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<h2 align="right">Strawman: RDF-Model-Summary-1</h2>

<h1>Resource Description Framework: Data Model Summary</h1>

<h3>RDF Interest Group Discussion Document</h3>

<dl>
<dt>This Version:</dt>

<dd><a href="/2000/09/rdfmodel/1">
http://www.w3.org/2000/09/rdfmodel/1</a> $Date: 2000/09/08 13:45:48 $<br />
</dd>

<dt>Newest Version:</dt>

<dd><a href="/2000/09/rdfmodel/">
http://www.w3.org/2000/09/rdfmodel/</a><br />
</dd>

<dt>Editor (butcher ;-):</dt>

<dd>Dan Brickley <tt><a href="mailto:danbri@w3.org">
&lt;danbri@w3.org&gt;</a>, World Wide Web Consortium /
ILRT</tt></dd>

<dt>Rather crudely derrived from <a href=
"http://www.w3.org/TR/REC-rdf-syntax/">REC-rdf-syntax-19990222</a>,
whose editors were:</dt>

<dd>Ora Lassila <tt><a href=
"mailto:ora.lassila@research.nokia.com">
&lt;ora.lassila@research.nokia.com&gt;</a></tt>, Nokia Research
Center<br />
Ralph R. Swick <tt><a href="mailto:swick@w3.org">
&lt;swick@w3.org&gt;</a></tt>, World Wide Web Consortium</dd>
</dl>

<p><font size="-1"><a href=
"http://www.w3.org/Consortium/Legal/ipr-notice.html#Copyright">
Copyright</a>&nbsp;&copy;&nbsp;1997,1998,1999,2000 <a href=
"http://www.w3.org">W3C</a> (<a href=
"http://www.lcs.mit.edu">MIT</a>, <a href="http://www.inria.fr/">
INRIA</a>, <a href="http://www.keio.ac.jp/">Keio</a> ), All Rights
Reserved. W3C <a href=
"http://www.w3.org/Consortium/Legal/ipr-notice.html#LegalDisclaimer">
liability,</a> <a href=
"http://www.w3.org/Consortium/Legal/ipr-notice.html#W3CTrademarks">
trademark</a>, <a href=
"http://www.w3.org/Consortium/Legal/copyright-documents.html">
document use</a> and <a href=
"http://www.w3.org/Consortium/Legal/copyright-software.html">
software licensing</a> rules apply.</font></p>

<h2>Status of This Document</h2>

<p><strong>Important:</strong> this document should <em>not</em> be
mistaken for a W3C Specification. The text below was crudely
excerpted from the <a href=
"http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/">RDF Model and
Syntax REC</a>. Unlike that document, this current text is not a
complete work.</p>

<p>This document has been prepared (solely as a personal contribution by
the editor) as a strawman discussion
document for consideration by the <a href="/RDF/Interest/">RDF
Interest Group</a>.
</p>

<p>It was produced by taking the RDF Model and
Syntax specification and removing most of the content that relates
to the RDF 1.0 XML grammar, examples, and other material not
directly relevant to the specification of the RDF data model. It
should be noted that the initial version of this excerpted 'RDF
Model Overview' was <em>not</em> produced with any great attention
to detail, and serves solely as a 'proof of concept' or strawman
sketch.</p>

<p><strong>RDF implementors health warning</strong>: please don't
use this work as a reference document, tempting as it may be. The
sole use for this is to futher the <a href=
"http://lists.w3.org/Archives/Public/www-rdf-interest/2000Sep/0066.html">
discussion</a> on www-rdf-interest and refine the <a href=
"/2000/03/rdf-tracking/">RDF Issues List</a>. It should also be emphasised
that no commitment to any ongoing
maintainance or refinement of this document has been made.

