23:59:59</validuntil>
6    <certifiedby src="http://authority.example/powder.xml" />
7    <supportedby src="http://service.example.com?id=abc" />
8  </attribution>

POWDER-S [RDF/XML]

1  <rdf:Description rdf:about="">
2    <wdrs:issuedby rdf:resource="http://example.org/company.rdf#me" />
3    <dcterms:issued>2008-12-23T00:00:00</dcterms:issued>
4    <wdrs:validfrom>2008-01-01T00:00:00</wdrs:validfrom>
5    <wdrs:validuntil>2008-12-31T23:59:59</wdrs:validuntil>
6    <wdrs:certifiedby rdf:resource="http://authority.example/powder.xml" />
7    <wdrs:supportedby rdf:resource="http://service.example.com?id=abc" />
8  </rdf:Description>

3 Description Resource Semantics

Description Resources use vocabularies defined in RDF and/or plain string literals (tags) to describe resources de-referenced from instances of the IRI set. Since descriptor set elements are not involved in the specification of the IRI set itself, they are transferred verbatim from POWDER to POWDER-BASE. Example 3-1 below shows a generic example of a DR in which the IRI set has been elided for clarity (the semantics of the IRI set is discussed in Section 4 below).

Example 3-1: A Generic Example of A Descriptor Set and Tag Set within a DR [XML]

  

1  <dr>
2    <iriset>…</iriset>

3    <descriptorset>
4      <ex:finish rdf:resource="http://example.org/vocab#shiny"/>
5      <ex:shape>square</ex:shape>
6    </descriptorset>

7    <tagset>
8      <tag>red</tag>
9      <tag>light</tag>
10   </tagset>
11 </dr>

The ex:finish element specifies that the ex:finish relation holds between all resources specified by iriset and the http://example.org/vocab#shiny resource.

The content of ex:shape is interpreted as a string literal. The ex:shape element specifies that all resources in iriset has the value "square" for the ex:shape dataproperty.

tag is a string property defined by POWDER. Its content is a single string literal, possibly including spaces.

The overall description of the resources in iriset is the union of the descriptions in the descriptorset and the tagset. In our example these are:

an ex:finish relation to http://example.org/vocab#shiny
AND
an ex:shape of "square"
AND
the tags "red" and "light"

We formally interpret the above as follows: there is an OWL class containing all resources that share all of these properties, and there is an OWL class of all resources denoted by iriset, and the latter is a subset of the former. In POWDER-S we say:

Example 3-2 The POWDER-S Encoding of Example 3-1 [RDF/XML]

1  <owl:Class rdf:nodeID="iriset_1">
2    all resources specified by <iriset>...</iriset>
3  </owl:Class>

4  <owl:Class rdf:nodeID="descriptorset_1">
5    <owl:intersectionOf rdf:parseType="Collection">
6      <owl:Restriction>
7        <owl:onProperty rdf:resource="http://example.org/vocab#finish"/>
8        <owl:hasValue rdf:resource="http://example.org/vocab#shiny"/>
9      </owl:Restriction>
10     <owl:Restriction>
11       <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
12       <owl:hasValue>square</owl:hasValue>
13     </owl:Restriction>
14   </owl:intersectionOf>
15 </owl:Class>

16 <owl:Class rdf:nodeID="tagset_1">
17   <owl:intersectionOf rdf:parseType="Collection">
18     <owl:Restriction>
19       <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#tag"/>
20       <owl:hasValue>red</owl:hasValue>
21     </owl:Restriction>
22     <owl:Restriction>
23       <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#tag"/>
24       <owl:hasValue>light</owl:hasValue>
25     </owl:Restriction>
26   </owl:intersectionOf>
27 </owl:Class>

28 <owl:Class rdf:nodeID="iriset_1">
29   <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
30   <rdfs:subClassOf rdf:nodeID="tagset_1"/>
31 </owl:Class>

It is possible to have more than one iriset element, in which case a resource receives all of the descriptions by belonging to any one of the corresponding IRI sets. For example:

Example 3-3: A DR with Multiple IRI Sets in its Scope [XML]

1  <dr>
2   <iriset>.1.</iriset>
3   <iriset>.2.</iriset>

4   <descriptorset xml:id="silver">
5     <ex:finish rdf:resource="http://example.org/vocab#shiny"/>
6   </descriptorset>
7  </dr>

receives the following semantics:

Example 3-4: The POWDER-S encoding of Example 3-3 [RDF/XML]

1  <owl:Class rdf:nodeID="iriset_1">
2     all resources specified by <iriset>.1.</iriset>
3  </owl:Class>
  
4  <owl:Class rdf:nodeID="iriset_2">
5     all resources specified by <iriset>.2.</iriset>
6  </owl:Class>

7  <owl:Class rdf:ID="silver">
8    <owl:intersectionOf rdf:parseType="Collection">
9      <owl:Restriction>
10       <owl:onProperty rdf:resource="http://example.org/vocab#finish"/>
11       <owl:hasValue rdf:resource="http://example.org/vocab#shiny"/>
12     </owl:Restriction>
13   </owl:intersectionOf>
14 </owl:Class>

15 <owl:Class rdf:nodeID="iriset_1">
16   <rdfs:subClassOf rdf:resource="#silver"/>
17 </owl:Class>

18 <owl:Class rdf:nodeID="iriset_2">
19   <rdfs:subClassOf rdf:resource="#silver"/>
20 </owl:Class>

Examples 3-3 and 3-4 also show that if a descriptorset element has an ID of its own, this is used in the POWDER-S document. This is reflected in the way that the sub class relationship (lines 15 - 20) is asserted. Where no xml:id attribute is set in the original POWDER document, the transform assigns rdf:nodeID identifiers for the blank nodes. As a result, rdf:nodeID attributes are used within the POWDER-S document in the sub class assertion (see lines 28 -31 in Example 3-2). However, where the original POWDER document includes an xml:id attribute, as in line 4 of Example 3-3, the sub class assertions in lines 16 and 19 of Example 3-4 is correctly asserted using the rdf:resource attribute.

A POWDER processor is free to choose any traversal policy for treating multiple iriset elements in a DR: first match wins, last match wins, shortest iriset first, and so on, as long as all iriset elements are tried before deciding that DR does not apply to a candidate resource (candidate resource is defined in the Grouping of Resources document [GROUP]). However, DR authors may use the order of the iriset elements to suggest an efficient scope evaluation strategy, by putting the iriset with the widest coverage first, so that a processor that chooses to follow the iriset elements in document order is more likely to terminate the evaluation after fewer checks.

