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1084 lines
45 KiB
1084 lines
45 KiB
<?xml version="1.0" encoding="iso-8859-1"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<head>
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<title>CBD - Concise Bounded Description</title>
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<style type="text/css">
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pre.box { border: 1px solid; background-color: #E8E8E8; padding-left: 1mm; padding-right: 1mm; padding: 2mm; page-break-inside: avoid }
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</style>
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<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-Member-SUBM.css" />
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</head>
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<body>
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<div class="head">
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<p><a href="http://www.w3.org/"><img
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src="http://www.w3.org/Icons/w3c_home" alt="W3C" height="48"
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width="72" /></a> <a href="http://www.w3.org/Submission/">
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<img height="48" width="211" alt="W3C Member Submission"
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src="http://www.w3.org/Icons/member_subm" /></a></p>
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<h1 id="cbd">CBD - Concise Bounded Description</h1>
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<h2><a name="w3c-doctype" id="w3c-doctype"></a>W3C Member
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Submission 3 June 2005</h2>
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<dl>
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<dt>This version:</dt>
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<dd><a href="http://www.w3.org/Submission/2005/SUBM-CBD-20050603/">http://www.w3.org/Submission/2005/SUBM-CBD-20050603/</a></dd>
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<dt>Latest version:</dt>
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<dd><a href="http://www.w3.org/Submission/CBD/">http://www.w3.org/Submission/CBD/</a></dd>
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<dt>Previous version:</dt>
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<dd><a href="http://www.w3.org/Submission/2004/SUBM-CBD-20040930/">http://www.w3.org/Submission/2004/SUBM-CBD-20040930/</a></dd>
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<dt>Author:</dt>
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<dd>Patrick Stickler, Nokia, <a href="mailto:patrick.stickler@nokia.com">patrick.stickler@nokia.com</a></dd>
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</dl>
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<p class="copyright">
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Copyright <a href="http://www.nokia.com">Nokia</a> 2003-2004.<br /> This document is available under the <a href="http://www.w3.org/Consortium/Legal/copyright-documents">W3C
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Document License</a>. See the <a
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href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">W3C
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Intellectual Rights Notice and Legal Disclaimers</a> for additional information.
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</p>
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</div>
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<hr />
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<h2><a name="abstract">Abstract</a></h2>
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<p>
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This document <a href="#r1">[1]</a> defines a <em>concise bounded description</em> of a resource in
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terms of an RDF graph, as a general and broadly optimal unit of specific knowledge
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about that resource to be utilized by, and/or interchanged between,
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semantic web agents.
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</p>
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<div>
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<h2><a name="status" id="status">Status of this Document</a></h2>
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<p><em>This section describes the status of this document at the
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time of its publication. Other documents may supersede this
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document.</em></p>
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<p>By publishing this document, W3C acknowledges that Nokia has
|
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made a formal submission to W3C for discussion. Publication of this
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document by W3C indicates no endorsement of its content by W3C, nor
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that W3C has, is, or will be allocating any resources to the issues
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addressed by it. This document is not the product of a chartered W3C
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group, but is published as potential input to the <a
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href="/Consortium/Process">W3C Process</a>. Publication of
|
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acknowledged Member Submissions at the W3C site is one of the benefits
|
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of <a href="/Consortium/Prospectus/Joining">W3C Membership</a>. Please
|
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consult the requirements associated with Member Submissions of <a
|
|
href="http://www.w3.org/Consortium/Patent-Policy-20030520.html#sec-submissions">section
|
|
3.3 of the W3C Patent Policy</a>. Please consult the complete <a
|
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href="/Submission">list of acknowledged W3C Member
|
|
Submissions</a>. See also <a href="/Submission/2004/06/">Submission request</a> and
|
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<a href="/Submission/2004/06/Comment">Team Comment</a>.</p>
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</div>
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<hr />
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<h2><a name="contents">Table of contents</a></h2>
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<blockquote>
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<ol>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#definition">Definition</a></li>
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<li><a href="#example">Example</a></li>
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<li><a href="#application">Application Issues</a></li>
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<li><a href="#alternatives">Alternative Forms of Description</a></li>
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<li><a href="#references">Notes and References</a></li>
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</ol>
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</blockquote>
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<hr />
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<h2><a name="introduction">Introduction</a></h2>
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<blockquote>
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<p>
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As the semantic web <a href="#r2">[2]</a> emerges and the behavior of automated software
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agents becomes increasingly directed by formally defined knowledge about resources
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<a href="#r3">[3]</a>
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gathered from disparate sources <a href="#r4">[4]</a>, the need for optimal and consistent
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interchange of knowledge about specific resources between agents
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becomes critical to achieving an efficient, globally scalable, and ubiquitous semantic web.
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</p>
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<p>
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This document defines a <em>concise bounded description</em> of a resource in
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terms of an RDF graph <a href="#r5">[5]</a>, as a general and broadly optimal unit of specific knowledge
|
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about that resource to be utilized by, and/or interchanged between,
|
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semantic web agents.
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</p>
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<p>
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Given a particular node in a particular RDF graph, a <em>concise bounded description</em>
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is a subgraph consisting of those statements which together constitute a
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focused body of knowledge about the resource denoted by that particular node. The
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precise nature of that subgraph will hopefully become clear given the definition,
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discussion and example provided below.
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</p>
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<p>
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Optimality is, of course, application dependent and it is not presumed that a <em>concise
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bounded description</em> is an optimal form of description for every application;
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however, it is presented herein as a reasonably general and broadly optimal form
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of description for many applications, and unless otherwise warranted,
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constitutes a reasonable default response to the request "tell me about this resource".
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</p>
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</blockquote>
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<hr />
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<h2><a name="definition">Definition</a></h2>
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<blockquote>
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<p>
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Given a particular node (the starting node) in a particular RDF graph (the
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source graph), a subgraph
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of that particular graph, taken to comprise a <em>concise bounded description</em>
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of the resource denoted by the starting node, can be identified
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as follows:
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</p>
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<ol>
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<li>
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Include in the subgraph all statements in the source graph where the subject of the
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statement is the starting node;
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</li>
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<li>
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Recursively, for all statements identified in the subgraph
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thus far having a blank node object, include in the subgraph
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all statements in the source graph where the subject of the statement is the blank
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node in question and which are not already included in the subgraph.