</p>

<p>Comments on this discussion document may be sent to &lt;<a href=
"mailto:www-rdf-interest@w3.org">www-rdf-interest@w3.org</a>&gt;,
the mailing list of the <a href="/RDF/Interest/">RDF Interest
Group</a>.</p>

<hr width="100%" />
<h2><a id="TOC" name="TOC"></a>Table of Contents</h2>

<ol>
<li><a href="#intro">Introduction</a></li>

<li><a href="#basic">Basic RDF Model</a></li>

<li><a href="#container">RDF containers</a></li>

<li><a href="#statements">RDF statements</a></li>

<li><a href="#model">Formal Model for RDF</a></li>

<li><a href="#glossary">Glossary</a></li>

<li><a href="#references">Appendix: References</a></li>
</ol>

<hr width="100%" />
<h2><a id="intro" name="intro"></a>1. Introduction</h2>

<p>The World Wide Web was originally built for human consumption,
and although everything on it is <i>machine-readable</i>, this data
is not <i>machine-understandable</i>. It is very hard to automate
anything on the Web, and because of the volume of information the
Web contains, it is not possible to manage it manually. The
solution proposed here is to use <i>metadata</i> to describe the
data contained on the Web. Metadata is "data about data" (for
example, a library catalog is metadata, since it describes
publications) or specifically in the context of this specification
"data describing Web resources". The distinction between "data" and
"metadata" is not an absolute one; it is a distinction created
primarily by a particular application, and many times the same
resource will be interpreted in both ways simultaneously.</p>

<a id="basic" name="basic"></a>

<h2>2. Basic RDF Model</h2>

<p>The foundation of RDF is a model for representing named
properties and property values. The RDF model draws on
well-established principles from various data representation
communities. RDF properties may be thought of as attributes of
resources and in this sense correspond to traditional
attribute-value pairs. RDF properties also represent relationships
between resources and an RDF model can therefore resemble an
entity-relationship diagram. (More precisely, RDF Schemas &mdash;
which are themselves instances of RDF data models &mdash; are ER
diagrams.) In object-oriented design terminology, resources
correspond to objects and properties correspond to instance
variables.</p>

<p>The RDF data model is a syntax-neutral way of representing RDF
expressions. The data model representation is used to evaluate
equivalence in meaning. Two RDF expressions are equivalent if and
only if their data model representations are the same. This
definition of equivalence permits some syntactic variation in
expression without altering the meaning. (See <a href=
"#stringComparison">Section 6.</a> for additional discussion of
string comparison issues.)</p>

<p>The basic data model consists of three object types:</p>

<table width="90%" summary="presentational table">
<tr valign="top">
<td><a id="resource" name="resource"></a>Resources</td>
<td>All things being described by RDF expressions are called <i>
resources</i>. A resource may be an entire Web page; such as the
HTML document "http://www.w3.org/Overview.html" for example. A
resource may be a part of a Web page; e.g. a specific HTML or XML
element within the document source. A resource may also be a whole
collection of pages; e.g. an entire Web site. A resource may also
be an object that is not directly accessible via the Web; e.g. a
printed book. Resources are always named by URIs plus optional
anchor ids (see [<a href=
"http://www.ietf.org/internet-drafts/draft-fielding-uri-syntax-04.txt">
URI</a>]). Anything can have a URI; the extensibility of URIs
allows the introduction of identifiers for any entity
imaginable.</td>
</tr>

<tr valign="top">
<td><a id="propertyType" name="propertyType"></a><a id="property"
name="property"></a>Properties</td>
<td>A <i>property</i> is a specific aspect, characteristic,
attribute, or relation used to describe a resource. Each property
has a specific meaning, defines its permitted values, the types of
resources it can describe, and its relationship with other
properties. This document does not address how the characteristics
of properties are expressed; for such information, refer to the <a
href="/TR/1998/WD-rdf-schema">RDF Schema specification</a>).</td>
</tr>