3.1 Multiple Description Resources in a Single POWDER Document

A POWDER document may have any number of DRs, all of which are simultaneously asserted and ordering is not important. So, for example:

Example 3-5: A POWDER Document Containing Multiple DRs [XML]

1  <powder>

2    <dr>
3      <iriset>.1.<code>iriset</code>
4      <descriptorset>
5        <ex:shape>square</ex:shape>
6      </descriptorset>
7    </dr>

8    <dr>
9      <iriset>.2.<code>iriset</code>
10     <descriptorset>
11       <ex:finish rdf:resource="http://example.org/vocab#shiny"/>
12     </descriptorset>
13   </dr>

14 </powder>

receives the following semantics:

Example 3-6: The POWDER-S Encoding of Example 3-5 [RDF/XML]

1  <owl:Class rdf:nodeID="iriset_1">
2    all resources specified by <iriset>.1.</iriset>
3  </owl:Class>

4  <owl:Class rdf:nodeID="descriptorset_1">
5    <owl:intersectionOf rdf:parseType="Collection">
6     <owl:Restriction>
7        <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
8        <owl:hasValue>square</owl:hasValue>
9      </owl:Restriction>
10   </owl:intersectionOf>
11 </owl:Class>

12 <owl:Class rdf:nodeID="iriset_1">
13   <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
14 </owl:Class>

15 <owl:Class rdf:nodeID="iriset_2">
16   all resources specified by <iriset>.2.</iriset>
17 </owl:Class>

18 <owl:Class rdf:nodeID="descriptorset_2">
19   <owl:intersectionOf rdf:parseType="Collection">
20     <owl:Restriction>
21       <owl:onProperty rdf:resource="http://example.org/vocab#finish"/>
22       <owl:hasValue rdf:resource="http://example.org/vocab#shiny"/>
23     </owl:Restriction>
24    </owl:intersectionOf>
25 </owl:Class>

26 <owl:Class rdf:nodeID="iriset_2">
27   <rdfs:subClassOf rdf:nodeID="descriptorset_2"/>
28 </owl:Class>

The owl:intersectionOf of a singleton collection in both descriptor sets, although redundant, is a result of the GRDDL transformation. As noted above, syntactically different but semantically equivalent representations are equally valid.

Note that iriset_1 and iriset_2 are not necessarily disjoint — some resources may be both shiny AND square.

A POWDER document may have an ol element which is an ordered list of DRs. Such a list receives first-match semantics, that is, when seeking the description of a candidate IRI, processors extract the descriptor set from the first DR in the ordered list in which it is in scope. ol elements allow the easy expression of exceptions to more general rules. So, for example:

Example 3-7: An Ordered List of DRs [XML]

1  <ol>

2    <dr>
3     <iriset>.1.</iriset>
4     <descriptorset>
5       <ex:shape>square</ex:shape>
6     </descriptorset>
7    </dr>

8    <dr>
9     <iriset>.2.</iriset>
10    <descriptorset>
11      <ex:shape>round</ex:shape>
12    </descriptorset>
13   </dr>

14   <dr>
15    <iriset>.3.</iriset>
16    <descriptorset>
17      <ex:shape>triangle</ex:shape>
18    </descriptorset>
19   </dr>

20 </ol>

receives the following semantics, where belonging to description_1 automatically precludes belonging to description_2 and description_3; and belonging to description_2 automatically precludes belonging to description_3:

Example 3-8: The POWDER-S Encoding of Example 3-7 [RDF/XML]

1  <owl:Class rdf:nodeID="iriset_1">
2    all resources specified by <iriset>.1.</iriset>
3  </owl:Class>

4  <owl:Class rdf:nodeID="descriptorset_1">
5    <owl:intersectionOf rdf:parseType="Collection">
6      <owl:Restriction>
7        <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
8         <owl:hasValue>square</owl:hasValue>
9      </owl:Restriction>
10   </owl:intersectionOf>
11 </owl:Class>

12 <owl:Class rdf:nodeID="iriset_1">
13   <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
14 </owl:Class>

15 <owl:Class rdf:nodeID="iriset_2">
16   all resources specified by <iriset>.2.</iriset>
17 </owl:Class>

18 <owl:Class rdf:nodeID="descriptorset_2">
19   <owl:intersectionOf rdf:parseType="Collection">
20     <owl:Restriction>
21       <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
22       <owl:hasValue>round</owl:hasValue>
23     </owl:Restriction>
24   </owl:intersectionOf>
25 </owl:Class>

26 <owl:Class>
27   <owl:intersectionOf rdf:parseType="Collection">
28     <owl:Class rdf:nodeID="iriset_2"/>
29     <owl:Class>
30       <owl:complementOf rdf:parseType="Collection">
31         <owl:Class rdf:nodeID="iriset_1"/>
32       </owl:complementOf>
33     </owl:Class>
34   </owl:intersectionOf>
35   <rdfs:subClassOf rdf:nodeID="descriptorset_2"/>
36 </owl:Class>

37 <owl:Class rdf:nodeID="iriset_3">
38   all resources specified by <iriset>.3.</iriset>
39 </owl:Class>

40 <owl:Class rdf:nodeID="descriptorset_3">
41   <owl:intersectionOf rdf:parseType="Collection">
42     <owl:Restriction>
43       <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
44       <owl:hasValue>triangular</owl:hasValue>
45     </owl:Restriction>
46   </owl:intersectionOf>
47 </owl:Class>

48 <owl:Class>
49   <owl:intersectionOf rdf:parseType="Collection">
50     <owl:Class rdf:nodeID="iriset_3"/>
51     <owl:Class>
52       <owl:complementOf rdf:parseType="Collection">
53         <owl:Class>
54           <owl:unionOf rdf:parseType="Collection">
55             <owl:Class rdf:nodeID="iriset_2"/>
56             <owl:Class rdf:nodeID="iriset_1"/>
57           </owl:unionOf>
58         </owl:Class>
59       </owl:complementOf>
60     </owl:Class>
61   </owl:intersectionOf>
62   <rdfs:subClassOf rdf:nodeID="descriptorset_3"/>
63 </owl:Class>

3.2 Descriptor Set Semantics

3.2.1 Descriptor Sets expressed as RDF Properties and Values

In the simplest case, a descriptor set contains RDF properties that have literals or RDF Resources as their values as shown in Example 3-9 below. Note that that http://example.org/vocab#shiny MUST NOT identify an RDFS or OWL class.

Example 3-9: A Descriptor Set Containing an RDF Property with a Literal Value and One with an RDF Resource as its Value.

POWDER [XML]

1  <descriptorset>
2    <ex:shape>square</ex:shape>
3    <ex:finish rdf:resource="http://example.org/vocab#shiny"/>
4  </descriptorset>

POWDER-S [RDF/XML]

1  <owl:Class rdf:nodeID="descriptorset_1">
2    <owl:intersectionOf rdf:parseType="Collection">
3      <owl:Restriction>
4        <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
5        <owl:hasValue>square</owl:hasValue>
6      </owl:Restriction>
7      <owl:Restriction>
8        <owl:onProperty rdf:resource="http://example.org/vocab#finish"/>
9        <owl:hasValue rdf:resource="http://example.org/vocab#shiny"/>
10     </owl:Restriction>
11   </owl:intersectionOf>
12 </owl:Class>

These simple cases will normally be what is required for Description Resources.