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</li>
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<li>
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Recursively, for all statements included in the subgraph
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thus far, for all reifications of each statement in the source
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graph, include
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the <em>concise bounded description</em> beginning from
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the rdf:Statement node of each reification.
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</li>
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</ol>
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<p>
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This results in a subgraph where the object nodes are either URI references,
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literals, or blank nodes not serving as the subject of any
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statement in the graph.
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</p>
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</blockquote>
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<hr />
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<h2><a name="example">Example</a></h2>
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<blockquote>
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<p>
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<a name="sourcegraph">
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Given the following RDF graph as input (the source graph):
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</a>
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</p>
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<pre class="box">
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<?xml version="1.0"?>
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<rdf:RDF
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xmlns:rdf ="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
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xmlns:rdfs ="http://www.w3.org/2000/01/rdf-schema#"
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xmlns:owl ="http://www.w3.org/2002/07/owl#"
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xmlns:dc ="http://purl.org/dc/elements/1.1/"
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xmlns:dct ="http://purl.org/dc/terms/"
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xmlns:xsd ="http://www.w3.org/2001/XMLSchema#"
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xmlns:foaf ="http://xmlns.com/foaf/0.1/"
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xmlns:ex ="http://example.com/">
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<rdf:Description rdf:about="http://example.com/aReallyGreatBook">
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<dc:title>A Really Great Book</dc:title>
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<dc:publisher>Examples-R-Us</dc:publisher>
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<dc:creator>
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<rdf:Description>
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<rdf:type rdf:resource="http://xmlns.com/foaf/0.1/Person"/>
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<foaf:name>John Doe</foaf:name>
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<foaf:mbox>john@example.com</foaf:mbox>
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<foaf:img>
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<rdf:Description rdf:about="http://example.com/john.jpg">
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<rdf:type rdf:resource="http://xmlns.com/foaf/0.1/Image"/>
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<dc:format>image/jpeg</dc:format>
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<dc:extent>1234</dc:extent>
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</rdf:Description>
|
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</foaf:img>
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<foaf:phone rdf:resource="tel:+1-999-555-1234"/>
|
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</rdf:Description>
|
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</dc:creator>
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<dc:contributor>
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<rdf:Description>
|
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<rdf:type rdf:resource="http://xmlns.com/foaf/0.1/Person"/>
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<foaf:name>Jane Doe</foaf:name>
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</rdf:Description>
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</dc:contributor>
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<dc:language>en</dc:language>
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<dc:format>application/pdf</dc:format>
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<dc:rights>Copyright (C) 2004 Examples-R-Us. All rights reserved.</dc:rights>
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<dct:issued rdf:datatype="http://www.w3.org/2001/XMLSchema#date">2004-01-19</dct:issued>
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<rdfs:seeAlso rdf:resource="http://example.com/anotherGreatBook"/>
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</rdf:Description>
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|
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<rdf:Statement>
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<rdf:subject rdf:resource="http://example.com/aReallyGreatBook"/>
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<rdf:predicate rdf:resource="http://purl.org/dc/elements/1.1/format"/>
|
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<rdf:object>application/pdf</rdf:object>
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<rdfs:isDefinedBy rdf:resource="http://example.com/book-formats.rdf"/>
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</rdf:Statement>
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<rdf:Statement>
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<rdf:subject rdf:resource="http://xmlns.com/foaf/0.1/Image"/>
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<rdf:predicate rdf:resource="http://purl.org/dc/elements/1.1/format"/>
|
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<rdf:object>image/jpeg</rdf:object>
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<rdfs:isDefinedBy rdf:resource="http://example.com/image-formats.rdf"/>
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</rdf:Statement>
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<rdf:Description rdf:about="http://example.com/anotherGreatBook">
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<dc:title>Another Great Book</dc:title>
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<dc:publisher>Examples-R-Us</dc:publisher>
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<dc:creator>June Doe (june@example.com)</dc:creator>
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<dc:format>application/pdf</dc:format>
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<dc:language>en</dc:language>
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<dc:rights>Copyright (C) 2004 Examples-R-Us. All rights reserved.</dc:rights>
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<dct:issued rdf:datatype="http://www.w3.org/2001/XMLSchema#date">2004-05-03</dct:issued>
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<rdfs:seeAlso rdf:resource="http://example.com/aReallyGreatBook"/>
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</rdf:Description>
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<rdf:Description rdf:about="http://example.com/aBookCritic">
|
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<ex:likes rdf:resource="http://example.com/aReallyGreatBook"/>
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<ex:dislikes rdf:resource="http://example.com/anotherGreatBook"/>
|
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</rdf:Description>
|
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|
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<rdf:Property rdf:about="http://xmlns.com/foaf/0.1/mbox">
|
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<rdf:type rdf:resource="http://www.w3.org/2002/07/owl#InverseFunctionalProperty"/>
|
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</rdf:Property>
|
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|
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</rdf:RDF>
|
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|
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</pre>
|
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|
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<p>
|
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The <em>concise bounded description</em> of
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<tt>http://example.com/aReallyGreatBook</tt>