<tr valign="top">
<td><a id="statement" name="statement">
</a>Statements&nbsp;&nbsp;</td>
<td>A specific resource together with a named property plus the
value of that property for that resource is an RDF <i>
statement</i>. These three individual parts of a statement are
called, respectively, the <i>subject</i>, the <i>predicate</i>, and
the <i>object</i>. The object of a statement (i.e., the property
value) can be another resource or it can be a literal; i.e., a
resource (specified by a URI) or a simple string or other primitive
datatype defined by XML. In RDF terms, a <i>literal</i> may have
content that is XML markup but is not further evaluated by the RDF
processor. There are some syntactic restrictions on how markup in
literals may be expressed; see <a href="#quoting">Section
2.2.1.</a></td>
</tr>
</table>

<a name="container" id="container"></a>

<h2>3. Container Model</h2>

<p>RDF defines three types of container objects:</p>

<table width="90%" summary="presentational table">
<tr valign="top">
<td><a id="Bag" name="Bag"></a>Bag</td>
<td>An unordered list of resources or literals. <i>Bag</i>s are
used to declare that a property has multiple values and that there
is no significance to the order in which the values are given. <i>
Bag</i> might be used to give a list of part numbers where the
order of processing the parts does not matter. Duplicate values are
permitted.</td>
</tr>

<tr valign="top">
<td><a id="Sequence" name="Sequence"></a>Sequence</td>
<td>An ordered list of resources or literals. <i>Sequence</i> is
used to declare that a property has multiple values and that the
order of the values is significant. <i>Sequence</i> might be used,
for example, to preserve an alphabetical ordering of values.
Duplicate values are permitted.</td>
</tr>

<tr valign="top">
<td><a id="Alternative" name="Alternative">
</a>Alternative&nbsp;&nbsp;</td>
<td>A list of resources or literals that represent alternatives for
the (single) value of a property. <i>Alternative</i> might be used
to provide alternative language translations for the title of a
work, or to provide a list of Internet mirror sites at which a
resource might be found. An application using a property whose
value is an <i>Alternative</i> collection is aware that it can
choose any one of the items in the list as appropriate.</td>
</tr>
</table>

<blockquote><i>Note: The definitions of</i> Bag <i>and</i> Sequence
<i>explicitly permit duplicate values. RDF does not define a core
concept of</i> Set<i>, which would be a</i> Bag <i>with no
duplicates, because the RDF core does not mandate an enforcement
mechanism in the event of violations of such constraints. Future
work layered on the RDF core may define such
facilities.</i></blockquote>

<p>To represent a collection of resources, RDF uses an additional
resource that identifies the specific collection (an <i>
instance</i> of a collection, in object modeling terminology). This
resource must be declared to be an instance of one of the container
object types defined above. The <i>type</i> property, defined
below, is used to make this declaration. The membership relation
between this container resource and the resources that belong in
the collection is defined by a set of properties defined expressly
for this purpose. These membership properties are named simply
"_1", "_2", "_3", etc. Container resources may have other
properties in addition to the membership properties and the <i>
type</i> property. Any such additional statements describe the
container; see <a href="#distributedReferents">Section 3.3</a>,
Distributive Referents, for discussion of statements about each of
the members themselves.</p>

<p>A common use of containers is as the value of a property. When
used in this way, the statement still has a single statement object
regardless of the number of members in the container; the container
resource itself is the object of the statement.</p>

<a name="statements" id="statements"></a> <a id="higherorder" name=
"higherorder"></a>

<h2>4. Modeling Statements and Statements about Statements</h2>

<p>In addition to making statements about Web resources, RDF can be
used for making statements about other RDF statements; we will
refer to these as <i>higher-order statements</i>. In order to make
a statement about another statement, we actually have to build a
model of the original statement; this model is a new resource to
which we can attach additional properties.</p>

<p>Statements are made about resources. A model of a statement is
the resource we need in order to be able to make new statements
(higher-order statements) about the modeled statement.</p>

<p>For example, let us consider the sentence</p>

<blockquote><i>Ora Lassila is the creator of the resource
http://www.w3.org/Home/Lassila.</i></blockquote>

<p>RDF would regard this sentence as a fact. If, instead, we write
the sentence</p>

<blockquote><i>Ralph Swick says that Ora Lassila is the creator of
the resource http://www.w3.org/Home/Lassila.</i></blockquote>