The remainder of section 3.2.1 is a Feature at Risk. See the Status section.

More complex RDF descriptions are possible but are likely to cause problems and should not normally be used. The following example highlights the semantic problem of using blank nodes within a POWDER descriptor set.

Example 3-10: A Descriptor Set Containing a Blank Node

POWDER [XML]

1  <descriptorset>
2    <ex:material>
3      <ex:Wood>
4        <ex:finish rdf:resource="http://example.org/vocab#shiny"/>
5        <ex:madeof>cedar</ex:madeof>
6      </ex:Wood>
7     </ex:material>
8  </descriptorset>

POWDER-S [RDF/XML]

1   <owl:Class rdf:nodeID="descriptorset_1">
2    <owl:intersectionOf rdf:parseType="Collection">
3      <owl:Restriction>
4         <owl:onProperty rdf:resource="http://example.org/vocab#material"/>
5         <owl:hasValue>
6           <ex:Wood>
7             <ex:finish rdf:resource="http://example.org/vocab#shiny"/>
8             <ex:madeof>cedar</ex:madeof>
9           </ex:Wood>
10        </owl:hasValue>
11      </owl:Restriction>
12    </owl:intersectionOf>
13  </owl:Class>

The semantics of the blank node (ex:Wood) are that there is at least one identifiable resource that is of the type ex:Wood that has the color brown, and that this is the value filler for the properties ex:finish and ex:madeof. Although this may be semantically valid, it is very unlikely that such a construct will be appropriate for use in POWDER. This is because it is possible for a descriptor set to be defined independently of any Description Resource and therefore not associated with any resources directly. Equally, a descriptor set may be part of a DR for which there are no resources that are within its scope at the time of its publication. It is the nature of POWDER that descriptions may well be published before or after the resources that any given DR describes. This lack of direct connection means that the use of blank nodes is strongly discouraged.

Identifying nodes within a descriptor set does not cause any of the semantic problems discussed for blank nodes; however, it does effectively create or duplicate vocabulary terms every time a DR is processed. Consider the following example, which is a variation on the previous one.

Example 3-11: A Descriptor Set Containing an Identified Node

POWDER [XML]

1  <descriptorset>
2    <ex:material>
3      <ex:Wood rdf:about="http://my.example.org/myVocab#PolishedCedar">
4        <ex:finish rdf:resource="http://example.org/vocab#shiny"/>
5        <ex:madeof>cedar</ex:madeof>
6      </ex:Wood>
7    </ex:material>
8  </descriptorset>

POWDER-S [RDF/XML]

1   <owl:Class rdf:about="http://my.example.org/myVocab#PolishedCedar">
2     <owl:intersectionOf rdf:parseType="Collection">
3       <owl:Restriction>
4         <owl:onProperty rdf:resource="http://example.org/vocab#finish"/>
5         <owl:hasValue rdf:resource="http://example.org/vocab#shiny"/>
6       </owl:Restriction>
7       <owl:Restriction>
8         <owl:onProperty rdf:resource="http://example.org/vocab#madeof/">
9         <owl:hasValue>cedar</owl:hasValue>
10      </owl:Restriction>
11    </owl:intersectionOf>
12  </owl:Class>

13  <owl:Class rdf:nodeID="descriptorset_1">
14    <owl:intersectionOf rdf:parseType="Collection">
15      <owl:Restriction>
16        <owl:onProperty rdf:resource="http://example.org/vocab#material"/>
17        <owl:hasValue rdf:resource="http://my.example.org/myVocab#PolishedCedar"/>
18      </owl:Restriction>
19    </owl:intersectionOf>
20  </owl:Class>

By adding rdf:about="http://my.example.org/myVocab#PolishedCedar" to line 3 of the descriptor set, a new class of PolishedCedar has been created in the http://my.example.org/myVocab# vocabulary. It is this that is described as having a shiny finish and made of cedar. Creating new vocabulary terms in this way should only be done where the DR author has no alternative since the triples concerning the new class will be created repeatedly as the document is processed. It is always better to create a separate new vocabulary and use that or, better still, to re-use an existing one. Nevertheless, POWDER does support the usage shown in Example 3-11.

As a further point, notice that the IRI used to identify the Polished Cedar class is an absolute one. A relative URI, or a value supplied to the RDF/XML attribute rdf:ID in line 3, would be relative to the candidate IRI (as defined in the Grouping of Resources document [GROUP]) — i.e. each and every IRI that is described by the DR of which the descriptor set is a part. This is almost certainly not what is intended.

To summarize the discussion of examples 3-9, 3-10 and 3-11: the semantics of POWDER descriptor sets work well with properties that take literals or RDF Resources as values. More complex RDF constructs are likely to lead to unintended or unintelligible results.

3.2.2 Asserting the rdf:type Relationship

Asserting the rdf:type property, i.e. that all elements within an IRI set are instances of a particular OWL or RDFS Class, is achieved most easily using the typeof element which takes the URI of the class as the value of its src attribute as shown in Example 3-12 below. The POWDER-S translation of the descriptorset element intersects typeof classes with the property restrictions (if any) in the descriptorset.

Example 3-12: A Descriptor Set Asserting that IRIs within its Scope are Instances of a Class.

POWDER [XML]

1  <descriptorset>
2    <typeof src="http://example.org/vocab#Conformance_Class" />
3    <ex:shape>square</ex:shape>
4  </descriptorset>

POWDER-S [RDF/XML]

1  <owl:Class rdf:nodeID="descriptorset_1">
2    <owl:intersectionOf rdf:parseType="Collection">
3      <owl:Class rdf:about="http://example.org/vocab#Conformance_Class" />
4      <owl:Restriction>
5        <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
6        <owl:hasValue>square</owl:hasValue>
7      </owl:Restriction>
8    </owl:intersectionOf>
9  </owl:Class>

 

This is particularly useful in the context of the POWDER use cases [USECASES] when claiming that resources on a Web site conform to a published set of criteria. In such situations, multiple criteria can be grouped together by defining the class of resources that satisfy all the criteria as the intersection of a number of property restrictions; series of increasingly stricter conformance levels can be defined as a subsumption hierarchy of such classes.

If used directly, the rdf:type property will be treated in the same way as the typeof element in the POWDER to POWDER-S transform.

3.2.3 Referring to External Descriptor Sets

A descriptor set may defer to a second descriptor set in another POWDER document using the src attribute. However, this cannot express POWDER semantics since, at the time of processing, the remote document may be unknown, unavailable or not a valid POWDER document. Therefore the formal semantics are limited as shown below.

Example 3-13: A Descriptor Set Referring to one in an External Document

POWDER [XML]

<descriptorset src="http://remote.example.org/powder2.xml#d1" />

POWDER-S [RDF/XML]

<owl:Class rdf:nodeID="descriptorset_1">
  <rdfs:seeAlso rdf:resource="http://remote.example.org/powder2.xml#d1" />
</owl:Class>

Informally, a processor MAY apply full semantics to a descriptor set referred to in this way if it is known to be part of a valid POWDER document, but only once it too has been transformed into POWDER-S.