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corresponds to the following subgraph:
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</p>
|
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|
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<pre class="box">
|
|
|
|
<?xml version="1.0"?>
|
|
|
|
<rdf:RDF
|
|
xmlns:rdf ="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
|
|
xmlns:rdfs ="http://www.w3.org/2000/01/rdf-schema#"
|
|
xmlns:owl ="http://www.w3.org/2002/07/owl#"
|
|
xmlns:dc ="http://purl.org/dc/elements/1.1/"
|
|
xmlns:dct ="http://purl.org/dc/terms/"
|
|
xmlns:xsd ="http://www.w3.org/2001/XMLSchema#"
|
|
xmlns:foaf ="http://xmlns.com/foaf/0.1/">
|
|
|
|
<rdf:Description rdf:about="http://example.com/aReallyGreatBook">
|
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<dc:title>A Really Great Book</dc:title>
|
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<dc:publisher>Examples-R-Us</dc:publisher>
|
|
<dc:creator>
|
|
<rdf:Description>
|
|
<rdf:type rdf:resource="http://xmlns.com/foaf/0.1/Person"/>
|
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<foaf:name>John Doe</foaf:name>
|
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<foaf:mbox>john@example.com</foaf:mbox>
|
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<foaf:img rdf:resource="http://example.com/john.jpg"/>
|
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<foaf:phone rdf:resource="tel:+1-999-555-1234"/>
|
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</rdf:Description>
|
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</dc:creator>
|
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<dc:contributor>
|
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<rdf:Description>
|
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<rdf:type rdf:resource="http://xmlns.com/foaf/0.1/Person"/>
|
|
<foaf:name>Jane Doe</foaf:name>
|
|
</rdf:Description>
|
|
</dc:contributor>
|
|
<dc:language>en</dc:language>
|
|
<dc:format>application/pdf</dc:format>
|
|
<dc:rights>Copyright (C) 2004 Examples-R-Us. All rights reserved.</dc:rights>
|
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<dct:issued rdf:datatype="http://www.w3.org/2001/XMLSchema#date">2004-01-19</dct:issued>
|
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<rdfs:seeAlso rdf:resource="http://example.com/anotherGreatBook"/>
|
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</rdf:Description>
|
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|
|
<rdf:Statement>
|
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<rdf:subject rdf:resource="http://example.com/aReallyGreatBook"/>
|
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<rdf:predicate rdf:resource="http://purl.org/dc/elements/1.1/format"/>
|
|
<rdf:object>application/pdf</rdf:object>
|
|
<rdfs:isDefinedBy rdf:resource="http://example.com/book-formats.rdf"/>
|
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</rdf:Statement>
|
|
|
|
</rdf:RDF>
|
|
|
|
</pre>
|
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</blockquote>
|
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|
|
<hr />
|
|
|
|
<h2><a name="application">Application Issues</a></h2>
|
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<blockquote>
|
|
|
|
<h3 id="Representations_versus_Descriptions">Representations versus Descriptions</h3>
|
|
<blockquote>
|
|
|
|
<p>
|
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The web, as traditionally defined <a href="#r6">[6]</a>, <a href="#r7">[7]</a>,
|
|
is concerned with providing representations
|
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of resources. Representations can essentially be anything
|
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associated with the resource which is considered useful in
|
|
understanding, accessing, interacting, and/or manipulating that
|
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resource, such that the representation conveys information about the state of
|
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the resource. This is fine for visual presentation to
|
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humans browsing the web, who can deal with a great amount of variety,
|
|
inconsistency, and ambiguity while still deriving benefit from the representations
|
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provided. It is this flexibility and unconstrained richness of what representations
|
|
can be that is one of the factors responsible for the tremendous success of the web.
|
|
</p>
|
|
|
|
<p>
|
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However, semantic web agents (at present at least) are not
|
|
able to deal as well with the broad range of possible representations
|
|
which might be associated with a resource; and in nearly all cases,
|
|
are unable to make any use of such representations, as they are
|
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typically intended for human rather than machine consumption. Semantic
|
|
web agents, not being anywhere near as intelligent as most humans, require
|
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information which is explicit and formally defined. In short,
|
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semantic web agents need concise, bounded descriptions of resources, expressed
|
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in a machine understandable language, rather than seemingly arbitrary representations which
|
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to agents are usually semantically opaque.
|
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</p>
|
|
|
|
<p>
|
|
Concise bounded descriptions of resources can be considered to be a form
|
|
of representation, however they are a highly specialized form and not
|
|
the most usual or obvious form in a web primarily intended for human consumption.
|
|
They are, however, a key form of representation which semantic web
|
|
agents need in order to reason about such resources and adjust their behavior
|
|
accordingly.
|
|
</p>
|
|
</blockquote>
|
|
|
|
<h3 id="Determination_of_the_Source_Graph">Determination of the Source Graph</h3>
|
|
<blockquote>
|
|
|
|
<p>
|
|
The starting point for obtaining a <em>concise bounded description</em> is a
|
|
particular node in a particular RDF graph.
|
|
Determination of the particular source graph from which the description
|
|
is to be obtained is (primarily) a <strong>system issue</strong>.
|
|
</p>
|
|
|
|
<p>
|
|
The source graph does not necessarily correspond exactly to the set
|
|
of statements persistently managed by a given system in a triple
|
|
store, or set of triple stores.
|
|
There may be statements maintained by the system which
|
|
are sensitive and not intended to be accessible to the public at large.
|
|
There may be statements maintained by the system which are
|
|
specific only to the internal functioning of the system, and not
|
|
relevant or interesting to most agents.
|
|
The system may employ a reasoning component which is able to
|
|
dynamically infer new statements from those persistently stored.
|
|
The system may allow access to certain statements to certain users
|
|
but not to others.
|
|
</p>
|
|
|
|
<p>
|
|
So, in practice, the actual graph which a particular service utilizes
|
|
as the source graph for returning <em>concise bounded
|
|
descriptions</em> in response to query requests to that service, may be a virtual
|
|
graph that only exists in a transient state at run-time, during
|
|
the execution of a particular query.
|
|
</p>
|
|
|
|
<p>
|
|
Thus, issues such as security, visibility, inference, etc. which
|
|
may govern the determination of the particular
|
|
graph employed by a given service in a given context to satisfy
|
|
a given request are distinct
|
|
from the identification of a particular <em>concise bounded
|
|
description</em> once that source graph has been determined.
|
|
</p>
|
|
|
|
<p>
|
|
However, while such issues are functionally disjunct, that does not mean they cannot
|
|
be addressed in parallel, when processing a request for a <em>concise
|
|
bounded description</em>. E.g., in the
|
|
Nokia Semantic Web Server <a href="#r8">[8]</a> implementation <a href="#r9">[9]</a>,
|
|
identification of statements belonging to a particular
|
|
<em>concise bounded description</em> are determined using inference
|
|
rules, which may be evaluated in parallel with select RDFS <a href="#r5">[5]</a>,
|
|
OWL <a href="#r10">[10]</a>,
|
|
and service specific inference rules. The source graph is not
|
|
first constructed, by applying the RDFS, OWL, and other inference
|
|
rules, from which the <em>concise bounded description</em> is
|
|
then afterwards obtained; rather, all are performed as a single
|
|
logical step, implemented using the same reasoning machinery.
|
|
</p>
|
|
</blockquote>
|
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|
|
<h3 id="Query_and_Application_Programming_Interfaces">Query and Application Programming Interfaces</h3>
|
|
<blockquote>
|
|
|
|
<p>
|
|
Again, as stated above, the starting point for obtaining a
|
|
<em>concise bounded description</em> is a particular node in a
|
|
particular RDF graph.
