[This archive copy from the posting to XML-DEV mailing list, June 22, 1997]

XML-Data.html

Position Paper from Microsoft
20 June 1997

XML-Data

Authors:

Andrew Layman, Microsoft Corporation
Jean Paoli, Microsoft Corporation
Steve De Rose, Inso Corporation
Henry S. Thompson, University of Edinburgh

Acknowledgements:

We thank Paul Grosso (Arbortext), Sharon Adler (Inso Corporation), Anders Berglund (Inso Corporation), François Chahuneau (AIS/Berger-Levrault), and Edward Jung (Microsoft) for their help and contributions to this proposal.

Copyright (c) 1997 Microsoft Corp.

Abstract

This document provides the specification for exchanging structured and networked data on the Web. This specification uses XML, the Extensible Markup Language for describing data as well as data about data. We expect this specification to be useful for a wide range of applications such as describing database transfers, digital signatures or remotely-located web resources.

1. Introduction

The Internet holds the potential to integrate all information in a global network (with many private but integrated domains). The Internet promises access to information any time and, with wireless technology, anywhere. Today, however, the Internet is merely an access medium to text and pictures. To actualize the Internet's potential, we need to add intelligent search, data exchange, adaptive presentation, and personalization. The Internet must go beyond setting an information access standard, and must set an information understanding standard, which means: a standard way of representing data so that software can better search, move, display, and otherwise manipulate information currently hidden in contextual obscurity.

XML is an important step in this direction. It offers a standard syntax for textual structure of tagged data, based on extensive industry and theoretical experience. Its lexical format easily depicts a tree structure. A tree is a natural format that is richer than a simple flat list, yet (compared to a generalized graph) also respectful of cognitive and data processing requirements for economy and simplicity.

Looking at this point in more detail, there are several ways of structuring data. One is a flat tagging system. In this system, sets of keywords are applied to data elements. This is a simple form of data structure, but it does not capture any relationships between the keywords.

A more advanced means of structuring information is a tree. A tree allows expression of subsumption, containment, or any other single (contextual) relationship such as "manages." Trees correspond to object-oriented class hierarchies, file system hierarchies, organizational hierarchies and so forth. Trees are relatively easy to understand and to construct. Trees are efficient to process, and there is a linear (e.g. textual) structure that a program can parse incrementally, and determine when it is finished. This makes trees particularly useful as a transmission format for asynchronous, distributed systems such as the Internet, and also for display purposes where the single relationship (usually visual containment) enables incremental display.

A still more elaborate structure is a directed graph. A graph allows expression of arbitrary binary relationships, that is, many relationships between two things. A graph can express subsumption, containment, and any number of other relationships simultaneously. It is therefore a superset of a tree. This makes graphs very expressive for real-world semantics, but it also makes them harder to understand, more difficult to construct, and less efficient to process than trees. There is no efficient linear (e.g. textual) structure of a graph that can be incrementally processed. Therefore, while they are particularly useful for representing (and instrumenting) the complete semantics of a system, they are typically not suitable for transmission, display, or immediate processing.

The tree structure is proved broadly implementable and easy to deploy, not just in theory but also widely in practice. Industrial implementations, in the SGML community and elsewhere, demonstrate its intrinsic quality and industrial strength, e.g. aircraft (ATA), automotive (J2008), banking (OFX), and semiconductors (Pinnacles PCIS).

This proposal shows how to add a single convention to XML so that graph arcs are easily added into a lexical tree structure, without requiring decomposition of tree format into a "lowest common denominator" nodes-and-arcs structure. (For a quick look at the difference, see the XML-Data versus MCF in XML comparison.)

XML-Data consists of a collection of related technologies. First, it unifies lexical trees with graph structures. Second, it builds on this to define a representation for schemata based on XML instance syntax. It offers a mechanism to organize element types into a hierarchy, and proposes a small set of basic types. Finally, it adds facilities for lexical typing and proposes a small collection of lexical types.

