Resource Description Framework (RDF)

Resource Description Framework (RDF) Model and Syntax Specification
http://www.w3.org/TR/REC-rdf-syntax/
Resource Description Framework (RDF) Schema Specification 1.0
http://www.w3.org/TR/2000/CR-rdf-schema-20000327/

RDF Model and Syntax

Language to describe resources
Use metadata (data about data) to describe Web resources
Provides interoperability between applications that exchange machine-understandable information on the Web
Use XML as a syntax

Application Areas

Resource discovery - better search engine capabilities
Cataloging - describe content and content relationship (web pages)
Intelligent software agents - knowledge sharing
Content Rating - PICS
Collections of pages
Intellectual property rights
Privacy preferences and policies
Digital signatures - build the "Web of Trust"

Basic RDF Model

Three object types:

Resources - Things being described by RDF expressions. Resources are always named by URIs

HTML Document
Specific XML element within the document source.
Collection of pages
Book

Properties - Specific aspect, characteristic, attribute or relation used to describe a resource

Creator
Title
Name

Statements - Resource (Subject) +
Property (Predicate) +
Property Value (Object)

Statement Example

John Punin is the creator of the web page http://www.cs.rpi.edu/~puninj/XMLJ/

Subject (Resource) - http://www.cs.rpi.edu/~puninj/XMLJ/
Predicate (Property) - Creator
Object (Literal) - John Punin

Statement Example (2)

John Punin whose e-mail is puninj@cs.rpi.edu is the creator of the web page http://www.cs.rpi.edu/~puninj/XMLJ

RDF statement figure

Basic Serialization Syntax

The RDF root element and namespace:

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
    .
    .
    .
   </rdf:RDF>

The RDF Description Element

Attributes

about - refers to a URI of an existing resource
ID - signals the creation of a new resource

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
      .
      .
      .
     </rdf:Description>
   </rdf:RDF>

RDF Property Elements

Property names must be associated with a schema.
Qualify property names with a namespace prefix
Production propertyElt : <PropertyName> Value </PropertyName>
Value - Description element or String

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:dc="http://purl.org/dc/elements/1.1/">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <dc:creator>John Punin</dc:creator>
     </rdf:Description>
   </rdf:RDF>

Describing Multiple Properties

Describing creator, title and date of the resource http://www.cs.rpi.edu/~puninj/XMLJ/

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:dc="http://purl.org/dc/elements/1.1/">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <dc:creator>John Punin</dc:creator>
       <dc:title>Programming XML in Java</dc:title>
       <dc:date>2001-04-10</dc:date>
     </rdf:Description>
   </rdf:RDF>

Describing Multiple Resources

Use multiple Description elements

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:dc="http://purl.org/dc/elements/1.1/">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <dc:creator>John Punin</dc:creator>
       <dc:title>Programming XML in Java</dc:title>
       <dc:date>2001-04-10</dc:date>
     </rdf:Description>

     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XGMML/">
       <dc:creator>John Punin</dc:creator>
       <dc:title>Extensible Graph Markup and Modeling Language</dc:title>
       <dc:date>2001-04-04</dc:date>
     </rdf:Description>
   </rdf:RDF>

Nesting Resources

Values of properties are Description elements

   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:dc="http://purl.org/dc/elements/1.1/"
	    xmlns:vcard="http://imc.org/vCard/3.0#">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <dc:creator>
          <rdf:Description>
             <vcard:FN>John Punin</vcard:FN>
             <vcard:EMAIL>puninj@cs.rpi.edu</vcard:EMAIL>
          </rdf:Description>
       </dc:creator>
     </rdf:Description>
   </rdf:RDF>

Referring to Resources by Reference

Production propertyElt :
<PropertyName> Value </PropertyName> or
<PropertyName rdf:resource="URI" />

   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:dc="http://purl.org/dc/elements/1.1/"
	    xmlns:vcard="http://imc.org/vCard/3.0#">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <dc:creator rdf:resource="http://www.cs.rpi.edu/students/puninj"/>
     </rdf:Description>