|
|
How you identify that
|
|
particular node in the source graph is an <strong>interface issue</strong>.
|
|
</p>
|
|
|
|
<p>
|
|
Also, in some cases, the source graph may be explicitly named <a href="#r11">[11]</a>
|
|
and therefore identification of the source graph by name may also be
|
|
a potential interface issue.
|
|
</p>
|
|
|
|
<p>
|
|
Different query and application programming interfaces can provide different
|
|
means by which the particular graph and/or the particular node in that graph
|
|
might be identified and referred to when requesting a <em>concise bounded
|
|
description</em>. And while different interfaces may identify that node
|
|
and that graph by different means, once that node and graph are identified,
|
|
obtaining a <em>concise bounded description</em> can proceed the same
|
|
for all cases, irrespective of how the starting point was arrived at.
|
|
</p>
|
|
|
|
<p>
|
|
However, insofar as particular forms of descriptions are concerned,
|
|
it is important to note that the form of description
|
|
can introduce requirements on the functionality of the interface
|
|
(and visa versa)
|
|
which must be satisfied if all statements contained
|
|
in a given source graph are to be accessible via at least one query
|
|
expressable via that interface.
|
|
</p>
|
|
|
|
<p>
|
|
For example, to provide access to <em>concise bounded descriptions</em>,
|
|
a service requires only the simplest, minimal query interface -- one
|
|
in which only a single, known, and predefined graph is
|
|
accessible (and need not be explicitly specified)
|
|
and where the URI label of a subject node is specified as the only
|
|
query parameter. An example of such a query interface is the basic
|
|
URIQA query interface <a href="#r4">[4]</a>. Given a source RDF graph
|
|
which is accessible via the basic URIQA interface, which takes a URI as
|
|
its sole query parameter and returns a <em>concise
|
|
bounded description</em> (if successful), every statement in that source
|
|
graph is accessible
|
|
via that simple interface such that, if every URI which
|
|
occurs as the subject of a statement in that source graph is known,
|
|
then if the results of executing a
|
|
basic URIQA query for each such URI are merged together, the merged
|
|
graph will equal the source graph being queried.
|
|
</p>
|
|
|
|
<p>
|
|
Some alternative forms of description, such as those
|
|
presented <a href="#alternatives">below</a>, which explicitly accomodate
|
|
inverse functional
|
|
properties, require slightly more comprehensive query interfaces if all
|
|
statements in the source graph are to be accessible via the same query
|
|
interface. Specifically, in addition to basic URIQA-like queries, they
|
|
must provide for queries comprised minimally of
|
|
a unique arc label for the inverse functional property as well as an object
|
|
node label. If a query service provided access to an RDF graph using only the basic
|
|
URIQA query interface, but returned <em>inverse functional concise bounded descriptions</em>,
|
|
rather than the more general <em>concise bounded descriptions</em>, then there
|
|
could be statements in the source graph relating to resources denoted by
|
|
blank nodes which are inaccessible via that query interface, because they
|
|
would not be included in any <em>concise bounded description</em>
|
|
accessible via the basic URIQA interface, due to the occurrence of inverse
|
|
functional properties in statements about such a resource, and
|
|
one cannot otherwise explicitly inquire about that resource using
|
|
the URIQA query interface, as that interface does not enable one
|
|
to directly inquire about a resource denoted by a blank node.
|
|
</p>
|
|
|
|
<p>
|
|
This should not be seen as a shortcoming inherent in either the query interface
|
|
or the form of description employed, but simply as an
|
|
example of how the interface via which requests about resources
|
|
are made and the form of description provided in responses to such
|
|
requests must be functionally matched. For some applications, requirements
|
|
may dictate first selecting a particular interface, in which case the form(s)
|
|
of description employed in responses should take into account the
|
|
capabilities of the interface to ensure that all statements are accessible
|
|
via that interface in some response. For other applications, requirements may
|
|
dictate first selecting
|
|
the form(s) of description, in which case the interface must take into
|
|
account the form(s) of the description to ensure that all statements
|
|
are accessible via that interface in some response.
|
|
</p>
|
|
|
|
<p>
|
|
The more general and capable the query interface, the more options
|
|
one has regarding suitable forms of descriptions. Though it should
|
|
be noted that the more general the query interface, and the more
|
|
variable the forms of description returned, the greater the implementational
|
|
and processing burden for both the requesting and responding agents.
|
|
That said, even a service employing a fully general query interface
|
|
and supporting numerous possible forms of descriptions can offer significant
|
|
benefit to arbitrary semantic web agents by providing a
|
|
<em>concise bounded description</em> as its default form of description.
|
|
</p>
|
|
</blockquote>
|
|
|
|
<h3 id="Managing_Magnitude">Managing Magnitude</h3>
|
|
<blockquote>
|
|
|
|
<p>
|
|
In certain cases, for certain source graphs, a <em>concise bounded description</em>
|
|
may exceed what is a useful magnitude for a given application or requesting
|
|
agent, and the service may choose to employ various means to limit the number
|
|
of statements included in a response in order to address practical capacity and performance
|
|
issues.
|
|
</p>
|
|
|
|
<p>
|
|
Such practical limitations on the magnitude of responses should be
|
|
employed only when absolutely necessary, and while a partial description may
|
|
still be returned, the service should clearly indicate to the requesting
|
|
agent that such limits have been reached and that the response does not
|
|
constitute a complete <em>concise bounded description</em> per the original
|
|
request. The requesting agent, being made aware of the magnitude limitations,
|
|
can then make informed decisions about how or whether to proceed based on
|
|
the incomplete information it has been provided.
|
|
</p>
|
|
|
|
<p>
|
|
A system may also allow the requesting agent to specify its preferred
|
|
limits, where applicable. Though, whether the limits are defined
|
|
by the system or specified by the requesting agent, it should
|
|
nevertheless be clear in the response whether any such limit was
|
|
reached, and that the response does not otherwise constitute a complete
|
|
description.
|
|
</p>
|
|
|
|
<p>
|
|
The following are only a few possible ways to limit the
|
|
magnitude of responses. There are surely others.