XML-Data can encode the content, semantics and schemata for a gamut of cases, from simple and prosaic to complex and sophisticated:

• An ordinary document
• A structured record, such as a appointment record or purchase order
• An object, with data and methods
• A data record, such as the result set of a query
• Information in a database or a web site (e.g. CDF)
• Graphical presentation (e.g. an application user interface)
• Upper ontology (standard schema entities and types)
• UberWeb (all the links between information and people on the web)

The resulting flexibility of a single homogenous data representation system allows any reader to uniformly determine the structural semantics of a data element. Information can then be reused for new purposes and in novel contexts. For example, a record from a database of restaurants and a record from a client contact database might be reused in the context of an appointment, say in setting a lunch date with a client. The relationships between the restaurant and contact data do not reside in the schema data described by either database individually, but are extensions defined by the instance of the appointment.

This proposal, building on the earlier Web Collections in XML proposal, shows how to use a single syntax for a broad range of data, using that syntax for data and schemata, permitting the expressiveness of graph data when such power is required, but retaining the benefits of lexical trees.

2. Examples of XML-Data

Data

The following example shows a simple order from a bookstore for several books, a record, and a cup of coffee.

<ORDER>
  <SOLD-TO>
    <PERSON><LASTNAME>Layman</PERSON>
            <FIRSTNAME>Andrew</FIRSTNAME>
    </PERSON>
  </SOLD-TO>
  <SOLD-ON>19970317</SOLD-ON>
  <ITEM>
    <PRICE>5.95</PRICE>
    <BOOK>
      <TITLE>Number, the Language of
Science</TITLE>
      <AUTHOR>Dantzig, Tobias</AUTHOR>
    </BOOK>
  </ITEM>
  <ITEM>
    <PRICE>12.95</PRICE>
    <BOOK>
      <TITLE>Introduction to Objectivist
Epistemology</TITLE>
      <AUTHOR>Rand, Ayn</AUTHOR>
    </BOOK>
  </ITEM>
  <ITEM>
    <PRICE>12.95</PRICE>
    <RECORD>

<TITLE><COMPOSER>Tchaikovsky's</COMPOSER
> First Piano Concerto</TITLE>
      <ARTIST>>Janos</ARTIST>
    </RECORD>
  </ITEM>
  <ITEM>
    <PRICE>1.50</PRICE>
    <COFFEE>
      <SIZE>small</SIZE>
      <STYLE>cafe macchiato</STYLE>
    </COFFEE>
  </ITEM>
</ORDER>

XML-Data is flexible enough to encode heterogeneous structures, for example books, records and coffee all within one sales order. These different kinds of items do not need to all have the same internal parts. For example, books have titles, coffee generally doesn't. XML-Data allows values to be expressed as element content (for example the book titles shown) or with a value attribute (for example the author and artist elements). Properties of elements can be expressed as attributes (e.g. size and style of coffee) or as sub-elements (e.g. author, artist). XML-Data can appear in separate documents or within other documents (such as HTML pages).

Data about Other Data

XML-Data is suitable for complex, self-contained data structures such as the book order, and also for information such as the Channel Definition Format, which describes remotely-located web resources, many of which are themselves data:

<CHANNEL>
  <ITEM
HREF="http://www.zoosports.com/intro.htm"
level="2"
precache="NO">
    <A
HREF="http://www.zoosports.com/page1.htm">
This is a link to page 1.</A>
    <TITLE>Welcome to ZooSports!</TITLE>
    <ABSTRACT>ZooSports articles, news, and promotional
offers</ABSTRACT>
  </ITEM>
  <SCHEDULE ENDDATE="1994-11-05">
    <INTERVALTIME DAY="1"/>
    <EARLIESTTIME HOUR="12"/>
    <LATESTTIME HOUR="18"/>
  </SCHEDULE>
</CHANNEL>

PICS-NG Labels

XML-Data can express PICS-NG Labels:

(This uses the Layman-Bray proposal for namespaces.)