<p>we have said nothing about the resource
http://www.w3.org/Home/Lassila; instead, we have expressed a fact
about a statement Ralph has made. In order to express this fact to
RDF, we have to model the original statement as a resource with
four properties. This process is formally called <i>reification</i>
in the Knowledge Representation community. A model of a statement
is called a <i>reified statement</i>.</p>

<p>To model statements RDF defines the following properties:</p>

<table width="90%" summary="presentational table">
<tr>
<td valign="top"><a id="propObj" name="propObj"></a><a id="subject"
name="subject"></a>subject</td>
<td>The <i>subject</i> property identifies the resource being
described by the modeled statement; that is, the value of the <i>
subject</i> property is the resource about which the original
statement was made (in our example,
http://www.w3.org/Home/Lassila).</td>
</tr>

<tr>
<td valign="top"><a id="propName" name="propName"></a><a id=
"predicate" name="predicate"></a>predicate&nbsp;&nbsp;</td>
<td>The <i>predicate</i> property identifies the original property
in the modeled statement. The value of the <i>predicate</i>
property is a resource representing the specific property in the
original statement (in our example, creator).</td>
</tr>

<tr>
<td valign="top"><a id="value" name="value"></a><a id="object"
name="object"></a>object</td>
<td>The <i>object</i> property identifies the property value in the
modeled statement. The value of the <i>object</i> property is the
object in the original statement (in our example, "Ora
Lassila").</td>
</tr>

<tr>
<td valign="top"><a id="instanceOf" name="instanceOf"></a><a id=
"type" name="type"></a>type&nbsp;&nbsp;&nbsp;</td>
<td>The value of the <i>type</i> property describes the type of the
new resource. All reified statements are instances of
RDF:Statement; that is, they have a <i>type</i> property whose
object is RDF:Statement. The <i>type</i> property is also used more
generally to declare the type of any resource, as was shown in
Section 3, "Containers".</td>
</tr>
</table>

<p>A new resource with the above four properties represents the
original statement and can both be used as the object of other
statements and have additional statements made about it. The
resource with these four properties is not a replacement for the
original statement, it is a model of the statement. A statement and
its corresponding reified statement exist independently in an RDF
graph and either may be present without the other. The RDF graph is
said to contain the fact given in the statement if and only if the
statement is present in the graph, irrespective of whether the
corresponding reified statement is present.</p>

<p>To model the example above, we could attach another property to
the reified statement (say, "attributedTo") with an appropriate
value (in this case, "Ralph Swick").</p>

<p>...</p>

<p>Reification is also needed to represent explicitly in the model
the statement grouping implied by <tt>Description</tt> elements.
The RDF graph model does not need a special construct for <tt>
Description</tt>s; since <tt>Description</tt>s really are
collections of statements, a <tt>Bag</tt> container is used to
indicate that a set of statements came from the same (syntactic)
<tt>Description</tt>. Each statement within a <tt>Description</tt>
is reified and each of the reified statements is a member of the
Bag representing that <tt>Description</tt>. As an example, the RDF
fragment</p>

<h2><a id="model" name="model"></a>5. Formal Model for RDF</h2>

<p>The RDF Model and Syntax specification shows three
representations of the data model; as 3-tuples (triples), as a
graph, and in XML. These representations have equivalent meaning.
The mapping between the representations used in this specification
is not intended to constrain in any way the internal representation
used by implementations.</p>

<p>The RDF data model is defined formally as follows:</p>

<table border="1" width="90%" summary="presentational table">
<tr>
<td>
<ol>
<li>There is a set called <i>Resources</i>.</li>

<li>There is a set called <i>Literals</i>.</li>

<li>There is a subset of <i>Resources</i> called <i>
Properties</i>.</li>

<li><a id="triple" name="triple"></a>There is a set called <i>
Statements</i>, each element of which is a triple of the form