|
|
</p>
|
|
|
|
<h4><a name="bnodebloat">Limit the (over)use of Blank Nodes</a></h4>
|
|
<blockquote>
|
|
|
|
<p>
|
|
The most common cause of unmanagable magnitudes in responses
|
|
corresponding to a <em>concise bounded description</em> is when
|
|
the source graph constitutes a large body of knowledge which
|
|
extensively employs blank nodes to denote "significant"
|
|
resources, and which relies on the use of inverse functional
|
|
properties to uniquely refer to such "significant" blank
|
|
node denoted resources. Many FOAF <a href="#r12">[12]</a> specific
|
|
knowledge bases exhibit such characteristics.
|
|
</p>
|
|
|
|
<p>
|
|
Thus, it is conceivable that for a given source graph, a
|
|
request for a <em>concise bounded description</em> of a
|
|
resource per a particular node in that graph may result
|
|
in a subgraph that contains nearly all of the statements
|
|
in the source graph, or is even equal to the source graph,
|
|
because the description did not terminate at a useful
|
|
boundary, given the extensive use of blank nodes.
|
|
</p>
|
|
|
|
<p>
|
|
A simple way to rectify this particular problem is,
|
|
obviously, to limit the use of blank nodes.
|
|
</p>
|
|
|
|
<p>
|
|
This can be done in several ways. One can simply assign
|
|
dynamically generated URIs employing UUIDs, e.g.</p>
|
|
<blockquote>
|
|
<p>
|
|
<tt>http://example.com/id/12bf7f7a-2698-461a-b20a-587cfcadba80</tt>
|
|
</p>
|
|
</blockquote>
|
|
<p>
|
|
to all otherwise unnamed ("blank") nodes, thus
|
|
providing explicit points of termination in <em>concise
|
|
bounded descriptions</em> which otherwise would not terminate
|
|
at a blank node; as well as explicit means of reference
|
|
by which access to descriptions about those particular resources
|
|
is provided.
|
|
</p>
|
|
|
|
<p>
|
|
One could also adopt the commendable practice of always
|
|
using specifically assigned <tt>http:</tt> URIs <a href="#r6">[6]</a>
|
|
to denote resources which are expected to occur as the object of multiple
|
|
statements and/or be commonly referred to
|
|
in communication between agents, even if statements about such resources
|
|
might otherwise employ inverse functional properties.
|
|
</p>
|
|
</blockquote>
|
|
|
|
<h4 id="Limiting_Path_Length">Limiting Path Length</h4>
|
|
<blockquote>
|
|
|
|
<p>
|
|
The system can limit the length of any non-cyclic path from the starting
|
|
node to any other node in the description subgraph; effectively limiting
|
|
the "depth" of the subgraph from the starting node.
|
|
</p>
|
|
</blockquote>
|
|
|
|
<h4 id="Limiting_Total_Number_of_Statements">Limiting Total Number of Statements</h4>
|
|
<blockquote>
|
|
|
|
<p>
|
|
The system can limit the total number of statements included in the
|
|
description subgraph; possibly employing a particular sequence by
|
|
which statements are added to the description subgraph so that the
|
|
most "significant" or "nearest" statements are included first.
|
|
</p>
|
|
</blockquote>
|
|
|
|
<h4 id="Excluding_or_Limiting_Reifications">Excluding or Limiting Reifications</h4>
|
|
<blockquote>
|
|
|
|
<p>
|
|
The system can exclude statements relating to reifications from
|
|
the description subgraph; or may limit such statements to only
|
|
those relevant to a particular purpose, such as provenance.
|
|
</p>
|
|
</blockquote>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<hr />
|
|
|
|
<h2><a name="alternatives">Alternative Forms of Description</a></h2>
|
|
<blockquote>
|
|
|
|
<p>
|
|
As has been mentioned above, it is expected that some applications may find
|
|
other forms of resource description more optimal than a <em>concise bounded description</em>.
|
|
The following are some possible alternative forms of description which can
|
|
be seen as derivations of the basic form of <em>concise bounded description</em>
|
|
described above, which may be more suitable for particular applications.
|
|
These alternative forms of description are not, however, considered to be as general
|
|
or broadly optimal as a <em>concise bounded description</em>.
|
|
</p>
|
|
|
|
<h3><a name="scbd">Symmetric Concise Bounded Description</a></h3>
|
|
<blockquote>
|
|
|
|
<p>
|
|
A <em>concise bounded description</em>, as defined above, can be seen as an "asymmetric"
|
|
view of a resource; such that it only includes statements which are expressed along arc paths
|
|
in the graph which originate from the starting node (outbound arcs);
|
|
and does not include statements which are expressed along (non-circular) arc paths which
|
|
terminate at the starting node (inbound arcs).
|
|
</p>
|
|
<p>
|
|
Statements which are expressed as outbound arcs from a particular node
|
|
provide <em>primary</em> knowledge about the resource denoted by that
|
|
node, where that resource is of primary focus (the subject). Statements which are expressed
|
|
as inbound
|
|
arc paths to a given node provide <em>secondary</em> knowledge
|
|
about the resource denoted by that node, where that resource is of secondary focus (the object).
|
|
While some applications might be concerned about secondary knowledge about a given
|
|
resource, it is expected that most will be concerned only about primary knowledge.
|
|
Furthermore, there are particular
|
|
magnitude issues for descriptions containing such secondary knowledge, as discussed
|
|
further below.
|
|
</p>
|
|
<p>
|
|
There is a special form of secondary knowledge which has a direct relation to
|
|
(possibly implicit) primary knowledge; and that concerns the use of <em>symmetric</em>
|
|
properties. E.g. consider the properties <tt>dct:hasPart</tt> and <tt>dct:isPartOf</tt>.
|
|
For any statement 'X dct:hasPart Y' we can infer 'Y dct:isPartOf X', and visa versa.
|
|
It may be the case that in a given graph, there are only statements utilizing one
|
|
of a pair of symmetric properties, and therefor, a <em>concise bounded description</em>
|
|
may omit a statement where the starting node occurs as object and thus implicit
|
|
knowledge which could be inferred from knowledge about the symmetric pair of properties
|
|
is not accessible in the description.
|
|
E.g., if in a given graph a given node occurs as
|
|
the object of a <tt>dct:isPartOf</tt> statement but no symmetrical <tt>dct:hasPart</tt> statement
|
|
exists, a <em>concise bounded description</em> beginning at that node will not
|
|
include the <tt>dct:isPartOf</tt> statement and thus the agent recieving the description
|
|
will not have the necessary knowledge (the <tt>dct:isPartOf</tt> statement) from which it
|
|
might infer the symmetrical <tt>dct:hasPart</tt> statement.