<xml>
  <xml:schema>
    <namespaceDcl
href="http://purl.org/Schemas"
name="purl"/>
    <namespaceDcl
href="http://www.foo.com"
name="foo"/>
  </xml:schema>
  <xml:data>
    <purl:description1
href="http://purl.color.org/document.html">
;
      <title>Light and Dark: A study of
color</title>
      <subject><LCSH>
          <for>Color and Color
Palettes</for></LCSH> </subject>
      <author> <foo:author>
                            <name>John
Smith</name>

<affiliation>thedarkside</affiliation>

<email>john@thedarkside</email></foo:aut
hor>
               <foo:author>
                            <name>Smith, Jane
Q.</name>

<affiliation>thelightregion</affiliation>

<email>jane@thelightregion</email></foo:
author></purl:description1>
  </xml:data>
</xml>

Digital Signatures, Security &Authentication

Returning to the bookstore example, this is the same order with a digital signature added. The structured nature of XML-Data makes it easy to sign whole elements or parts of them.

<ORDER>
  <dsig:DSIG>

<MANIFEST>>80183589575795589189518915</MANIFEST
>
    <SIG
href="http://XYX/Joe@company.com"/>
  </dsig:DSIG>
  <SOLD-TO>
    <PERSON><LASTNAME>>Layman</PERSO>
            <FIRSTNAME>>Andrew</FIRSTNAME>
    </PERSON>
  </SOLD-TO>
  <SOLD-ON>>19970317</SOL>
  <ITEM>
    <PRICE>5.95</PRICE>
    <BOOK>
      <TITLE>Number, the Language of
Science</TITLE>
      <AUTHOR>Dantzig, Tobias</AUTHOR>
    </BOOK>
  </ITEM>
  <ITEM>
    <PRICE>12.95</PRICE>
    <BOOK>
      <TITLE>Introduction to Objectivist
Epistemology</TITLE>
      <AUTHOR>Rand, Ayn</AUTHOR>
    </BOOK>
  </ITEM>
  <ITEM>
    <PRICE>12.95</PRICE>
    <RECORD>

<TITLE><COMPOSER>Tchaikovsky's</COMPOSER
> First Piano Concerto</TITLE>
      <ARTIST>>Janos</ARTIST>
    </RECORD>
  </ITEM>
  <ITEM>
    <PRICE>1.50</PRICE>
    <COFFEE>
      <SIZE>small</SIZE>
      <STYLE>cafe macchiato</STYLE>
    </COFFEE>
  </ITEM>
</ORDER>

Database Information

While XML-Data can represent complex structures, it can also represent simple ones, for example a simple list of database records:

<BOOK-MASTER-LIST>
  <BOOK id="book1">
    <TITLE>Number, the Language of
Science</TITLE>
    <AUTHOR>>Dantzig, Tobias</AUTHOR>
  </BOOK>

  <BOOK id="book2">
    <TITLE>Introduction to Objectivist
Epistemology</TITLE>
    <AUTHOR>>Rand, Ayn</AUTHOR>
  </BOOK>

  <BOOK id="book3">
    <TITLE>I, The Jury</TITLE>
    <AUTHOR>>Spillane, Mickey</AUTHOR>
  </BOOK>

  <BOOK id="book4">
    <TITLE>Half Magic</TITLE>
    <AUTHOR>>Eager, Edward</AUTHOR>
  </BOOK>

  <BOOK id="book5">
    <TITLE>QED</TITLE>
    <AUTHOR>>Feynmann, Richard P.</AUTHOR>
  </BOOK>
<BOOK-MASTER-LIST>

Graph Structures

An XML-Data element may include links to resources outside the immediate tree. When it meets application needs, this href facility can be used to break up a single structure into multiple parts, with relations among them indicated by Universal Resource Identifier (URI) links. The references can be local or remote. In this example, they are inventory records from the database table we just looked at.