<p>{pred, sub, obj}</p>

<p>Where pred is a property (member of <i>Properties</i>), sub is a
resource (member of <i>Resources</i>), and obj is either a resource
or a literal (member of <i>Literals</i>).</p>
</li>
</ol>
</td>
</tr>
</table>

<p>We can view a set of statements (members of <i>Statements</i>)
as a directed labeled graph: each resource and literal is a vertex;
a triple {p, s, o} is an arc from s to o, labeled by p. This is
illustrated in figure 11.</p>

<center>
<p><img src="fig11.gif" alt="statement graph template" /><a href=
"fig11.html">D</a></p>
</center>

<center>
<p>Figure 11: Simple statement graph template</p>
</center>

<p>This can be read either</p>

<blockquote><i>o is the value of p for s</i></blockquote>

<p>or (left to right)</p>

<blockquote><i>s has a property p with a value o</i></blockquote>

<p>or even</p>

<blockquote><i>the p of s is o</i></blockquote>

<p>For example, the sentence</p>

<blockquote><i>Ora Lassila is the creator of the resource
http://www.w3.org/Home/Lassila</i></blockquote>

<p>would be represented graphically as follows:</p>

<center>
<p><img src="fig12.gif" alt="Simple statement graph" /><a href=
"fig12.html">D</a></p>
</center>

<center>
<p>Figure 12: Simple statement graph</p>
</center>

<p>and the corresponding triple (member of <i>Statements</i>) would
be</p>

<blockquote>{creator, [http://www.w3.org/Home/Lassila], "Ora
Lassila"}</blockquote>

<p>The notation [<i>I</i>] denotes the resource identified by the
URI <i>I</i> and quotation marks denote a literal.</p>

<p>Using the triples, we can explain how statements are reified (as
introduced in Section 4). Given a statement</p>

<blockquote>{creator, [http://www.w3.org/Home/Lassila], "Ora
Lassila"}</blockquote>

<p>we can express the reification of this as a new resource X as
follows:</p>

<blockquote>{type, [X], [RDF:Statement]}<br />
{predicate, [X], [creator]}<br />
{subject, [X], [http://www.w3.org/Home/Lassila]}<br />
{object, [X], "Ora Lassila"}</blockquote>

<p>From the standpoint of an RDF processor, facts (that is,
statements) are triples that are members of <i>Statements</i>.
Therefore, the original statement remains a fact despite it being
reified since the triple representing the original statement
remains in <i>Statements</i>. We have merely added four more
triples.</p>

<p>The property named "type" is defined to provide primitive
typing. The formal definition of type is:</p>

<a id="formalType" name="formalType"></a>

<table border="1" width="90%" summary="presentational table">
<tr>
<td>
<ol start="5">
<li>There is an element of <i>Properties</i> known as
RDF:type.</li>

<li>Members of <i>Statements</i> of the form {RDF:type, sub, obj}
must satisfy the following: sub and obj are members of <i>
Resources.</i> [<a href="/TR/1998/WD-rdf-schema">RDFSchema</a>]
places additional restrictions on the use of type.</li>
</ol>
</td>
</tr>
</table>

<p>Furthermore, the formal specification of reification is:</p>

<a id="formalReification" name="formalReification"></a>

<table border="1" width="90%" summary="presentational table">
<tr>
<td>
<ol start="7">
<li>There is an element of <i>Resources</i>, not contained in <i>
Properties</i>, known as RDF:Statement.</li>

<li>There are three elements in <i>Properties</i> known as
RDF:predicate, RDF:subject and RDF:object.</li>

<li>Reification of a triple {pred, sub, obj} of <i>Statements</i>
is an element r of <i>Resources</i> representing the reified triple
and the elements s<sub>1</sub>, s<sub>2</sub>, s<sub>3</sub>, and
s<sub>4</sub> of <i>Statements</i> such that

<p>s<sub>1</sub>: {RDF:predicate, r, pred}<br />
s<sub>2</sub>: {RDF:subject, r, subj}<br />
s<sub>3</sub>: {RDF:object, r, obj}<br />
s<sub>4</sub>: {RDF:type, r, [RDF:Statement]}</p>
</li>
</ol>
</td>
</tr>
</table>