3.2.4 Further Descriptors

There are two POWDER elements that can be included as child elements of descriptorset that are mapped to property restrictions in POWDER-S. In both cases the same string is used as the element name in POWDER and vocabulary term in POWDER-S:

sha1sum
A SHA-1 sum of the described resource
certified
An element of type xsd:boolean used when a DR certifies another resource.

The usage of both sha1sum and certified is shown in section 5.2 of the Description Resources document [DR].

We define further elements that can be included as child elements of descriptorset that, when transformed into POWDER-S, become annotation properties of the descriptive OWL class (not property restrictions).

displaytext
is transformed to dcterms:description. The text supplied as the value of this element may be displayed in user agents.
displayicon
has a src attribute, the value of which is a URI (or IRI) u which, in POWDER-S, becomes foaf:depiction rdf:resource="u". The referred-to image may be displayed in user agents.
seealso, label, comment
Each of these elements is transformed into an annotation of the OWL class using the similar term from the rdfs vocabulary with which it shares its name. For the avoidance of doubt:

<seealso src="http://www.example.com/page.html" />
<label>An example to us all</label>
<comment>Comments make code easier to read</comment>

are transformed into:

<rdfs:seeAlso rdf:resource="http://www.example.com/page.html" />
<rdfs:label>An example to us all</rdfs:label>
<rdfs:comment>Comments make code easier to read</rdfs:comment>

Usage of these elements is exemplified in the following section. As with rdf:type, they are provided as shortcuts within POWDER — the direct use of rdfs:seeAlso, rdfs:comment and rdfs:label will be rendered in exactly the same way (as annotations and not property restrictions) by the transform.

It is unlikely that other terms from the rdfs vocabulary can be used in a meaningful way in a POWDER context.

3.3 Tag Set Semantics

The semantics of the (free text) tags are similar to those for properties with literal values. Each tag given in a tagset element in a POWDER document is a value for the RDF datatype property wdrs:tag as shown below. Note also the use of the seealso (which puts the tags in context), label and comment elements described in the previous section.

Example 3-14: A Tag Set.

POWDER [XML]

1  <tagset>
2    <label>Tags for the London landmark</label>
3    <tag>London</tag>
4    <tag>Swiss Re</tag>
5    <tag>gherkin</tag>
6    <seealso src="http://encyclopaedia.example.com/gherkin.html" />
7    <seealso src="http://photo.example.com/gherkin.jpg" />
8    <comment>Tags are linked to specific resources that contextualize them</comment>
9  </tagset>

POWDER-S [RDF/XML]

1  <owl:Class rdf:nodeID="tagset_1">
2    <rdfs:label>Tags for the London landmark</rdfs:label>
3    <owl:intersectionOf rdf:parseType="Collection">
4      <owl:Restriction>
5        <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder#tag" />
6        <owl:hasValue>London</owl:hasValue>
7      </owl:Restriction>
8      <owl:Restriction>
9        <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder#tag" />
10       <owl:hasValue>Swiss Re</owl:hasValue>
11     </owl:Restriction>
12     <owl:Restriction>
13       <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder#tag" />
14       <owl:hasValue>gherkin</owl:hasValue>
15     </owl:Restriction>
16   </owl:intersectionOf>
17   <rdfs:seeAlso rdf:resource="http://encyclopaedia.example.com/gherkin.html" />
18   <rdfs:seeAlso rdf:resource="http://photo.example.com/gherkin.jpg" />
19   <rdfs:comment>Tags are linked to specific resources that contextualize them</rdfs:comment>
20 </owl:Class>

4 IRI Set Semantics

The previous sections have shown that the semantics of several elements of a POWDER document can be obtained by applying the GRDDL transform associated with the namespace to generate native RDF/OWL as POWDER-S. This is not so for the IRI set element which, although transformed into valid RDF/OWL syntax, does not express the full semantics.

The IRI constraints defined in the POWDER Grouping of Resources document [GROUP] are given regular-expression semantics by the first part of the GRDDL transform from POWDER to POWDER-BASE. Regular-expression IRI groups are, in their turn, given semantics using datarange restrictions by the POWDER-BASE to POWDER-S transformation. It is noteworthy that the value space of POWDER's IRI constraints is, for the most part, a white space separated list of alternative values. This makes POWDER in its XML form relatively simple, but the implications for the semantics are substantial.

4.1 White Space and List Pre-Processing

Many elements of a POWDER IRI set definition have white space separated lists of strings as their value. White space is any of U+0009, U+000A, U+000D and U+0020. A space-separated list is a string in which the items are separated by one or more space characters (in any order). The string may also be prefixed or suffixed with zero or more of those characters. The GRDDL transform associated with the POWDER namespace converts these into components of a regular expression for use in POWDER-BASE and POWDER-S by following the steps set out below:

The resulting string is used in a template regular expression to give the element and list's desired semantics. For example

<includehosts>example.com example.org </includehosts>

becomes

<owl:Restriction>
  <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
  <owl:hasValue  rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com|example\.org)(\:([0-9]+))?\/</owl:hasValue>
</owl:Restriction>

4.2 POWDER and POWDER-BASE IRI Set Semantics

POWDER's use cases involve information resources available on the Web, identified by IRIs containing host names, directory paths, IP addresses, port numbers, and so on. To make it as easy as possible to create IRI sets we define a series of IRI constraints in the Grouping of Resources document [GROUP]. These all receive semantics through being mapped to includeregex and excluderegex elements in POWDER-BASE.

Re-visiting the example given in the previous section, the POWDER element

<iriset>
  <includehosts>example.com example.org</includehosts>
</iriset>

is expressed in POWDER-BASE as:

<iriset>
  <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com|example\.org)(:([0-9]+))?\/<includeregex>
</iriset>

IRIs are always interpreted as strings, even if they include numerical parts such as ports and IP numbers as shown in the following example:

Example 4-1: The POWDER and POWDER-BASE Encoding of an Example IRI Set

POWDER: [XML]

<iriset>
  <includehosts>example.com example.org</includehosts>
  <includeports>80 8080 8081 8082</includeports>
</iriset>

POWDER-BASE: [XML]

<iriset>
  <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com|example\.org)(\:([0-9]+))?\/</includeregex>
  <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)*[^\:\/\?\#\@]+\:(80|8080|8081|8082)\/</includeregex>
</iriset>

This approach is applied to several of the POWDER IRI set elements. The following table shows these and their associated template regular expressions. In each case, var means the value of the POWDER element after processing as defined in Section 4.1.