<ORDER id="order1">
   <dsig:DSIG>

<MANIFEST>>80183589575795589189518915</MANIFEST
>
     <SIG
href="http://XYX/Joe@company.com"/>
   </dsig:DSIG>
   <SOLD-TO>

<PERSON><LASTNAME>>Layman</PERSO>
              <FIRSTNAME>>Andrew</FIRSTNAME>
      </PERSON>
    </SOLD-TO>
    <SOLD-ON>19970317<</SOLD-ON>
    <ITEM
href="http://bigbookstore.com/data/bookmaster?XML-XPTR=book
1">
      <PRICE>5.95</PRICE>
    </ITEM>
    <ITEM
href="http://bigbookstore.com/data/bookmaster?XML-XPTR=book
2">
      <PRICE>12.95</PRICE>
    </ITEM>
    <ITEM
href="http://bigbookstore.com/data/musicmaster?XML-XPTR=cd1
">
      <PRICE>12.95</PRICE>
    </ITEM>
    <ITEM>
      <PRICE>1.50</PRICE>
      <COFFEE>
        <SIZE>small</SIZE>
        <STYLE>cafe macchiato</STYLE>
      </COFFEE>
    </ITEM>
</ORDER>

Notice that each of the ITEM elements establishes a relationship between the ORDER and a BOOK, and that the relationship itself has attributes, in this case the price at which the book was sold. Relations can have attributes, can contain elements and the process can be carried to any needed level of detail.

Discontiguous Information (propertyOf)

Information about an element can be contained in the element, but also can sit outside it. For example, the following applies a digital signature to a sales order without actually modifying the order:

<dsig:DSIG>
  <xml:propertyOf
href="http://bigbookstore.com/data/orders?XML-XPTR=order1&q
uot;/>
  <MANIFEST
>80183589575795589189518915</MANIFEST>
  <SIG
href="http://XYX/Joe@company.com"/>
</dsig:DSIG>

Schema

Every data object, such as a purchase order, contains certain parts, such as sold-to, sold-on date, items, etc. We can write a formal description of what these parts are and which are allowed where. This is called a "schema" and is written using a form of XML-Data:

<xml:schema ID="BookOrderSchema">
  <!-- This schema is digitally signed. Schemas are a form of data,
       so they, too, can be signed. -->
  <dsig:DSIG>
    <MANIFEST
>*(&#&$&@*$&%*&@*$&$*@</M
ANIFEST>
    <SIG
href="http://XYX/Jane@company.com"/>
  </dsig:DSIG>

  <!-- Here are all the element types, their contents,
       attributes and relations. -->
  <elementType id="ORDER">
    <relation href="#SOLD-TO"/>
    <relation href="#SOLD-ON"/>
    <relation href="#ITEM"
occurs="STAR"/>
  </elementType>
  <relationType id="SOLD-TO">
    <elt href="#PERSON"/>
  </relationType>
  <relationType id="SOLD-ON">  
    <pcdata/>
    <!-- Date is YYYYMMDD -->
    <attribute name="lextype"
default="DATE.ISO8061"
presence="fixed"/>
  </relationType>
  <elementType id="PERSON">
    <relation href="#LASTNAME"/>
    <relation href="#FIRSTNAME"/>
  </elementType>
  <elementType id="LASTNAME">
    <pcdata/>
  </elementType>
  <elementType id="FIRSTNAME">
    <pcdata/>
  </elementType>
  <relationType id="PRICE">
    <pcdata/>
  </relationType>
  <relationType id="ITEM">
    <any/>
    <relation href="#PRICE"/>
    <range href="#BOOK"/>
    <range href="#RECORD"/>
    <range href="#COFFEE"/>
  </relationType>
  <elementType id="BOOK">
    <relation href="#TITLE"/>
    <relation href="#AUTHOR"/>
  </elementType>
  <elementType id="RECORD">
    <relation href="#TITLE"/>
    <relation href="#ARTIST"/>
  </elementType>
  <relationType id="SIZE">
    <pcdata/>
  </relationType>
  <relationType id="STYLE">
    <pcdata/>
  </relationType>
  <elementType id="COFFEE">
    <relation href="#SIZE"/>
    <relation href="#STYLE"/>
  </elementType>
  <elementType id="TITLE">
    <mixed><elt
href="#COMPOSER"/></mixed>
  </elementType>
  <relationType id="AUTHOR">
    <pcdata/>
  </relationType>
  <relationType id="ARTIST">
    <pcdata/>
  </relationType>
  <relationType id="COMPOSER">
    <pcdata/>
  </relationType>
</xml:schema>