<p>The resource r in the definition above is called the <i>reified
statement</i>. When a resource represents a reified statement; that
is, it has an RDF:type property with a value of RDF:Statement, then
that resource must have exactly one RDF:subject property, one
RDF:object property, and one RDF:predicate property.</p>

<p>As described in Section 3, it is frequently necessary to
represent a collection of resources or literals; for example to
state that a property has an ordered sequence of values. RDF
defines three kinds of collections: ordered lists, called <i>
Sequences</i>, unordered lists, called <i>Bags</i>, and lists that
represent alternatives for the (single) value of a property, called
<i>Alternatives</i>.</p>

<p>Formally, these three collection types are defined by:</p>

<a id="formalCollection" name="formalCollection"></a>

<table border="1" width="90%" summary="presentational table">
<tr>
<td>
<ol start="10">
<li>There are three elements of <i>Resources</i>, not contained in
<i>Properties</i>, known as RDF:Seq, RDF:Bag, and RDF:Alt.</li>

<li>There is a subset of <i>Properties</i> corresponding to the
ordinals (1, 2, 3, ...) called <i>Ord</i>. We refer to elements of
<i>Ord</i> as RDF:_1, RDF:_2, RDF:_3, ...</li>
</ol>
</td>
</tr>
</table>

<p>To represent a collection <i>c</i>, create a triple {RDF:type,
<i>c</i>, <i>t</i>} where <i>t</i> is one of the three collection
types RDF:Seq, RDF:Bag, or RDF:Alt. The remaining triples {RDF:_1,
<i>c</i>, <i>r</i><sub>1</sub>}, ..., {RDF:_n, <i>c</i>, <i>
r</i><sub>n</sub>}, ... point to each of the members <i>
r</i><sub>n</sub> of the collection. For a single collection
resource there may be at most one triple whose predicate is any
given element of <i>Ord</i> and the elements of <i>Ord</i> must be
used in sequence starting with RDF:_1. For resources that are
instances of the RDF:Alt collection type, there must be exactly one
triple whose predicate is RDF:_1 and that is the default value for
the Alternatives resource (that is, there must always be at least
one alternative).</p>

<a id="glossary" name="glossary"></a>

<h2>Glossary</h2>

<dl>
<dt>Arc</dt>

<dd>A representation of a property in a graph form; specifically
the edges in a directed labeled graph.</dd>

<dt>Attribute</dt>

<dd>A characteristic of an object. In Chapter 6 this term refers to
a specific XML syntactic construct; the <tt>name="value"</tt>
portions of an XML tag.</dd>

<dt>Element</dt>

<dd>As used here, this term refers to a specific XML syntactic
construct; i.e., the material between matching XML start and end
tags.</dd>

<dt>Literal</dt>

<dd>The most primitive value type represented in RDF, typically a
string of characters. The content of a literal is not interpreted
by RDF itself and may contain additional XML markup. Literals are
distinguished from Resources in that the RDF model does not permit
literals to be the subject of a statement.</dd>

<dt>Node</dt>

<dd>A representation of a resource or a literal in a graph form;
specifically, a vertex in a directed labeled graph.</dd>

<dt><a href="#property">Property</a></dt>

<dd>A specific attribute with defined meaning that may be used to
describe other resources. A property plus the value of that
property for a specific resource is a <i>statement</i> about that
resource. A property may define its permitted values as well as the
types of resources that may be described with this property.</dd>

<dt><a href="#resource">Resource</a></dt>

<dd>An abstract object that represents either a physical object
such as a person or a book or a conceptual object such as a color
or the class of things that have colors. Web pages are usually
considered to be physical objects, but the distinction between
physical and conceptual or abstract objects is not important to
RDF. A resource can also be a component of a larger object; for
example, a resource can represent a specific person's left hand or
a specific paragraph out of a document. As used in this
specification, the term resource refers to the whole of an object
if the URI does not contain a fragment (anchor) id or to the
specific subunit named by the fragment or anchor id.</dd>