|
|
</p>
|
|
<p>
|
|
There are various ways to address symmetric properties. One way is to infer such
|
|
statements on the server side, such that the (inference derived) source graph would in fact
|
|
include the <tt>dct:hasPart</tt> statement and thus it would be included in a <em>concise
|
|
bounded description</em>.
|
|
This is the approach employed by the Nokia Semantic Web Server <a href="#r9">[9]</a>.
|
|
</p>
|
|
|
|
<p>
|
|
Another approach is to return an alternative form of description, which
|
|
includes all statements expressed along both outbound and inbound arc paths,
|
|
terminated in like fashion as a <em>concise bounded description</em> but
|
|
extending from the starting node in both directions;
|
|
thus enabling the requesting
|
|
agent to potentially infer itself any implicit statements based on symmetric
|
|
property pairs. We
|
|
can call this derivative of a <em>concise bounded description</em> a
|
|
<em>symmetric concise bounded description</em>.
|
|
</p>
|
|
|
|
<p>
|
|
Specifically, given a particular node (the starting node) in a particular RDF graph (the
|
|
source graph), a subgraph
|
|
of that particular graph, taken to comprise a <em>symmetric concise bounded description</em>
|
|
of the resource denoted by the starting node, can be identified as follows:
|
|
</p>
|
|
|
|
<ol>
|
|
<li>
|
|
Include in the subgraph all statements in the source graph
|
|
where the object of the statement is the starting node;
|
|
</li>
|
|
<li>
|
|
Recursively, for all statements identified in the subgraph
|
|
thus far having a blank node subject not equal to the starting
|
|
node, include in the subgraph
|
|
all statements in the source graph where the object of the statement is the blank
|
|
node in question and which are not already included in the subgraph.
|
|
</li>
|
|
<li>
|
|
Recursively, for all statements included in the subgraph
|
|
thus far, for all reifications of each statement in the
|
|
source graph, include
|
|
the <em>symmetric concise bounded description</em> beginning from
|
|
the rdf:Statement node of each reification.
|
|
</li>
|
|
<li>
|
|
Include in the subgraph the <em>concise bounded description</em>
|
|
beginning from the starting node.
|
|
</li>
|
|
</ol>
|
|
|
|
<p>
|
|
This produces a subgraph that includes a <em>concise bounded description</em>, given
|
|
the same starting point,
|
|
but in addition, includes all inbound arc paths, up to but not beyond a URIref subject node.
|
|
</p>
|
|
|
|
<p>
|
|
Thus, given the <a href="#sourcegraph">source RDF graph</a> presented above,
|
|
the <em>symmetric concise bounded description</em> of the resource denoted by
|
|
<tt>http://example.com/aReallyGreatBook</tt> corresponds to the
|
|
following subgraph:
|
|
</p>
|
|
|
|
<pre class="box">
|
|
|
|
<?xml version="1.0"?>
|
|
|
|
<rdf:RDF
|
|
xmlns:rdf ="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
|
|
xmlns:rdfs ="http://www.w3.org/2000/01/rdf-schema#"
|
|
xmlns:owl ="http://www.w3.org/2002/07/owl#"
|
|
xmlns:dc ="http://purl.org/dc/elements/1.1/"
|
|
xmlns:dct ="http://purl.org/dc/terms/"
|
|
xmlns:xsd ="http://www.w3.org/2001/XMLSchema#"
|
|
xmlns:foaf ="http://xmlns.com/foaf/0.1/"
|
|
xmlns:ex ="http://example.com/">
|
|
|
|
<rdf:Description rdf:about="http://example.com/aReallyGreatBook">
|
|
<dc:title>A Really Great Book</dc:title>
|
|
<dc:publisher>Examples-R-Us</dc:publisher>
|
|
<dc:creator>
|
|
<rdf:Description>
|
|
<rdf:type rdf:resource="http://xmlns.com/foaf/0.1/Person"/>
|
|
<foaf:name>John Doe</foaf:name>
|
|
<foaf:mbox>john@example.com</foaf:mbox>
|
|
<foaf:img rdf:resource="http://example.com/john.jpg"/>
|
|
<foaf:phone rdf:resource="tel:+1-999-555-1234"/>
|
|
</rdf:Description>
|
|
</dc:creator>
|
|
<dc:contributor>
|
|
<rdf:Description>
|
|
<rdf:type rdf:resource="http://xmlns.com/foaf/0.1/Person"/>
|
|
<foaf:name>Jane Doe</foaf:name>
|
|
</rdf:Description>
|
|
</dc:contributor>
|
|
<dc:language>en</dc:language>
|
|
<dc:format>application/pdf</dc:format>
|
|
<dc:rights>Copyright (C) 2004 Examples-R-Us. All rights reserved.</dc:rights>
|
|
<dct:issued rdf:datatype="http://www.w3.org/2001/XMLSchema#date">2004-01-19</dct:issued>
|
|
<rdfs:seeAlso rdf:resource="http://example.com/anotherGreatBook"/>
|
|
</rdf:Description>
|
|
|
|
<rdf:Statement>
|
|
<rdf:subject rdf:resource="http://example.com/aReallyGreatBook"/>
|
|
<rdf:predicate rdf:resource="http://purl.org/dc/elements/1.1/format"/>
|
|
<rdf:object>application/pdf</rdf:object>
|
|
<rdfs:isDefinedBy rdf:resource="http://example.com/book-formats.rdf"/>
|
|
</rdf:Statement>
|
|
|
|
<rdf:Description rdf:about="http://example.com/aBookCritic">
|
|
<ex:likes rdf:resource="http://example.com/aReallyGreatBook"/>
|
|
</rdf:Description>
|
|
|
|
</rdf:RDF>
|
|
|
|
</pre>
|
|
|
|
<p>
|
|
As mentioned above, there are cases where <em>symmetric concise bounded descriptions</em>
|
|
have magnitude problems; in particular, when a given node occurs as the object of
|
|
an exceptionally large number of statements. E.g. given a persistent triple
|
|
store contaning millions of triples and a source graph which is dynamically
|
|
inferred from
|
|
that triple store by a reasoner, which applies all RDF and RDFS closure rules, the <em>symmetric
|
|
concise bounded description</em> for cases such as <tt>rdf:Resource</tt> and
|
|
similar standard and/or proprietary RDF classes could be immense.