     <rdf:Description about="http://www.cs.rpi.edu/students/puninj">
         <vcard:FN>John Punin</vcard:FN>
         <vcard:EMAIL>puninj@cs.rpi.edu</vcard:EMAIL>
     </rdf:Description>
   </rdf:RDF>

Using XML in Property Elements

Set parseType attribute to Literal

   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns="http://www.w3.org/1998/Math/MathML"
	    xmlns:dc="http://purl.org/dc/elements/1.1/">
     <rdf:Description about="http://www.cs.rpi.edu/TR/tr01/">
       <dc:title rdf:parseType="Literal">
         Solving 
           <mrow>
            <mrow><msup><mi>x</mi><mn>2</mn></msup><mo>+</mo><mn>1</mn></mrow>
            <mo>=</mo><mn>0</mn>
           </mrow>
       </dc:title>
       <dc:date>2020-01-30</dc:date>
     </rdf:Description>
   <rdf:RDF>

Using Abbreviated RDF Syntax

First basic abbreviation syntax:

Property elements converted to attributes

<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	 xmlns:dc="http://purl.org/dc/elements/1.1/">
  <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
    <dc:creator>John Punin</dc:creator>
    <dc:title>Programming XML in Java</dc:title>
    <dc:date>2001-04-10</dc:date>
  </rdf:Description>
</rdf:RDF>


<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	 xmlns:dc="http://purl.org/dc/elements/1.1/">
  <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/"
    dc:creator="John Punin" dc:title="Programming XML in Java"
    dc:date="2001-04-10"/>
</rdf:RDF>

Using Abbreviated RDF Syntax (2)

Second basic abbreviation syntax:

The inner Description element and its contained properties can be written as attributes of the "creator" element

   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
            xmlns:dc="http://purl.org/dc/elements/1.1/"
            xmlns:vcard="http://imc.org/vCard/3.0#">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <dc:creator>
          <rdf:Description about="http://www.cs.rpi.edu/students/puninj">
             <vcard:FN>John Punin</vcard:FN>
             <vcard:EMAIL>puninj@cs.rpi.edu</vcard:EMAIL>
          </rdf:Description>
       </dc:creator>
     </rdf:Description>
   </rdf:RDF>

   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:dc="http://purl.org/dc/elements/1.1/"
	    xmlns:vcard="http://imc.org/vCard/3.0#">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <dc:creator rdf:resource="http://www.cs.rpi.edu/students/puninj"
                   vcard:FN="John Punin"
                   vcard:EMAIL="puninj@cs.rpi.edu"/>
     </rdf:Description>
   </rdf:RDF>

Using Abbreviated RDF Syntax (2)

Third basic abbreviation syntax:

New fact: http://www.cs.rpi.edu/~puninj/XMLJ/ is an instance of a WebPage
rdf:type element adds this new fact
value of the rdf:type can be used directly as element name


   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
   	    xmlns:dc="http://purl.org/dc/elements/1.1/">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <rdf:type rdf:resource="http://www.schemas.org/www/Webpage"/>
       <dc:creator>John Punin</dc:creator>
       <dc:title>Programming XML in Java</dc:title>
       <dc:date>2001-04-10</dc:date>
     </rdf:Description>
   </rdf:RDF>

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:dc="http://purl.org/dc/elements/1.1/"
	    xmlns:s="http://www.schemas.org/www/">
     <s:WebPage about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <dc:creator>John Punin</dc:creator>
       <dc:title>Programming XML in Java</dc:title>
       <dc:date>2001-04-10</dc:date>
     </s:WebPage>
   </rdf:RDF>

RDF Containers

Bag - An unordered list of resources or literals
Sequence - An ordered list of resources or literals
Alternative - A list of resources or literals that represent alternatives for the value of a property

Using the Bag Container

Statement: The authors of the book 0201000237 are Alfred, John and Jeffrey

Statement using Bag Container

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
   	    xmlns:dc="http://purl.org/dc/elements/1.1/">
     <rdf:Description about="urn:ISBN:0-201-00023-7">
       <dc:creator>
           <rdf:Bag>
	      <rdf:li>Alfred</rdf:li>
	      <rdf:li>John</rdf:li>
	      <rdf:li>Jeffrey</rdf:li>
           </rdf:Bag>
       </dc:creator>
     </rdf:Description>
   </rdf:RDF>

Using the Seq Container

Statement: The students of the course csci-2962 in alphabetical order are Elizabeth, George and John.