Table 3. Template regular expressions for IRI constraints that take a white space separated list of values.
POWDER IRI Constraint
(include/exclude...
POWDER-BASE Regular Expression
(used in includeregex/excluderegex)
schemes ^var\:\/\/
hosts \:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?var(\:([0-9]+))?\/
ports \:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)*[^\:\/\?\#\@]+\:var\/
exactpaths \:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]*)(\:([0-9]+))?var($|\?|\#)
pathcontains \:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]*)(\:([0-9]+))?\/[^\?\#]*var[^\?\#]*[\?\#]?
pathstartswith \:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]*)(\:([0-9]+))?var
pathendswith \:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]*)(\:([0-9]+))?\/[^\?\#]*var($|\?|\#)
resources ^var$

Note that the Grouping of Resources document [GROUP] sets out a canonicalization process that must be followed. This has particular implications for the matching of ports: where the port number is constrained, default port numbers for the relevant scheme must be taken into account.

Two further pairs of IRI set constraints defined in the Grouping of Resources document undergo additional processing when transformed from POWDER to POWDER-BASE: includequerycontains and includeiripattern (and their 'exclude' counterparts). Each of these maps to multiple elements in the POWDER-BASE document.

includequerycontains and excludequerycontains take a single value, not a white space separated list of values. Furthermore, an attribute delimiter takes a single character that delimits the name/value pairs in the query string. If no such attribute is set, the ampersand (&) character is used as the default. To transform these elements from POWDER to POWDER-BASE regular expressions the following steps are carried out:

This transformation is exemplified below.

Example 4-2: The POWDER and POWDER-BASE Encoding of an Example IRI Set including an includequerycontains Constraint

POWDER: [XML]

<iriset>
  <includehosts>example.org</includehosts>
  <includequerycontains>id=123456&group=abcdefg</includequerycontains>
</iriset>

POWDER-BASE: [XML]

<iriset>
  <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.org)(\:([0-9]+))?\/</includeregex>
  <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]*)(\:([0-9]+))?\/[^\?\#]*\?([^\#]*\&)?id=123456(\&|$)</includeregex>
  <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]*)(\:([0-9]+))?\/[^\?\#]*\?([^\#]*\&)?group=abcdefg(\&|$)</includeregex>
</iriset>

includeiripattern and excludeiripattern also take a single value, not a white space separated list of values, and generate includeregex and excluderegex elements in POWDER-S as follows:

The following example shows these steps.

Example 4-3: The POWDER and POWDER-BASE Encoding of an Example IRI Set including an includeiripattern Constraint

POWDER: [XML]

<iriset>
  <includeiripattern>http://*.example.org:8080</includeiripattern>
</iriset>

POWDER-BASE: [XML]

<iriset>
  <includeregex>^http\:\/\/([^\:\/\?\#\@]+\.)?example.org:8080</includeregex>
</iriset>

Incidentally, the IRI set defined here is 'all resources on all subdomains of example.org (but not on example.org) accessed via HTTP through port 8080.'

4.3 POWDER-S IRI Set Semantics

Providing OWL/RDF semantics for iriset elements is not directly possible, since RDF does not provide any means for accessing or manipulating the string representation of an IRI. We extend RDF with a datatype property wdrs:matchesregex as shown below.

wdrs:matchesregex rdf:type owl:DatatypeProperty .
wdrs:matchesregex rdf:type owl:Property .
wdrs:matchesregex rdfs:domain rdfs:Resource .
wdrs:matchesregex rdfs:range xsd:string .

We further stipulate that for triples:

x wdrs:matchesregex reg .

<x, reg> is in IEXT(I(wdrs:matchesregex)) if and only if:

It is now possible to express includeregex and excluderegex as a owl:hasValue restriction [OWL] on this dataproperty and build up an OWL Class to represent the IRI set in the POWDER-S encoding. Furthermore, the sub class relationship between the IRI set and the descriptor set is asserted.

The following example takes a complete example POWDER document through POWDER-BASE to POWDER-S. Note that the only change from POWDER to POWDER-BASE is in the elements within the IRI set.

Example 4-4: The Full Transformation of an Example from POWDER Through POWDER-BASE to POWDER-S

POWDER [XML]

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#" 
3          xmlns:ex="http://example.org/vocab#">

4    <attribution>
5      <issuedby src="http://authority.example.org/company.rdf#me" />
6      <issued>2007-12-14T00:00:00</issued>
7    </attribution>

8    <dr>
9      <iriset>
10       <includehosts>example.com example.org</includehosts>
11       <includeports>80 8080 8081 8082</includeports>
12     </iriset>
13     <descriptorset>
14      <ex:color>red</ex:color>
15      <ex:shape>square</ex:shape>
16      <displaytext>Everything on example.org and example.com is red and square</displaytext>
17      <displayicon src="http://example.org/icon.png" />
18    </descriptorset>
19   </dr>

20 </powder>

POWDER-BASE [XML]

1  <?xml version="1.0"?>
2  <powder xmlns="http://www.w3.org/2007/05/powder#" 
3          xmlns:ex="http://example.org/vocab#">

4    <attribution>
5      <issuedby src="http://authority.example.org/company.rdf#me" />
6      <issued>2007-12-14T00:00:00</issued>
7    </attribution>

8    <dr>
9      <iriset>
10       <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com|example\.org)(\:([0-9]+))?\/</includeregex>
11       <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)*[^\:\/\?\#\@]+\:(80|8080|8081|8082)\/</includeregex>
12     </iriset>
13     <descriptorset>
14      <ex:color>red</ex:color>
15      <ex:shape>square</ex:shape>
16      <displaytext>Everything on example.org and example.com is red and square</displaytext>
17      <displayicon src="http://example.org/icon.png" />
18    </descriptorset>
19   </dr>

20 </powder>

POWDER-S [RDF/XML]

1  <?xml version="1.0"?>
2  <rdf:RDF
3     xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4     xmlns:foaf="http://xmlns.com/foaf/0.1/"
5     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
6     xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
7     xmlns:owl="http://www.w3.org/2002/07/owl#"
8     xmlns:dcterms="http://purl.org/dc/terms/0.1/"
9     xmlns:ex="http://example.org/vocab#">
 
10    <rdf:Description rdf:about="">
11      <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
12      <dcterms:issued>2007-12-14</dcterms:issued>
13    </rdf:Description>
 
14    <owl:Class rdf:nodeID="iriset_1">
15      <owl:intersectionOf rdf:parseType="Collection">
16        <owl:Restriction>
17          <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
18          <owl:hasValue  rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com|example\.org)(\:([0-9]+))?\/</owl:hasValue>
19        </owl:Restriction>
20        <owl:Restriction>
21          <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
22          <owl:hasValue  rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)*[^\:\/\?\#\@]+\:(80|8080|8081|8082)\/</owl:hasValue>
23        </owl:Restriction>
24      </owl:intersectionOf>
25    </owl:Class>

26    <owl:Class rdf:nodeID="descriptorset_1">
27      <owl:intersectionOf rdf:parseType="Collection">
28        <owl:Restriction>
29          <owl:onProperty rdf:resource="http://example.org/vocab#color" />
30          <owl:hasValue>red</owl:hasValue>
31       </owl:Restriction>
32        <owl:Restriction>
33          <owl:onProperty rdf:resource="http://example.org/vocab#shape" />
34          <owl:hasValue>square</owl:hasValue>
35        </owl:Restriction>
36      </owl:intersectionOf>
37      <dcterms:description>Everything on example.org and example.com is red and square</dcterms:description>
38      <foaf:depiction rdf:resource="http://example.org/icon.png" />
39    </owl:Class>
 
40    <owl:Class rdf:nodeID="iriset_1">
41      <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
42    </owl:Class>
 
43 </rdf:RDF>

The formal encoding of an excluderegex element uses the owl:complementOf property to link to the IRI set specified by the regular expression as exemplified below (note line 8).