Type Extension

Sometimes some elements are variants of others, in which case we can organize the element types into a genus-species hierarchy using the extends attribute:

<xml:schema ID="ArtSchema">
  <elementType id="artistic-work">
    <relation href="#TITLE"/>
  </elementType>
  <elementType id="BOOK"
extends="#artistic-work">
    <relation href="#AUTHOR"/>
  </elementType>
  <elementType id="RECORD"
extends="#artistic-work">
    <relation href="#ARTIST"/>
    <relation href="#COMPOSER"
occurs="OPTIONAL"/>
  </elementType>
  <relationType id="AUTHOR">
    <pcdata/>
  </relationType>
  <relationType id="COMPOSER"
extends="#AUTHOR"/>
  <relationType id="ARTIST">
    <pcdata/>
  </relationType>
</xml:schema>

Here we see that books and records are both types of artistic work, and that a composer is a type of author.

Schema Extension

We can use also use this ability to customize a schema that has useful features, but which is too general. In this example, we show a general schema for orders, then another one that is customized for our bookstore:

<xml:schema
ID="GenericOrderSchema">
  <elementType id="ORDER">
    <relation href="#SOLD-TO"/>
    <relation href="#SOLD-ON"/>
  </elementType>
  <relationType id="SOLD-TO">
    <elt href="#PERSON"/>
  </relationType>
  <elementType id="PERSON">
    <relation href="#LASTNAME"/>
    <relation href="#FIRSTNAME"/>
  </elementType>
  <relationType id="LASTNAME">
    <pcdata/>
  </relationType>
  <relationType id="FIRSTNAME">
    <pcdata/>
  </relationType>
</xml:schema>  


<xml:schema id="BookOrderSchema">
  <elementType id="ORDER"
extends="http://generic.com/genericOrder?XML-XPTR=ID(ORDER)
">
    <relation href="#ITEM"
occurs="STAR"/>
  </elementType>

  <relationType id="ITEM">
    <any/>
    <relation
href="http://generic.com/genericOrder?XML-XPTR=ID(ORDER)"/>
    <range
href="http://art.com/schemata?XML-XPTR=ID(BOOK)&qu
ot;/>
    <range
href="http://art.com/schemata?XML-XPTR=ID(RECORD)&
quot;/>
    <range href="#COFFEE"/>
  </relationType>

  <relationType id="SIZE">
    <pcdata/>
  </relationType>

  <relationType id="STYLE">
    <pcdata/>
  </relationType>

  <elementType id="COFFEE">
    <relation href="#SIZE"/>
    <relation href="#STYLE"/>
  </elementType>
</xml:schema>

<?XML version='1.0' rmd='all'?>
<!doctype schema SYSTEM
"http://www.w3c.org/pub/sotr/schema.dtd">
<xml:schema id='ExampleSchema'>
  <!-- schema goes here. -->
</xml:schema>

<elementType id="example">  <!-- element
example (p*) -->
    <elt href="#p" occurs="STAR"/>
</elementType>
<elementType id="p">       <!-- element p
((#PCDATA|p)*) -->
    <mixed><elt href="#p"/></mixed> 
</elementType>

<elementType id="ARTIST">
  <pcdata/>
</elementType>

<elementType id="animalFriends" >
  <group groupType="OR" occurs="STAR">
    <group groupType="OR" occurs="PLUS">
      <elt href="#cat"/>
      <elt href="#dog"/>
    </group>
    <elt href="#bird"/>
    <elt href="#rabbit"/>
    <elt href="#pig"/>
    <elt href="#fish"/>
  </group>
</elementType>