Statement using Seq Container

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:s="http://www.schemas.org/Course/">
     <rdf:Description about="http://www.cs.rpi.edu/courses/csci-2962">
       <s:students>
           <rdf:Seq>
	      <rdf:li rdf:resource="http://www.cs.rpi.edu/students/er"/>
	      <rdf:li rdf:resource="http://www.cs.rpi.edu/students/gl"/>
	      <rdf:li rdf:resource="http://www.cs.rpi.edu/students/js"/>
           </rdf:Seq>
       </s:students>
     </rdf:Description>
   </rdf:RDF>

Using the Alt Container

Statement: The format of the book 0201000237 are plain text, html and postscript.

Statement using the Alt Container

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:dc="http://purl.org/dc/elements/1.1/">
     <rdf:Description about="urn:ISBN:0-201-00023-7">
       <dc:format>
           <rdf:Alt>
	      <rdf:li>text/html</rdf:li>
	      <rdf:li>text/plain</rdf:li>
	      <rdf:li>application/postscript</rdf:li>
           </rdf:Alt>
       </dc:format>
     </rdf:Description>
   </rdf:RDF>

Making Statements about Containers

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:s="http://www.schemas.org/Course/"
	    xmlns:dc="http://purl.org/dc/elements/1.1/">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <s:homeworks>
           <rdf:Bag ID="pages">
	      <rdf:li rdf:resource="http://www.cs.rpi.edu/~puninj/XMLJ/hw1.html">
	      <rdf:li rdf:resource="http://www.cs.rpi.edu/~puninj/XMLJ/hw2.html">
           </rdf:Bag>
       </s:homeworks>
     </rdf:Description>

     <rdf:Description about="#pages">
        <dc:creator>John Punin</dc:creator>
     </rdf:Description>
   </rdf:RDF>

Making Statements about the Items in a Container

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:s="http://www.schemas.org/Course/"
	    xmlns:dc="http://purl.org/dc/elements/1.1/">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <s:homeworks>
           <rdf:Bag ID="pages">
	     <rdf:li rdf:resource="http://www.cs.rpi.edu/~puninj/XMLJ/hw1.html"/>
	     <rdf:li rdf:resource="http://www.cs.rpi.edu/~puninj/XMLJ/hw2.html"/>
           </rdf:Bag>
       </s:homeworks>
     </rdf:Description>

     <rdf:Description aboutEach="#pages">
        <dc:creator>John Punin</dc:creator>
     </rdf:Description>
   </rdf:RDF>

Statements about Statements

Using Reification

RDF defines the following properties:

rdf:subject - The resource being described
rdf:predicate - The original property
rdf:object - The property value in the statement
rdf:type - rdf:Statement

Gerard Uffelman says that John Punin is the creator of the web page http://www.cs.rpi.edu/~puninj/XMLJ/

Statemens about Statments Diagram

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:dc="http://purl.org/dc/elements/1.1/"
	    xmlns:s="http://www.schemas.org/schema/">
     <rdf:Description>
       <rdf:subject rdf:resource="http://www.cs.rpi.edu/~puninj/XMLJ/"/>
       <rdf:predicate rdf:resource="http://purl.org/dc/elements/1.1/creator"/>
       <rdf:object>John Punin</rdf:object>
       <rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Statement"/>
       <s:attributedTo>Gerard Uffelman</s:attributedTo>
     </rdf:Description>
   </rdf:RDF>

Non-Binary Relations

RDF data model only supports binary relations (relation between two resources)
Solution: Use an intermediate resource with additional properties

Statement: The cost of the book 0201000237 is $45.00

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:s="http://www.schemas.org/Units/">
     <rdf:Description about="urn:ISBN:0-201-00023-7">
       <s:price rdf:parseType="Resource">
          <rdf:value>45.00</rdf:value>
          <s:units rdf:resource="http://www.schemas.org/Units/USdollar"/>
       </s:price>
     </rdf:Description>
   </rdf:RDF>