Example 4-5: The POWDER, POWDER-BASE and POWDER-S Encoding of excluderegex Elements

POWDER [XML]

1  <iriset>
2    <incudehosts>example.org</includehosts>
3    <excludeports>8080</excludeports>
4  </iriset>

POWDER-BASE [XML]

1  <iriset>
2    <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.org)(\:([0-9]+))?\/</includeregex>
3    <excluderegex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)*[^\:\/\?\#\@]+\:(8080)\/</excluderegex>
4  </iriset>

POWDER-S [RDF/XML]

1  <owl:Class rdf:nodeID="iriset_1">
2    <owl:intersectionOf rdf:parseType="Collection">

3      <owl:Restriction>
4        <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
5        <owl:hasValue  rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.org)(\:([0-9]+))?\/</owl:hasValue>
6      </owl:Restriction>

7      <owl:Class>
8        <owl:complementOf>
9          <owl:Restriction>
10           <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
11           <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)*[^\:\/\?\#\@]+\:(80|8080|8081|8082)\/</owl:hasValue>
12         </owl:Restriction>
13       </owl:complementOf>
14     </owl:Class>

15   </owl:intersectionOf>
16 </owl:Class>

4.4 Direct Descriptions

POWDER and, consequently, POWDER-BASE documents might include descriptorset elements that are not inside a dr element but directly subsumed by the document's root. Such descriptions are not meant to be implicitly applied to any IRI groups, but are only made available by explicit reference by resources, as explained in Section 2.5 of the Description Resources document [DR].

In POWDER-S, such descriptions are translated into classes, but no subsumption of an IRI group is asserted, as shown in Example 4-6. Note, however, that as shown in Section 3, each derived OWL class is given an rdf:ID equivalent to the xml:id in the original POWDER document, not an rdf:nodeID, so that it can be referred to from outside.

Example 4-6: The Semantics of Direct Description Elements

POWDER [XML]

1  <descriptorset xml:id="square">
2    <ex:finish rdf:resource="http://example.org/vocab#shiny"/>
3    <ex:shape>square</ex:shape>
4  </descriptorset>

5  <descriptorset xml:id="round">
6    <ex:finish rdf:resource="http://example.org/vocab#matt"/>
7    <ex:shape>round</ex:shape>
8  </descriptorset>

9  <descriptorset xml:id="hexagonal">
10   <ex:finish rdf:resource="http://example.org/vocab#eggshell"/>
11   <ex:shape>hexagonal</ex:shape>
12 </descriptorset>

POWDER-S [RDF/XML]

1  <owl:Class rdf:ID="square">
2    <owl:intersectionOf rdf:parseType="Collection">
3      <owl:Restriction>
4        <owl:onProperty rdf:resource="http://example.org/vocab#finish"/>
5        <owl:hasValue rdf:resource="http://example.org/vocab#shiny"/>
6      </owl:Restriction>
7      <owl:Restriction>
8        <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
9        <owl:hasValue>square</owl:hasValue>
10     </owl:Restriction>
11   </owl:intersectionOf>
12 </owl:Class>

13 <owl:Class rdf:ID="round">
14   <owl:intersectionOf rdf:parseType="Collection">
15     <owl:Restriction>
16       <owl:onProperty rdf:resource="http://example.org/vocab#finish"/>
17       <owl:hasValue rdf:resource="http://example.org/vocab#matt"/>
18     </owl:Restriction>
19     <owl:Restriction>
20       <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
21       <owl:hasValue>round</owl:hasValue>
22     </owl:Restriction>
23   </owl:intersectionOf>
24 </owl:Class>

25 <owl:Class rdf:ID="hexagonal">
26   <owl:intersectionOf rdf:parseType="Collection">
27     <owl:Restriction>
28       <owl:onProperty rdf:resource="http://example.org/vocab#finish"/>
29       <owl:hasValue rdf:resource="http://example.org/vocab#eggshell"/>
30     </owl:Restriction>
31     <owl:Restriction>
32       <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
33       <owl:hasValue>hexagonal</owl:hasValue>
34     </owl:Restriction>
35   </owl:intersectionOf>
36 </owl:Class>

4.5 Semantics of abouthosts and aboutregex

The value of abouthosts is a whitespace-separated list of hosts. This list receives identical semantics to the value of the includehosts grouping element. In consequence, the transformation from POWDER to POWDER-BASE transforms the abouthosts elements into an aboutregex element using the same processing steps as for transforming includehosts into includeregex, as described in Section 4.2 above.

The aboutregex element of POWDER-BASE documents sets an outer limit on the resources described by the DRs within the document. That is to say, it restricts the resources that may receive a description not only implicitly (via subsumption by an IRI set) but also explicitly as described in Section 2.5 of the Description Resources document [DR].

In order to capture these semantics, the POWDER-BASE to POWDER-S transformation must add an implicit restriction to all descriptor sets in the document, effectively subsuming all resource classes created by descriptorset elements under the resource class that is created by the aboutregex element. In this manner, the implicit or explicit assignment of a description to a resource (cf. Section 2.5 of the Description Resources document [DR]) will create an inconsistency if the resource lies outside the aboutregex class.

It should be noted that iriset classes are not implicitly intersected with the aboutregex class, and it is the responsibility of the POWDER document author to ensure that all IRI sets in the document are within the scope defined by abouthosts/aboutregex.

This process is demonstrated by Example 4.7. Notice that in the POWDER-S document, each descriptor class is intersected with the 'aboutset' (lines 31, 51 and 63). A logical inconsistency will arise (i.e. an error) if an IRI set is not a subset of the aboutset class.