 <relationType id="favoriteFood"
><mixed/></relationType>
 <relationType id="chases"
><any/></relationType>

 <elementType id="dog" >
   <any/>
   <attribute name="name"/>
   <relation href="favoriteFood"/>
   <relation href="chases"/>
 </elementType>

<elementType id="p1">       <!-- element
p1 ((#PCDATA|p1)*) -->
    <mixed><elt href="#p"/></mixed> 
    <attribute name='id' type='ID'/>  <!-- attlist p id
ID=#IMPLIED
                                                        exm (a|b|c) 'c'
                                                        x CDATA FIXED
'y' -->
    <attribute name='exm' type='ENUMERATION' values='a b
c'default='c'/>
    <attribute name='x' defType='FIXED' default='y'/>
</elementType>

<intEntityDcl name="LTG">
    <entityDef>Language Technology Group</entityDef>
</intEntityDcl>

<extEntityDcl name="dilbert">
    <notation href="#gif"/>
    <systemId
href="http://www.ltg.ed.ac.uk/~ht/dilb.gif"/>
</extEntityDcl>

<notationDcl name="gif">
    <systemId href='http://who.knows.where/'/>
</notationDcl>

<xml:schema>
  <elementType id="animalFriends" >
    <elt href="#pet" occurs="PLUS" />
  </elementType>

  <elementType id="pet" >
    <any/>
  </elementType>

  <elementType id="cat" extends="#pet"/>

  <elementType id="dog"  extends="#pet"/>

</xml:schema>

<animalFriends>
  <cat/>
  <dog/>
  <cat/>
</animalFriends>

<elementType id="dog"
extends="#pet"/>
  <attribute name="age"/>
</elementType>

<xml:schema>
  <elementType id="animalFriends" >
    <elt href="#pet" occurs="PLUS" />
  </elementType>

  <elementType id="pet">
    <any/>
    <attribute id='name'/>
    <attribute id='owner'/>
  </elementType>

  <elementType id="cat" extends="#pet"/>
    <elt href='#kittens'/>
    <attribute id='lives' type='NMTOKEN'/>
  </elementType>

  <elementType id="dog" extends="#pet"/>
    <elt href='#puppies'/>
    <attribute id='breed'/>
  </elementType>
<xml:schema>

<animalFriends>
  <cat name="Fluffy" lives='9'/>
  <pet name="Diego"/>
  <dog name="Gromit" owner='Wallace' breed='mutt'/>
</animalFriends>

<relationType id="height">
  <any/>
</relationType>

<relationType id="#favoriteCatFood"
extends="#favoriteFood"/>

<elementType id="cat" extends="#pet"/>
  <relation href="#height"/>
  <relation href="#favoriteCatFood"
default="Fish"/>
</elementType>

<relationType id="eyeColor"
extends="http://whereever.org/#eyeColor">
    <pcdata/>
</relationType>

<elementType id="cat"
extends="http://whereever.org/#cat"/>
  <relation href="#eyeColor"/>
</elementType>

<relationType id="favoriteFood" >
  <mixed/>
</relationType>

<relationType id="weight" >
  <mixed/>
</relationType>

<elementType id="cat" >
  <implements href="http://whereever.org/#pet" />
  <attribute name="name"/>
  <relation href="#favoriteFood" />
  <relation href="#weight" />
</elementType>

<birthday
lextype="DATE.ISO8061">19541022</birthday>

<relationType id="birthday">
  <attribute name="lextype"
default="DATE.ISO8061"
presence="fixed"/>
</relationType>

<attribute name="price"
presence="REQUIRED"
lextypeIs="number"/>

<relationType id="birthday">
  <attribute name="lextype"
default="DATE.ATTR-YMD"
presence="specified"/>
</relationType>
...
<birthday year="1954"
month="10"
day="22" >

