W3C

SKOS Simple Knowledge Organization System
eXtension for Labels (XL)

Editor's Draft 14 April 2008

This version:
http://www.w3.org/2006/07/SWD/SKOS/xl/20080414
Latest version:
http://www.w3.org/2006/07/SWD/SKOS/xl/
Previous version:
None
Editors:
Alistair Miles, STFC Rutherford Appleton Laboratory / University of Oxford
Contributors:
Antoine Isaac, Vrije Universiteit Amsterdam

Copyright © 2008 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.

Abstract

This note defines an extension for the Simple Knowledge Organization System, providing additional support for describing and linking lexical entities. A special class of lexical entities, called xl:Label, is defined such that each instance of this class has a single RDF plain literal form, but two instances this class are not necessarily the same individual if they share the same literal form. Three alternate patterns for linking lexical entities are defined, being (1) n-ary relations between RDF plain literals, (2) binary relations between instances of xl:Label, and (3) n-ary relations between instances of xl:Label. These three patterns are presented as equally valid alternatives, in the hope that further implementation experience will shed light on which patterns are best suited to which situations, which will feed into future standardization work.

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 document is an editor's draft produced by the Semantic Web Deployment Working Group for internal review purposes only. It has no formal status whatsoever within any W3C process.

Editors' note: Anything marked in the document with an "@@" indicates something still to be done or fixed. For example, "@@TODO" indicates an outstanding task, "@@REF" indicates a reference that needs to be properly cited, and "@@X" indicates numbering that needs to be completed.

Table of Contents

1. Introduction

@@TODO

2. Vocabulary

The XL vocabulary is the set of URIs given in the left-hand column in the table below.

Table @@X. XL Vocabulary
URI Definition
xl:Label @@TODO
xl:literalForm @@TODO
xl:prefLabel @@TODO
xl:altLabel @@TODO
xl:hiddenLabel @@TODO
xl:NaryLiteralRelation @@TODO
xl:literalRelated @@TODO
xl:NaryLabelRelation @@TODO
xl:labelRelated @@TODO
xl:binaryLabelRelation @@TODO
xl:seeRelation @@TODO

Here, "the SKOS+XL vocabulary" refers to the union of the SKOS vocabulary and the XL vocabulary. "The SKOS+XL data model" refers to the union of the data model defined in the SKOS Reference [@@REF] and the class and property definitions stated in this document.

3. The xl:Label Class

3.1. Preamble

The class xl:Label is a special class of lexical entities.

An instance of the class xl:Label is a resource and may be named with a URI.

An instance of the class xl:Label has a single literal form. This literal form is an RDF plain literal, which is a string of UNICODE characters and an optional language tag. The property xl:literalForm is used to give the literal form of an xl:Label.

If two instances of the class xl:Label have the same literal form, they are not necessarily the same resource.

3.2. Class and Property Definitions

@@X xl:Label is an instance of owl:Class.
@@X xl:Label is disjoint with each of the classes skos:Concept, skos:ConceptScheme, skos:Collection.
@@X xl:literalForm is an instance of owl:DatatypeProperty.
@@X The rdfs:domain of xl:literalForm is the class xl:Label.
@@X The rdfs:range of xl:literalForm is the class of RDF plain literals.
@@X xl:literalForm is an instance of owl:FunctionalProperty.

3.3. Examples

The example below describes an xl:Label named with the URI <http://example.com/A>, with the literal form "foo" in English. This example is consistent with the XL data model.

Example @@X
<A> rdf:type xl:Label ; xl:literalForm "foo"@en .

The three examples below are each not consistent with the XL data model, because an xl:Label is described with two different literal forms.

Example @@X
<B> rdf:type xl:Label ; xl:literalForm "foo" ; xl:literalForm "bar" .
Example @@X
<B> rdf:type xl:Label ; xl:literalForm "foo"@en ; xl:literalForm "foo"@fr .
Example @@X
<B> rdf:type xl:Label ; xl:literalForm "foo"@en-GB ; xl:literalForm "foo"@en-US .

3.4. Notes

3.4.1. Identity and Entailment

As stated above, each instance of the class xl:Label has one and only one literal form. In other words, there is a function mapping the extension of xl:Label to the set of RDF plain literals. This function is, in fact, defined by the property extension of xl:literalForm. Note especially two facts about this function.

First, the function is not injective. In other words, there is not a one-to-one mapping from instances of xl:Label to the set of RDF plain literals. This means that two instances of xl:Label which have the same literal form are not necessarily the same individual.

This means that, for example, the entailment illustrated below is not supported by the XL data model.

Example @@X
<A> xl:literalForm "foo"@en .
<B> xl:literalForm "foo"@en .

does not entail

<A> owl:sameAs <B> .

Second, the function is not surjective. In other words, there may be no instances of xl:Label with a given literal form.

3.4.2. Membership of Concept Schemes

For some knowledge organization systems, it could be useful to assert the membership of an instance of xl:Label within a SKOS concept scheme. Currently, the property skos:inScheme has a domain which is incompatible with its use for this purpose. No alternative is provided in this specification. Whether to relax the domain of skos:inScheme so it could be used for this purpose is an open issue.

@@TODO raise a SWD issue to consider relaxing domain of skos:inScheme?

4. Preferred, Alternate and Hidden xl:Labels

4.1. Preamble

The three properties xl:prefLabel, xl:altLabel and xl:hiddenLabel are used to assign the preferred, alternate and hidden labels of a resource respectively, where those labels are instances of the class xl:Label. These properties are analogous to the properties of the same local name defined in the SKOS vocabulary, and there are logical dependencies between these two sets of properties defined below.