Example 4-7: The Semantics of abouthosts

POWDER Document [XML]

1  <attribution>
2    <issuedby src="http://authority.example.org/company.rdf#me" />
3    <abouthosts>example.org example.com</abouthosts>
4  </attribution>

5  <dr>
6    <iriset>
7      <includehosts>square.example.org</includehosts>
8    </iriset>
9    <descriptorset>
10     <ex:shape>square</ex:shape>
11   </descriptorset>
12 </dr>

13 <dr>
14   <iriset>
15     <includehosts>round.example.com</includehosts>
16   </iriset>
17   <descriptorset>
18     <ex:shape>round</ex:shape>
19   </descriptorset>
20 </dr>

21 <descriptorset xml:id="silver">
22   <ex:finish rdf:resource="http://example.org/vocab#shiny"/>
23   <ex:shape>square</ex:shape>
24 </descriptorset>

POWDER-BASE Document [XML]

1  <attribution>
2    <maker ref="http://authority.example.org/company.rdf#me" />
3    <aboutregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.org|example\.com)(\:([0-9]+))?\/</aboutregex>
4  </attribution>

5  <dr>
6    <iriset>
7      <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(square\.example\.org)(\:([0-9]+))?\/</includeregex>
8    </iriset>
9    <descriptorset>
10     <ex:shape>square</ex:shape>
11   </descriptorset>
12 </dr>

13 <dr>
14   <iriset>
15     <includeregex>\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(round\.example\.com)(\:([0-9]+))?\/</includeregex>
16   </iriset>

17   <descriptorset>
18     <ex:shape>round</ex:shape>
19   </descriptorset>
20 </dr>

21 <descriptorset xml:id="silver">
22   <ex:finish rdf:resource="http://example.org/vocab#shiny"/>
23   <ex:shape>square</ex:shape>
24 </descriptorset>

POWDER-S Document [RDF/XML]

1  <?xml version="1.0"?>
2  <rdf:RDF
3     xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
5     xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
6     xmlns:owl="http://www.w3.org/2002/07/owl#"
7     xmlns:dcterms="http://purl.org/dc/terms/"
8     xmlns:ex="http://example.org/vocab#">
 
9     <rdf:Description rdf:about="">
10      <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
11      <dcterms:issued>2007-12-14</dcterms:issued>
12    </rdf:Description>

13    <owl:Class rdf:nodeID="aboutset"> <!-- From the abouthosts element -->
14      <owl:intersectionOf rdf:parseType="Collection">
15        <owl:Restriction>
16          <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
17          <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.org|example\.net)(\:([0-9]+))?\/</owl:hasValue>
18        </owl:Restriction>
19      </owl:intersectionOf>
20    </owl:Class>

21    <owl:Class rdf:nodeID="iriset_1">
22      <owl:intersectionOf rdf:parseType="Collection">
23        <owl:Restriction>
24          <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregexp" />
25          <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(square\.example\.org)(\:([0-9]+))?\/</owl:hasValue>
26       </owl:Restriction>
27     </owl:intersectionOf>
28    </owl:Class>

29    <owl:Class rdf:nodeID="descriptorset_1">
30      <owl:intersectionOf rdf:parseType="Collection">
31        <owl:Class rdf:nodeID="aboutset"/>
32        <owl:Restriction>
33          <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
34          <owl:hasValue>square</owl:hasValue>
35        </owl:Restriction>
36      </owl:intersectionOf>
37    </owl:Class>

38    <owl:Class rdf:nodeID="iriset_1">
39      <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
40    </owl:Class>

41    <owl:Class rdf:nodeID="iriset_2">
42      <owl:intersectionOf rdf:parseType="Collection">
43        <owl:Restriction>
44          <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregexp" />
45          <owl:hasValue rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">\:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(round\.example\.com)(\:([0-9]+))?\/</owl:hasValue>
46        </owl:Restriction>
47      </owl:intersectionOf>
48    </owl:Class>

49    <owl:Class rdf:nodeID="descriptorset_2">
50      <owl:intersectionOf rdf:parseType="Collection">
51        <owl:Class rdf:nodeID="aboutset"/>
52        <owl:Restriction>
53          <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
54          <owl:hasValue>round</owl:hasValue>
55        </owl:Restriction>
56      </owl:intersectionOf>
57    </owl:Class>

58    <owl:Class rdf:nodeID="iriset_2">
59      <rdfs:subClassOf rdf:nodeID="descriptorset_2"/>
60    </owl:Class>

61    <owl:Class rdf:ID="silver">
62      <owl:intersectionOf rdf:parseType="Collection">
63        <owl:Class rdf:nodeID="aboutset"/>
64        <owl:Restriction>
65 	        <owl:onProperty rdf:resource="http://example.org/vocab#finish"/>
66          <owl:hasValue rdf:resource="http://example.org/vocab#shiny"/>
67        </owl:Restriction>
68        <owl:Restriction>
69          <owl:onProperty rdf:resource="http://example.org/vocab#shape"/>
70          <owl:hasValue>square</owl:hasValue>
71        </owl:Restriction>
72      </owl:intersectionOf>
73    </owl:Class>

74 </rdf:RDF>

As discussed in Section 3.2.3, a DR MAY refer to descriptorset elements in other POWDER documents. In such a situation, although it is not possible using XSLT (the technology used to effect the POWDER transforms) to generate aboutregex elements for POWDER-BASE as shown in the previous example, a conformant POWDER Processor MUST take account of abouthosts elements in both documents. The abouthosts element is designed to place an outer limit on the scope of any description in a POWDER document so that publishers descriptions retain effective control over the assertions made using their descriptions, even when such assertions are not attributed to them. For example, publishers of descriptions can use abouthosts to state that their descriptions are only meaningful for a particular set of domains, and may not be used to describe anything else.

4.6 POWDER-BASE IRI Set Semantics in OWL 2 (Informative)

At the time of this writing, the OWL-2 [OWL2] working draft provides for user-defined datatypes, using the restriction facet mechanism in XSD 2 [XSD2]. As this includes regular expression patterns, it is possible to translate POWDER into OWL 2 requiring a simpler extension than the one in Section 4.3.

More specifically, we extend RDF semantics [RDF-SEMANTICS] with a datatype property hasIRI, defined as:

hasIRI rdf:type owl:DatatypeProperty .
hasIRI rdf:type owl:Property .
hasIRI rdfs:domain owl:Thing .
hasIRI rdfs:range xsd:anyURI .

and the further stipulation that:

<x, uuu> is in IEXT(I(wdrs:hasIRI)) if and only if uuu is in the domain of I, with I(uuu)=x

Such an extension makes it possible to provide semantics to iriset by constructing an RDF datatype for each iriset and restricting the values of hasIRI to this datatype's range. In this manner, the POWDER-S translation of Example 4-4 becomes as shown in Example 4-8, where iriset_1 is a class of abstract resources, the concrete IRI string of which is within a user-defined datarange (lines 20-25 and 31-36).