<ORDER id="order1">
  <SOLD-TO
unit="http:/people#person1"/>
  <SOLD-ON value="19970317"/>
  <ITEMS unit="sequence1"/>
</ORDER>

<BOOK id="book1">
  <TITLE value="Number, the Language of
Science"/>
  <AUTHOR unit="http:/people#person2"/>
</BOOK>

<SEQUENCE id="sequence1">
  <ORD UNIT="book1">
    <PRICE value="5.95"/>
  </ORD>
  <ORD UNIT="cd1">
    <PRICE value="12.95"/>
  </ORD>
  <ORD UNIT="book2">
    <PRICE value="6.95"/>
  </ORD>
  <ORD UNIT="food1">
    <PRICE value="1.50"/>
  </ORD>
</SEQUENCE>

<COFFEE id="food1">
  <size value="small"/>
  <style value="cafe macchiato"/>
</RECORD>

<RECORD id="cd1">
  <TITLE value="Rachmaninoff's Second Piano
Concerto"/>
  <ARTIST unit="http:/people#person3"/>
</RECORD>

<BOOK id="book2">
  <TITLE value="The Evolution of
Complexity"/>
  <AUTHOR unit="http:/people#person4"/>
</BOOK>

<ORDER id="order1">
  <SOLD-TO
href="http:/people#person1"/>
  <SOLD-ON value="9970317"/>
  <ITEM>
    <PRICE>5.95</PRICE>
    <BOOK id="book1">
      <TITLE >Number, the Language of
Science</TITLE>
      <AUTHOR
href="http:/people#person2"/>
    </BOOK>
  </ITEM>
  <ITEM>
    <PRICE>12.95</PRICE>
    <RECORD id="cd1">
    <TITLE >Rachmaninoff's Second Piano
Concerto</TITLE>
      <ARTIST
href="http:/people#person3"/>
    </RECORD>
  </ITEM>
  <ITEM>
    <PRICE>6.95</PRICE>
    <BOOK id="book2">
      <TITLE >The Evolution of
Complexity</TITLE>
      <AUTHOR
unit="http:/people#person4"/>
    </BOOK>  
  </ITEM>
  <ITEM>
    <PRICE>1.50</PRICE>
    <COFFEE>
      <SIZE>small</SIZE>
      <STYLE>cafe macchiato</STYLE>
    </COFFEE>
  </ITEM>
</ORDER>

<!ENTITY % nodeattrs 'id ID #IMPLIED'  >
<!-- href is as per XML-LINK, but is not required unless there is
      no content -->

<!ENTITY % exattrs   'extends CDATA #IMPLIED'  >

<!ENTITY % linkattrs 'id ID #IMPLIED
                      href CDATA #IMPLIED' >

<!-- The shared content model of elementType, linkType and
relationType -->
<!-- Omitted element type same as "empty." -->
<!ENTITY % extendedmodel 'implements*,
                          (elt|group|empty|any|pcdata|mixed)?,
                          (relation|attribute)*'>

<!-- The top-level container -->
<!element schema         ((elementType|propertyOf|linkType|
                          relationType|extendType|augmentElementType|
                          intEntityDcl|extEntityDcl|
                          notationDcl|extDcls|c)*)>
<!attlist schema %nodeattrs;>

<!-- Element Type Declarations -->
<!element elementType   (%extendedmodel)>
<!-- Either name or id must be present - - absent name defaults to id
-->
<!attlist elementType %nodeattrs;
                      %exattrs;
                name    CDATA      #IMPLIED>

<!-- Element types allowed in content model -->
<!-- Note this is just short for a model group with only one elt in
it -->
<!element elt           EMPTY>
<!-- Elements can have exponents as well as groups -->
<!-- The href is required -->
<!attlist elt   %linkattrs;
                occurs     (required|optional|star|plus) 'required'>

<!-- A group in a content model, sequential or disjunctive -->
<!element group         ((group|elt)+)>
<!attlist group         %nodeattrs;
                groupType (seq|or) 'seq'
                occurs  (required|optional|plus) 'required'>