4.2. Class and Property Definitions

@@X xl:prefLabel, xl:altLabel, xl:hiddenLabel and xl:labelRelated are each instances of owl:ObjectProperty.
@@X The rdfs:range of each of xl:prefLabel, xl:altLabel xl:hiddenLabel and xl:labelRelated is the class xl:Label.
@@X For any R, L, V, if <R, L> is in IEXT(I(xl:prefLabel)) and <L, V> is in IEXT(I(xl:literalForm)) then <R, V> is in IEXT(I(skos:prefLabel)).
@@X For any R, L, V, if <R, L> is in IEXT(I(xl:altLabel)) and <L, V> is in IEXT(I(xl:literalForm)) then <R, V> is in IEXT(I(skos:altLabel)).
@@X For any R, L, V, if <R, L> is in IEXT(I(xl:hiddenLabel)) and <L, V> is in IEXT(I(xl:literalForm)) then <R, V> is in IEXT(I(skos:hiddenLabel)).

4.3. Examples

The example below illustrates the use of all three XL labeling properties, and is consistent with the SKOS+XL data model.

Example @@X
<foo>
  xl:prefLabel <bar> ;
  xl:altLabel <baz> ;
  xl:hiddenLabel <qux> .

<bar> rdf:type xl:Label ;
  xl:literalForm "bar"@en .

<baz> rdf:type xl:Label ;
  xl:literalForm "baz"@en .

<qux> rdf:type xl:Label ;
  xl:literalForm "qux"@en .

4.4. Notes

4.4.1. Inference Rules for "Dumbing-Down" to SKOS Lexical Labels

The following table illustrates some inference patterns which capture the entailments relating the XL labeling properties with the SKOS labeling properties.

@@TODO explain notation conventions

Table @@X. Inference Rules
If graph contains then add
uuu xl:prefLabel vvv .
vvv xl:literalForm xxx .		 uuu skos:prefLabel xxx .
uuu xl:altLabel vvv .
vvv xl:literalForm xxx .		 uuu skos:altLabel xxx .
uuu xl:hiddenLabel vvv .
vvv xl:literalForm xxx .		 uuu skos:hiddenLabel xxx .

These rules can be used to "dumb-down" a graph using the XL vocabulary for labeling to a simpler graph using only the SKOS vocabulary for labeling. This is illustrated in the example below.

Example @@X
<foo>
  xl:prefLabel <bar> ;
  xl:altLabel <baz> ;
  xl:hiddenLabel <qux> .

<bar> rdf:type xl:Label ;
  xl:literalForm "bar"@en .

<baz> rdf:type xl:Label ;
  xl:literalForm "baz"@en .

<qux> rdf:type xl:Label ;
  xl:literalForm "qux"@en .

entails

<foo>
  skos:prefLabel "bar"@en ;
  skos:altLabel "baz"@en ;
  skos:hiddenLabel "qux"@en .

4.4.2. SKOS+XL Labeling Integrity

Note that there are two important integrity conditions on the SKOS labeling properties defined in the SKOS data model. First, the properties skos:prefLabel, skos:altLabel and skos:hiddenLabel are pairwise disjoint. Second, a resource has no more than one value of skos:prefLabel per language [@@REF]. Because of the logical dependency between the XL labeling vocabulary and the SKOS vocabulary defined above, the following four examples, whilst consistent w.r.t. the XL data model alone, are not consistent with the SKOS+XL data model.

Example @@X
<foo> xl:prefLabel <A> ; xl:prefLabel <B> .

<A> xl:literalForm "foo"@en .

<B> xl:literalForm "bar"@en .
Example @@X
<foo> xl:prefLabel <A> ; xl:altLabel <B> .

<A> xl:literalForm "foo"@en .

<B> xl:literalForm "foo"@en .
Example @@X
<foo> xl:altLabel <A> ; xl:hiddenLabel <B> .

<A> xl:literalForm "foo"@en .

<B> xl:literalForm "foo"@en .
Example @@X
<foo> xl:prefLabel <A> ; xl:hiddenLabel <B> .

<A> xl:literalForm "foo"@en .

<B> xl:literalForm "foo"@en .

8. Linking to N-ary Literal/Label Relations

8.1. Preamble

In some situations it is useful to assert links from other types of resource (e.g. instances of the class skos:Concept) to n-ary relations between literals or xl:Labels.

This section defines a general purpose property for asserting such links.

8.2. Class & Property Definitions

@@X xl:seeRelation is an instance of owl:ObjectProperty.

8.3. Examples

In the example below, xl:seeRelation is used to assert a link between a SKOS concept and an n-ary literal relation.

@@TODO

In the example below, xl:seeRelation is used to assert a link between a SKOS concept and an n-ary label relation.

@@TODO

8.4. Notes

8.4.1. Correspondance Between SKOS Lexical Labels and XL Relations

There are no integrity conditions on the property xl:seeRelation. This means that there does not necessarily have to be any correspondence whatsoever between the SKOS lexical labels of a resource, and the labels involved in an associated XL literal or label relation. The example below, which is consistent with the SKOS+XL data model, illustrates this.

Example @@X
<foo>
  skos:prefLabel "foo"@en ;
  skos:altLabel "bar"@en ;
  xl:seeRelation [ xl:literalRelated "baz"@en , "quux"@en ] .

Refinements of the xl:seeRelation property are free to define stricter integrity conditions for label correspondance.

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