|
|
The managability of this particular form of description will be proportional
|
|
to how frequently a given node occurs as the object of a statement. For
|
|
classificatory terms (as well as literal objects, etc.) applications will likely benefit more from either
|
|
an (asymmetrical) <em>concise bounded description</em> or from a precise
|
|
query regarding specific usage expressed in a more general purpose query
|
|
language.
|
|
</p>
|
|
|
|
</blockquote>
|
|
|
|
<h3><a name="ifcbd">Inverse Functional Concise Bounded Description</a></h3>
|
|
<blockquote>
|
|
|
|
<p>
|
|
As discussed <a href="#bnodebloat">above</a>, in order to managing magnitudes
|
|
of descriptions where the source graph reflects a substantial use of blank nodes
|
|
and inverse functional properties, one may employ a special form of description
|
|
(along with a sufficiently capable query interface) to terminate at, and
|
|
exclude particular statements about, blank nodes occuring as the subject
|
|
of one or more statements which are are expressed using an inverse functional property.
|
|
</p>
|
|
|
|
<p>
|
|
Specifically, given a particular node (the starting node) in a particular RDF graph
|
|
(the source graph), a subgraph
|
|
of that particular graph, taken to comprise an <em>inverse functional concise bounded description</em>
|
|
of the resource denoted by the starting node, can be identified as follows:
|
|
</p>
|
|
|
|
<ol>
|
|
<li>
|
|
Include in the subgraph all statements in the source graph where the subject
|
|
of the statement is the starting node;
|
|
</li>
|
|
<li>
|
|
Recursively, for all statements included in the subgraph
|
|
thus far, for each blank node object:
|
|
<blockquote>
|
|
<p>
|
|
IF there exists at least one statement in the source graph having the blank node
|
|
as subject and where the source graph includes a statement which
|
|
asserts that the predicate of that statement is rdf:type owl:InverseFunctionalProperty
|
|
<a href="#r10">[10]</a>,
|
|
</p>
|
|
<p>THEN</p>
|
|
<blockquote>
|
|
<p>
|
|
For each statement in the source graph having the blank node as subject
|
|
and where the source graph includes a statement which
|
|
asserts that the predicate of that statement is rdf:type owl:InverseFunctionalProperty:
|
|
</p>
|
|
<blockquote>
|
|
<ul>
|
|
<li>
|
|
Include the statement in the subgraph; and
|
|
</li>
|
|
<li>
|
|
If the object of the statement is a blank node, include the
|
|
<em>inverse functional concise bounded description</em> beginning from
|
|
that blank object node;
|
|
</li>
|
|
</ul>
|
|
</blockquote>
|
|
</blockquote>
|
|
<p>ELSE</p>
|
|
<blockquote>
|
|
<p>
|
|
For each statement in the source graph having the blank node as subject:
|
|
</p>
|
|
<blockquote>
|
|
<ul>
|
|
<li>
|
|
Include the statement in the subgraph; and
|
|
</li>
|
|
<li>
|
|
If the object of the statement is a blank node, include the
|
|
<em>inverse functional concise bounded description</em> beginning from
|
|
that blank object node;
|
|
</li>
|
|
</ul>
|
|
</blockquote>
|
|
</blockquote>
|
|
</blockquote>
|
|
</li>
|
|
<li>
|
|
Recursively, for all statements included in the subgraph
|
|
thus far, for all reifications of each statement in the
|
|
source graph, include
|
|
the <em>inverse functional concise bounded description</em>
|
|
beginning from the rdf:Statement node of each reification.
|
|
</li>
|
|
</ol>
|
|
|
|
<p>
|
|
Thus, given the <a href="#sourcegraph">source RDF graph</a> presented above,
|
|
the <em>inverse functional concise bounded description</em> of the resource denoted by
|
|
<tt>http://example.com/aReallyGreatBook</tt> corresponds to the
|
|
following subgraph:
|
|
</p>
|
|
|
|
<pre class="box">
|
|
|
|
<?xml version="1.0"?>
|
|
|
|
<rdf:RDF
|
|
xmlns:rdf ="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
|
|
xmlns:rdfs ="http://www.w3.org/2000/01/rdf-schema#"
|
|
xmlns:owl ="http://www.w3.org/2002/07/owl#"
|
|
xmlns:dc ="http://purl.org/dc/elements/1.1/"
|
|
xmlns:dct ="http://purl.org/dc/terms/"
|
|
xmlns:xsd ="http://www.w3.org/2001/XMLSchema#"
|
|
xmlns:foaf ="http://xmlns.com/foaf/0.1/"
|
|
xmlns:ex ="http://example.com/">
|
|
|
|
<rdf:Description rdf:about="http://example.com/aReallyGreatBook">
|
|
<dc:title>A Really Great Book</dc:title>
|
|
<dc:publisher>Examples-R-Us</dc:publisher>
|
|
<dc:creator>
|
|
<rdf:Description>
|
|
<foaf:mbox>john@example.com</foaf:mbox>
|
|
</rdf:Description>
|
|
</dc:creator>
|
|
<dc:contributor>
|
|
<rdf:Description>
|
|
<rdf:type rdf:resource="http://xmlns.com/foaf/0.1/Person"/>
|
|
<foaf:name>Jane Doe</foaf:name>
|
|
</rdf:Description>
|
|
</dc:contributor>
|
|
<dc:language>en</dc:language>
|
|
<dc:format>application/pdf</dc:format>
|
|
<dc:rights>Copyright (C) 2004 Examples-R-Us. All rights reserved.</dc:rights>
|
|
<dct:issued rdf:datatype="http://www.w3.org/2001/XMLSchema#date">2004-01-19</dct:issued>
|
|
<rdfs:seeAlso rdf:resource="http://example.com/anotherGreatBook"/>
|
|
</rdf:Description>
|
|
|
|
<rdf:Statement>
|
|
<rdf:subject rdf:resource="http://example.com/aReallyGreatBook"/>
|
|
<rdf:predicate rdf:resource="http://purl.org/dc/elements/1.1/format"/>
|
|
<rdf:object>application/pdf</rdf:object>
|
|
<rdfs:isDefinedBy rdf:resource="http://example.com/book-formats.rdf"/>
|
|
</rdf:Statement>
|
|
|
|
</rdf:RDF>
|
|
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>
|
|
Note that any <em>inverse functional concise bounded description</em> is a subgraph
|
|
of, or equivalent to, the <em>concise bounded description</em> obtained
|
|
when starting from the same node in the same RDF graph.