Example 4-8: The POWDER-S Encoding of Example 4-4 With User-defined Datatypes [RDF/XML]

1  <?xml version="1.0"?>
2  <rdf:RDF
3    xmlns:wdrs="http://www.w3.org/2007/05/powder-s#"
4    xmlns:dcterms="http://purl.org/dc/terms/"
5    xmlns:foaf="http://xmlns.com/foaf/0.1/"
6    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
7    xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
8    xmlns:owl="http://www.w3.org/2002/07/owl#"
9    xmlns:owl2="http://www.w3.org/2006/12/owl2#"
10   xmlns:ex="http://example.org/vocab#">
  
11   <rdf:Description rdf:about="">
12     <wdrs:issuedby rdf:resource="http://authority.example.org/company.rdf#me" />
13     <dcterms:issued>2007-12-14</dcterms:issued>
14   </rdf:Description>
 
15   <owl:Class rdf:nodeID="iriset_1">
16     <owl:intersectionOf rdf:parseType="Collection">
17       <owl:Restriction>
18         <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#hasIRI" />
19          <owl:hasValue>
20            <owl:DataRange>
21              <owl2:pattern rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">
22                \:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)?(example\.com|example\.org)(\:([0-9]+))?\/
23              </owl2:pattern>
24              <owl2:onDataRange rdf:resource="http://www.w3.org/2001/XMLSchema-datatypes#string"/>
25            </owl:DataRange>
26          </owl:hasValue>
27        </owl:Restriction>
28        <owl:Restriction>
29          <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#hasIRI" />
30          <owl:hasValue>
31            <owl:DataRange>
32              <owl2:pattern rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">
33                \:\/\/(([^\/\?\#]*)\@)?([^\:\/\?\#\@]+\.)*[^\:\/\?\#\@]+\:(80|8080|8081|8082)\/
34              </owl2:pattern>
35              <owl2:onDataRange rdf:resource="http://www.w3.org/2001/XMLSchema-datatypes#string"/>
36            </owl:DataRange>
37          </owl:hasValue>
38        </owl:Restriction>
39      </owl:intersectionOf>
40    </owl:Class>

41    <owl:Class rdf:nodeID="descriptorset_1">
42      <owl:intersectionOf rdf:parseType="Collection">
43        <owl:Restriction>
44          <owl:onProperty rdf:resource="http://example.org/vocab#color" />
45          <owl:hasValue>red</owl:hasValue>
46       </owl:Restriction>
47        <owl:Restriction>
48          <owl:onProperty rdf:resource="http://example.org/vocab#shape" />
49          <owl:hasValue>square</owl:hasValue>
20        </owl:Restriction>
51      </owl:intersectionOf>
52      <dcterms:description>Everything on example.org and example.com is red and square</dcterms:description>
53      <foaf:depiction rdf:resource="http://example.org/icon.png" />
54    </owl:Class>
 
55    <owl:Class rdf:nodeID="iriset_1">
56      <rdfs:subClassOf rdf:nodeID="descriptorset_1"/>
57    </owl:Class>

58 </rdf:RDF>

5 Acknowledgements

The Working Group would like to thank Jeremy Carroll for his substantial contribution to the development of the semantics of POWDER.

6 References

[WEBARCH]
Architecture of the World Wide Web, Volume One, Section 2.2. W3C Recommendation 15 December 2004. I. Jacobs, N. Walsh. This document is at http://www.w3.org/TR/webarch/#id-resources
[GRDDL]
Gleaning Resource Descriptions from Dialects of Languages (GRDDL), W3C Recommendation 11 September 2007. D. Connolly. This document is at http://www.w3.org/TR/grddl/
[XSLT]
XSL Transformations (XSLT), W3C Recommendation 16 November 1999. J. Clark. This document is at http://www.w3.org/TR/xslt
[DR]
Protocol for Web Description Resources (POWDER): Description Resources, P. Archer, A. Perego, K. Smith. This document is at http://www.w3.org/TR/powder-dr/
[GROUP]
Protocol for Web Description Resources (POWDER): Grouping of Resources, A. Perego, P. Archer This document is at http://www.w3.org/TR/powder-grouping/
[USECASES]
POWDER: Use Cases and Requirements W3C Working Group Note 31 October 2007, P. Archer . This document is at http://www.w3.org/TR/powder-use-cases/
[PRIMER]
Protocol for Web Description Resources (POWDER): Primer 2008, K. Scheppe, D. Pentecost. (URI TBC)
[TESTS]
Protocol for Web Description Resources (POWDER): Test Suite 2008, A. Kukurikos. (URI TBC)
[WDR]
Protocol for Web Description Resources (POWDER): Web Description Resources XML Schema (WDR), A. Perego, K. Smith. This document is at http://www.w3.org/2007/05/powder
[WDRS]
Protocol for Web Description Resources (POWDER): POWDER-S Vocabulary (WDRS), P. Archer. This document is at http://www.w3.org/2007/05/powder-s
[WDRD]
Protocol for Web Description Resources (POWDER): Web Description Resources Datatypes (WDRD), A. Perego, K. Smith. This document is at http://www.w3.org/TR/powder-xsd/
[PDR-GRDDL]
Protocol for Web Description Resources (POWDER): GRDDL Transform 2008, K. Smith, A. Perego (URI TBC)
[XSD2]
XML Schema Part 2: Datatypes Second Edition W3C Recommendation 28 October 2004. P V Biron, A. Malhotra. This document is at http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/datatypes.html#rf-pattern
[N3]
Notation3 (N3): A readable RDF syntax, T. Berners-Lee, D. Connolly. This document is at http://www.w3.org/TeamSubmission/n3/
[DC]
Dublin Core Metadata Initiative Terms. See http://dublincore.org/documents/dcmi-terms/
[FOAF]
FOAF Vocabulary Specification 0.9 Namespace Document 24 May 2007, D. Brickley, L. Miller. This document is at http://xmlns.com/foaf/spec/
[W3C DTF]
W3C Date and Time Formats W3C Note, 15 September 1997. M. Wolf, C. Wicksteed. This document is at http://www.w3.org/TR/NOTE-datetime
[OWL]
OWL Web Ontology Language, Semantics and Abstract Syntax, W3C Recommendation 10 February 2004. P. F. Patel-Schneider, I. Horrocks. This document is at http://www.w3.org/TR/owl-semantics/syntax.html#2.1
[OWL2]
OWL 2 Web Ontology Language: Primer, W3C Working Draft 11 April 2008. B. Parsia, P. F. Patel-Schneider. This document is at http://www.w3.org/TR/2008/WD-owl2-primer-20080411/
[RDF-SEMANTICS]
RDF Semantics, W3C Recommendation 10 February 2004, P. Hayes. The key text is in Section 1.3. This document is at http://www.w3.org/TR/2004/REC-rdf-mt-20040210/
[XQXP]
XQuery 1.0 and XPath 2.0 Functions and Operators, A. Malhotra, J. Melton, N. Walsh. W3C Recommendation 23 January 2007. This document is at http://www.w3.org/TR/xpath-functions/
[XSLT2]
XSL Transformations (XSLT) Version 2.0, W3C Recommendation 23 January 2007. M. Kay. This document is at http://www.w3.org/TR/xslt20/
[Rabin]
URI Pattern Matching for Groups of Resources., Draft 0.1 17 June 2006, J. Rabin. This document is at http://www.w3.org/2005/Incubator/wcl/matching.html
[RFC2119]
Key words for use in RFCs to Indicate Requirement Levels, RFC 2119, S. Bradner. This document is at http://tools.ietf.org/html/rfc2119

7 Change Log

7.1 Changes since First Public Working Draft