The Dublin Core Metadata

Facilitate discovery of electronic resources
http://www.purl.org/dc/ (http://dublincore.org/)
The defined Dublin Core properties are:

contributor
coverage
creator
date
description
format
identifier
language
publisher
relation
rights
source
subject
title
type

Types values used with the type element

collection
dataset
event
image
interactive resource
model
party
physical object
place
service
software
sound
text

RDF example:

   <?xml version="1.0"?>
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns:dc="http://purl.org/dc/elements/1.1/">
     <rdf:Description about="http://www.cs.rpi.edu/~puninj/XMLJ/">
       <dc:creator>John Punin</dc:creator>
       <dc:title>Programming XML in Java</dc:title>
       <dc:description>Home Page of Programming XML in Java course</dc:description>
       <dc:date>2001-04-10</dc:date>
       <dc:format>text/html</dc:format>
       <dc:identifier>http://www.cs.rpi.edu/~puninj/XMLJ/</dc:identifier>
       <dc:subject>XML, Java, Computer Science Course</dc:subject>
       <dc:publisher>RPI Computer Science Department</dc:publisher>
       <dc:language>en</dc:language>
       <dc:type>electronic document</dc:type>
     </rdf:Description>
   </rdf:RDF>

Content hiding for RDF inside HTML

Use RDF abbreviated syntax

   <html>
     <head>
       <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	        xmlns:dc="http://purl.org/dc/elements/1.1/">
          <rdf:Description about=""
                           dc:creator="John Punin"
			   dc:title="Programming XML in Java"
			   dc:date="2001-04-10"/>
       </rdf:RDF>
       <title>Programming XML in Java</title>
     </head>
     <body>
      .
      .
      .
     </body>
   </html>

RDF Resources and Tools

W3C RDF
http://www.w3.org/RDF/
List of RDF resources
http://www.ilrt.bris.ac.uk/discovery/rdf/resources/
SiRPAC - Simple RDF Parser & Compiler (Java)
http://www.w3.org/RDF/Implementations/SiRPAC/
Libwww - RDF Parser (C)
http://www.w3.org/Library/

RDF Schema

Basic vocabulary to describe RDF vocabularies
Defines properties of the resources (e.g., title, author, subject, etc)
Defines kinds of resources being describes (books, Web pages, people, etc)
XML Schema gives specific constraints on the structure of an XML document
RDF Schema provides information about the interpretation of the RDF statements

RDF Classes and Properties

Namespace prefix:'rdfs' URI:'http://www.w3.org/2000/01/rdf-schema#'
Resources may be instances of one or more classes
Indicated by rdf:type property
Resource http://www.cs.rpi.edu/students/puninj is rdf:type Student
Resource http://www.cs.rpi.edu/students/puninj is rdf:type Person
Student is rdfs:subClassOf Person
Property-centric approach

Define properties in terms of the classes of resource to which they apply

RDF Core Classes

rdfs:Resource - resources are instances of this class
rdf:Property - properties are instances of this class
rdfs:Class - similar to a Class in object-oriented programming language.

RDF Core Properties

rdf:type - indicates that a resource is a member of a class
rdfs:subClassOf - specifies subset/superset relation between classes
rdfs:subPropertyOf - specifies that a property is a specialization of another
rdfs:seeAlso - specifies a resource that provides additional information
rdfs:isDefinedBy - indicates the resource defining the subject resource

RDF Constraints

rdfs:range

Indicate the classes that the values of a property must be members.
The value of a range property is a rdf:Class

rdfs:domain

Indicate the classes on whose members a property can be used.
No domain property means that the property may be used with any resource.