<!element any           EMPTY>
<!element empty         EMPTY>
<!element pcdata	EMPTY>

<!-- mixed content is just a flat, non-empty list of elts -->
<!-- We don't need to say anything about #pcdata, it's implied -->
<!element mixed         (elt+)>
<!attlist mixed         %nodeattrs;> 

<!-- Attributes -->
<!-- default value must be present iff presence is specified or fixed
-->
<!-- presence defaults to specified if default is present, else
implied -->
<!-- name attribute is locally unique, defaults to id if absent
-->
<!element attribute  empty>
<!attlist attribute  %linkattrs;
                name    CDATA #IMPLIED
                type
(id|idref|idrefs|entity|entities|nmtoken|nmtokens|
                         enumeration|notation|cdata) 'cdata'
                default CDATA #IMPLIED
                values NMTOKENS #IMPLIED
                presence (implied|specified|required|fixed) #IMPLIED 
                lextypeIs CDATA #IMPLIED>

<!-- Relations - - relationTypes are pointed to from relations,
            just as elementTypes are pointed to from elts -->
<!element relationType  (%extendedmodel;,
                         range*)>
<!attlist relationType  %nodeattrs;
                        %exattrs;
                        name CDATA #IMPLIED >

<!element range empty >
<!attlist range %linkattrs; >

<!element relation  EMPTY>
<!attlist relation  %linkattrs;
                    default CDATA #IMPLIED
                    occurs (required|optional|star|plus) 'optional'>

<!-- For adding attributes to existing element types -->
<!element propertyOf    EMPTY>
<!attlist propertyOf    href CDATA #REQUIRED>

<!element augmentElementType
((relation|attribute)*)>
<!attlist augmentElementType %linkattrs;
                             %exattrs;>

<!-- Shorthand for simple XML-LINKs -->
<!element linkType (%extendedmodel;)>
<!attlist linkType %nodeattrs;
                   %exattrs;
                   name CDATA #IMPLIED
                   role CDATA #IMPLIED
                   title CDATA #IMPLIED
                   show (embed|replace|new) #IMPLIED
                   actuate (auto|user) #IMPLIED
                   behaviour CDATA #IMPLIED >

<!element implements EMPTY>
<!attlist implements href CDATA #REQUIRED>

<!-- Entity Declarations -->
<!-- Note as this is written only external entities
      can have structure without escaping it -->
<!-- Name defaults to id if absent -->
<!element intEntityDcl     (#PCDATA)>
<!attlist intEntityDcl %nodeattrs;
                name    CDATA #IMPLIED>

<!-- The entity will be treated as binary if a notation is present
-->
<!-- systemID and publicId (if present) must have the required syntax
-->
<!element extEntityDcl    ( systemId, publicId?)>
<!attlist extEntityDcl %nodeattrs;
                name    CDATA #IMPLIED
		notation CDATA #IMPLIED>

<!-- Pointers for above -->
<!element systemID      EMPTY>
<!attlist systemID      %linkattrs;>
<!-- Must be empty if href is used -->
<!element publicID      (#PCDATA) >
<!attlist publicID      %linkattrs;>

<!-- Notation Declarations -->
<!-- systemID and publicId (if present) must have the required syntax
-->
<!element notationDcl        (systemId, publicId?)>
<!attlist notationDcl   %linkattrs;
                name    CDATA #IMPLIED>

<!-- External entity with declarations to be included -->
<!-- systemID and publicId (if present) must have the required syntax
-->
<!element extDcls       empty>
<!attlist extDcls
                systemId CDATA #REQUIRED
                publicId CDATA #IMPLIED>

<!-- Namespace Declarations -->
<!-- systemID and publicId (if present) must have the required syntax
-->
<!element namespaceDcl  EMPTY>
<!attlist namespaceDcl  %linkattrs;
                name    CDATA #IMPLIED>