|
|
</p>
|
|
|
|
</blockquote>
|
|
|
|
<hr />
|
|
|
|
<h2><a name="references">Notes and References</a></h2>
|
|
<blockquote>
|
|
|
|
<p><a name="r1">[1]</a> Note on terminology:</p>
|
|
<blockquote>
|
|
<p>
|
|
This document does not constitute a mathematical presentation of concise
|
|
bounded descriptions. The use of terms such as "concise", "bounded",
|
|
"description", "definition", etc. are not to be interpreted according to
|
|
any special meaning or use as mathematical terms. This document is an
|
|
English language presentation of a number of ideas which are considered
|
|
to offer utility to implementors of semantic web or other applications dealing with
|
|
descriptive metadata. More precise mathematical specifications of these
|
|
ideas would be welcome and beneficial. This document does not provide them.
|
|
</p>
|
|
|
|
<!--
|
|
<p>From the Merriam-Webster Online Dictionary:
|
|
|
|
<blockquote>
|
|
<tt>
|
|
<p>
|
|
Main Entry: <b>con·cise</b><br>
|
|
Pronunciation: <tt>k&n-'sIs</tt><br>
|
|
Function: <em>adjective</em><br>
|
|
<b>1</b> <b>:</b> marked by brevity of expression or statement <b>:</b> free from all elaboration and superfluous detail<br>
|
|
<b>2</b> <b>:</b> cut short <b>: <font size="-1">BRIEF</font></b>
|
|
</p>
|
|
|
|
<p>
|
|
Main Entry: <b><sup>4</sup>bound</b><br>
|
|
Function: <em>transitive verb</em><br>
|
|
<b>1</b> <b>:</b> to set limits or bounds to <b>: <font size="-1">CONFINE</font></b><br>
|
|
<b>2</b> <b>:</b> to form the boundary of <b>: <font size="-1">ENCLOSE</font></b><br>
|
|
<b>3</b> <b>:</b> to name the boundaries of
|
|
</p>
|
|
|
|
<p>
|
|
Main Entry: <b>de·scrip·tion</b><br>
|
|
Pronunciation: <tt>di-'skrip-sh&n</tt><br>
|
|
Function: <em>noun</em><br>
|
|
<b>1 a</b> <b>:</b> an act of describing; <em>specifically</em> <b>:</b> discourse intended to give a mental image of something experienced <b>b</b> <b>:</b> a descriptive statement or account
|
|
</p>
|
|
|
|
<p>
|
|
Main Entry: <b>de·fine</b><br>
|
|
Pronunciation: <tt>di-'fIn</tt><br>
|
|
Function: <i>verb</i><br>
|
|
<i>transitive senses</i><br>
|
|
<b>1 a</b> <b>:</b> to determine or identify the essential qualities or meaning of <whatever <i>define</i><i>s </i>us as human> <b>b</b> <b>:</b> to discover and set forth the meaning of (as a word) <b>c</b> <b>:</b> to create on a computer <<i>define</i> a window> <<i>define</i> a procedure><br>
|
|
<b>2 a</b> <b>:</b> to fix or mark the limits of <b>: <font size="-1">DEMARCATE</font></b> <rigidly <i>defined </i>property lines> <b>b</b> <b>:</b> to make distinct, clear, or detailed especially in outline <the issues aren't too well <i>defined</i>><br>
|
|
</tt>
|
|
</blockquote>
|
|
-->
|
|
</blockquote>
|
|
|
|
<p><a name="r2">[2]</a> W3C Semantic Web Activity, <a href="http://www.w3.org/2001/sw/">http://www.w3.org/2001/sw/</a></p>
|
|
<p><a name="r3">[3]</a> Architecture of the WWW, Definition of a Resource, <a href="http://www.w3.org/TR/webarch/#def-resource">http://www.w3.org/TR/webarch/#def-resource</a></p>
|
|
<p><a name="r4">[4]</a> URIQA, <a href="http://sw.nokia.com/uriqa/URIQA.html">http://sw.nokia.com/uriqa/URIQA.html</a></p>
|
|
<p><a name="r5">[5]</a> RDF, RDFS, <a href="http://www.w3.org/RDF/">http://www.w3.org/RDF/</a></p>
|
|
<p><a name="r6">[6]</a> RFC2616 - HTTP 1.1, <a href="http://www.ietf.org/rfc/rfc2616.txt">http://www.ietf.org/rfc/rfc2616.txt</a></p>
|
|
<p><a name="r7">[7]</a> Architecture of the WWW, <a href="http://www.w3.org/TR/webarch/">http://www.w3.org/TR/webarch/</a></p>
|
|
<p><a name="r8">[8]</a> Nokia Semantic Web Server, <a href="http://sw.nokia.com">http://sw.nokia.com</a></p>
|
|
<p><a name="r9">[9]</a> Nokia Semantic Web Server open source distribution, <a href="http://swdev.nokia.com/tools">http://swdev.nokia.com/tools</a></p>
|
|
<p><a name="r10">[10]</a> OWL, <a href="http://www.w3.org/2004/OWL/">http://www.w3.org/2004/OWL/</a></p>
|
|
<p><a name="r11">[11]</a> Named Graphs, Provenance and Trust, <a href="http://www.hpl.hp.com/techreports/2004/HPL-2004-57.html">http://www.hpl.hp.com/techreports/2004/HPL-2004-57.html</a></p>
|
|
<p><a name="r12">[12]</a> FOAF (Friend of a Friend), <a href="http://www.foaf-project.org">http://www.foaf-project.org</a></p>
|
|
|
|
<!--
|
|
<p><a name="rXX">[XX]</a> Merriam-Webster Online Dictionary <a href="http://www.webster.com">http://www.webster.com</a></p>
|
|
-->
|
|
|
|
</blockquote>
|
|
<hr />
|
|
|
|
<p>
|
|
<a href="http://validator.w3.org/check?uri=referer">
|
|
<img src="http://www.w3.org/Icons/valid-xhtml10" alt="Valid XHTML 1.0!" height="31" width="88" />
|
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</a>
|
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</p>
|
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</body>
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