RDF Documentation

rdfs:comment - human-readable description of a resource
rdfs:label - human-readable version of a resource name

RDF Schema Example

   <?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#">

   <rdfs:Class rdf:ID="Person">
     <rdfs:comment>Person Class</rdfs:comment>
     <rdfs:subClassOf 
          rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Resource"/>
   </rdfs:Class>

   <rdfs:Class rdf:ID="Student">
     <rdfs:comment>Student Class</rdfs:comment>
     <rdfs:subClassOf rdf:resource="#Person"/>
   </rdfs:Class>

   <rdfs:Class rdf:ID="Teacher">
     <rdfs:comment>Teacher Class</rdfs:comment>
     <rdfs:subClassOf rdf:resource="#Person"/>
   </rdfs:Class>

   <rdfs:Class rdf:ID="Course">
     <rdfs:comment>Course Class</rdfs:comment>
     <rdfs:subClassOf rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Resource"/>
   </rdfs:Class>

   <rdf:Property rdf:ID="teacher">
      <rdfs:comment>Teacher of a course</rdfs:comment>
      <rdfs:domain rdf:resource="#Course"/>	      
      <rdfs:range rdf:resource="#Teacher"/>	      
   </rdf:Property>

   <rdf:Property rdf:ID="students">
      <rdfs:comment>List of Students of a course in alphabetical order</rdfs:comment>
      <rdfs:domain rdf:resource="#Course"/>	      
      <rdfs:range rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Seq"/>
   </rdf:Property>

   <rdf:Property rdf:ID="name">
      <rdfs:comment>Name of a Person or Course</rdfs:comment>
      <rdfs:domain rdf:resource="#Person"/>	      
      <rdfs:domain rdf:resource="#Course"/>	      
      <rdfs:range rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Literal"/>	      
   </rdf:Property>

   </rdf:RDF>

RDF Example

   <?xml version="1.0"?> 
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	    xmlns="http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#">
     <Course rdf:ID="csci_2962">
        <name>Programming XML in Java</name>
        <teacher>
            <Teacher rdf:ID="jp">
               <name>John Punin</name>
	    </Teacher>
        </teacher>
        <students>
            <rdf:Seq>
	        <rdf:li>
	           <Student rdf:ID="er">
		      <name>Elizabeth Roberts</name>
		   </Student>
	        </rdf:li>
	        <rdf:li>
	           <Student rdf:ID="gl">
		      <name>George Lucas</name>
		   </Student>
	        </rdf:li>
	        <rdf:li>
	           <Student rdf:ID="js">
		      <name>John Smith</name>
		   </Student>
	        </rdf:li>
            </rdf:Seq>
        </students>
     </Course>
   </rdf:RDF>

Triples of the RDF Example

Using Sirpac
Triples of the data model : number (subject, predicate, object)

   1 (online:#csci_2962, http://www.w3.org/1999/02/22-rdf-syntax-ns#type,
   http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#Course)
   2 (online:#csci_2962,
   http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#name Programming,
   XML in Java)
   3 (online:#jp, http://www.w3.org/1999/02/22-rdf-syntax-ns#type
   http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#Teacher)
   4 (online:#jp, http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#name,
   John Punin)
   5 (online:#csci_2962,
   http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#teacher,
   online:#jp)
   6 (online:#genid5, http://www.w3.org/1999/02/22-rdf-syntax-ns#type,
   http://www.w3.org/1999/02/22-rdf-syntax-ns#Seq)
   7 (online:#er, http://www.w3.org/1999/02/22-rdf-syntax-ns#type,
   http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#Student)
   8 (online:#er,
   http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#name, 
   Elizabeth Roberts)
   9 (online:#genid5, http://www.w3.org/1999/02/22-rdf-syntax-ns#_1,
   online:#er)
   10 (online:#gl, http://www.w3.org/1999/02/22-rdf-syntax-ns#type,
   http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#Student)
   11 (online:#gl,
   http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#name, George Lucas)
   12 (online:#genid5, http://www.w3.org/1999/02/22-rdf-syntax-ns#_2,
   online:#gl)
   13 (online:#js, http://www.w3.org/1999/02/22-rdf-syntax-ns#type,
   http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#Student)
   14 (online:#js, http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#name,
   John Smith)
   15 (online:#genid5, http://www.w3.org/1999/02/22-rdf-syntax-ns#_3,
   online:#js)
   16 (online:#csci_2962,
   http://www.cs.rpi.edu/~puninj/XMLJ/course_schema.rdf#students,
   online:#genid5)

   The number of triples = 16