<dt><a href="#statement">Statement</a></dt>

<dd>An expression following a specified grammar that names a
specific resource, a specific property (attribute), and gives the
value of that property for that resource. More specifically here,
an <i>RDF statement</i> is a statement using the RDF/XML grammar
specified in this document.</dd>

<dt><a href="#triple">Triple</a></dt>

<dd>A representation of a statement used by RDF, consisting of just
the property, the resource identifier, and the property value in
that order.</dd>
</dl>

<h2><a id="references" name="references"></a>Appendix:
References</h2>

<dl>
<dt>[Dexter94]</dt>

<dd>F. Halasz and M. Schwarz. The Dexter Hypertext Reference Model.
Communications of the ACM, 37(2):30--39, February 1994. Edited by
K. Gr&oslash;nb&aelig;ck and R. Trigg. <a href=
"http://www.acm.org/pubs/citations/journals/cacm/1994-37-2/p30-halasz/">
http://www.acm.org/pubs/citations/journals/cacm/1994-37-2/p30-halasz/</a></dd>

<dt>[HTML]</dt>

<dd>HTML 4.0 Specification, Raggett, Le Hors, Jacobs eds, World
Wide Web Consortium Recommendation; <a href=
"http://www.w3.org/TR/REC-html40/">
http://www.w3.org/TR/REC-html40</a></dd>

<dt>[ISO10646]</dt>

<dd>ISO/IEC 10646. The applicable version of this standard is
defined in the XML specification [<a href=
"/TR/REC-xml">XML</a>].</dd>

<dt>[NAMESPACES]</dt>

<dd>Namespaces in XML; Bray, Hollander, Layman eds, World Wide Web
Consortium Recommendation; <a href=
"http://www.w3.org/TR/1999/REC-xml-names-19990114">
http://www.w3.org/TR/1999/REC-xml-names-19990114</a>.</dd>

<dt>[PICS]</dt>

<dd>PICS Label Distribution Label Syntax and Communication
Protocols, Version 1.1, W3C Recommendation 31-October-96; <a href=
"http://www.w3.org/TR/REC-PICS-labels">
http://www.w3.org/TR/REC-PICS-labels</a>.</dd>

<dt>[RDFSchema]</dt>

<dd>Resource Description Framework (RDF) Schemas; Brickley, Guha,
Layman eds., World Wide Web Consortium Working Draft; <a href=
"/TR/1998/WD-rdf-schema">
http://www.w3.org/TR/1998/WD-rdf-schema</a></dd>

<dt><a id="rfc2119" name="rfc2119"></a>[RFC2119]</dt>

<dd>Key words for use in RFCs to Indicate Requirement Levels; S.
Bradner, March 1997; <a href=
"http://www.isi.edu/in-notes/rfc2119.txt">RFC2119</a>.</dd>

<dt>[Unicode]</dt>

<dd>The Unicode Standard. The applicable version of this standard
is the version defined by the XML specification [<a href=
"/TR/REC-xml">XML</a>].</dd>

<dt>[URI]</dt>

<dd>Uniform Resource Identifiers (URI): Generic Syntax;
Berners-Lee, Fielding, Masinter, Internet Draft Standard August,
1998; <a href="http://www.isi.edu/in-notes/rfc2396.txt">
RFC2396</a>.</dd>

<dt>[XML]</dt>

<dd>Extensible Markup Language (XML) 1.0; World Wide Web Consortium
Recommendation; <a href="http://www.w3.org/TR/REC-xml">
http://www.w3.org/TR/REC-xml</a>.</dd>

<dt>[XMLinHTML]</dt>

<dd>XML in HTML Meeting Report; Connolly, Wood eds.; World Wide Web
Consortium Note; <a href="http://www.w3.org/TR/NOTE-xh">
http://www.w3.org/TR/NOTE-xh</a>.</dd>
</dl>

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