An Introduction to RDF and the Semantic Web

1
An Introduction to RDF and the Semantic Web

• Dr. Randy Kaplan

2
Resource Description Framework

• RDF
• Least Understood standard to come from the W3C
• May be the most powerful
• In order that the web achieve its potential
• May be the most important
• In order that the web achieve its potential

3
Resource Description Framework

• Why RDF?
• With HTML and XML we can swap our documents
  easily
• No meaning is attached to them - they are just
  data
• RDF addresses the problem of meaning in the data
  on the web

4
What We Need To Know

• When we exchange data we need to know things
  like,
• Who wrote the data
• When was the data written
• When was the data last updated
• These pieces of data are not data per se but the
  data about the data or meta data

5
XML

• Promised to deliver us from the unstructured data
  that makes up the Internet
• XML brings structure to the data
• Because HTML combined the appearance of the
  document with the content of the document it, the
  content was extremely hard to extract
• XML separated content from presentation

6
XML

• XML specifically dealt with the data of the
  content

ltmusic genre classicalgt lttitlegtEine Kleine
Nacht Muziklt/titlegt ltcomposergtMozartlt/composergt ltk
eygtE Flatlt/keygt lttempogt2/4lt/tempogt lt/musicgt
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XML

• We could convey some of the same information with
  different data

ltdocument type classical musicgt ltnamegtEine
Kleine Nacht Muziklt/namegt ltauthorgtMozartlt/authorgt
lt/documentgt
8
XML

• What if we wanted to find all pieces of music
  composed by Mozart?
• We would have to find all documents where the
  ltcomposergt element had a value of Mozart.
• We would also have to find all documents where
  the ltauthorgt element had a value of Mozart.

9
XML

• If there was another element used to denote the
  creator of the music then that term would have to
  be searched for also
• In order to be able to find all compositions
  written by Mozart without having to identify all
  elements designating the creator of the music
  then the same term would have to be used to
  identify the creator

10
XML

• This problem could also be solved by indicating
  that when the term composer is used, it means the
  same when another document says written by, and
  another says created by
• This would be quite an undertaking though as it
  involves identifying all words and phrases in all
  languages having this meaning

11
Missing

• Our ability to know that one or more terms mean
  the same thing is the thing that is missing from
  the Internet
• If we can build this layer into the Internet, it
  will take the information to a fundamentally
  different level

12
Dublin Core

• 1995
• Conference in Dublin, Ohio
• Discussed issues of semantics
• Agreed to a core set of themes common to all
  documents
• Set of properties became known as the Dublin Core
  (DC) initiative

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Dublin Core

• 3 Core Properties
• DC.Title
• DC.Creator
• DC.Subject
• 15 core properties were defined in the Dublin
  core (originally)

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Dublin Core

• The Dublin Core can be applied to XML

ltmusic genre classicalgt lttitlegtEine Kleine
Nacht Muziklt/titlegt ltCreatorgtMozartlt/Creatorgt ltkey
gtE Flatlt/keygt lttempogt2/4lt/tempogt lt/musicgt
ltdocument type classical musicgt ltnamegtEine
Kleine Nacht Muziklt/namegt ltCreatorgtMozartlt/Creator
gt lt/documentgt
15
Dublin Core

• Even though we now have used the same element to
  identify the entity responsible for creating the
  we dont know if the meaning of Creator is the
  same in both of these instances
• The only way to be sure is to use a very precise
  mechanism to identify the element being used

16
Dublin Core

• The Dublin Core can be applied to XML

ltmusic genre classicalgt lttitlegtEine Kleine
Nacht Muziklt/titlegt ltdc.Creator
xmlnsdchttp//purl.org/dc/elements/1.1/gtMozart
lt/dc.Creatorgt ltkeygtE Flatlt/keygt lttempogt2/4lt/tempogt
lt/musicgt
ltdocument type classical musicgt ltnamegtEine
Kleine Nacht Muziklt/namegt ltdc.Creator
xmlnsdchttp//purl.org/dc/elements/1.1/gtMozart
lt/dc.Creatorgt lt/documentgt

• Now we can see that these elements refer to
  exactly the same concept

17
CD Database

• Suppose you keep a small database of CDs on your
  computer
• There is a table in the database as below

Primary Key Album Name Artist
1 The Ecleftic Two Sides II a Book Wyclef Jean
2 Eine Kleine Nacht Muzik Mozart
3 Soultrane John Coltrane
4 The Real Eminem
18
Another CD Database

• There is a second database kept by another person
  who has a CD collection
• A table in the database is shown below

Key Title Performer
1 Eine Kleine Nacht Muzik Mozart
2 The Ecleftic Wyclef Jean
3 Kind of Blue Miles Davis
19
Comparing Databases

• Exchanging Information
• If we wanted to share information there would be
  a problem since the tuple names are different
• The same solution we used in the XML can be used
  in the database - the unique identifier

20
Another CD Database

• There is a second database kept by another person
  who has a CD collection
• A table in the database is shown below

Primary Key http//purl.org/dc/elements/1.1/Title http//purl.org/dc/elements/1.1/Creator
1 The Ecleftic Two Sides II a Book Wyclef Jean
2 Eine Kleine Nacht Muzik Mozart
3 Soultrane John Coltrane
4 The Real Eminem
21
Another CD Database

• There is a second database kept by another person
  who has a CD collection
• A table in the database is shown below

Key http//purl.org/dc/elements/1.1/Title http//purl.org/dc/elements/1.1/Creator
1 Eine Kleine Nacht Muzik Wolfgang Amadeus Mozart
2 The Ecleftic Wyclef Jean
3 Kind of Blue Miles Davis
22
URIs

• Uniform Resource Identifiers (URIs) give us a
  way to insure that the meaning of the column of
  data between databases is the same so long as the
  column is labeled with the same URI

23
Other Problems

• Unfortunately when we look at the databases we
  notice some other problems

Primary Key http//purl.org/dc/elements/1.1/Title http//purl.org/dc/elements/1.1/Creator
1 The Ecleftic Two Sides II a Book Wyclef Jean
2 Eine Kleine Nacht Muzik Mozart
3 Soultrane John Coltrane
4 The Real Eminem
Key http//purl.org/dc/elements/1.1/Title http//purl.org/dc/elements/1.1/Creator
1 Eine Kleine Nacht Muzik Wolfgang Amadeus Mozart
2 The Ecleftic Wyclef Jean
3 Kind of Blue Miles Davis
24
Other Problems

• Problem 1
• Albums which may be the same have different names
• Problem 2
• Different names are used to denote the same
  composers

25
Taxonomies

• These problems can be solved through the use of
  taxonomy
• A taxonomy is a -
• Controlled vocabulary of words
• Usually about a constrained topic
• Unique identifiers are key to developing
  taxonomies

26
Taxonomies

• If we were to devise a controlled classification
  list so we could tell which CDs were which genre
  then we would avoid problems like having one CD
  labeled as classical and another CD labeled as
  classic

27
Taxonomies

• CD Taxonomy
• Jazz
• Classical
• Soul
• Pop
• Hip Hop
• Folk

28
Taxonomies

• We are not limited to taxonomies of of music
• We could have type of performance, i.e., play,
  movie, live performance, etc.

29
Moving the Problem

• We really didnt solve the problem we described
  earlier
• We only moved the problem up a level
• We now have the problem with having more than one
  taxonomy for the same thing

30
Moving the Problem

• Consider
• http//taxonomies.org/Plays/PorgyAndBess
• http//taxonomies.org/Albums/PorgyAndBess
• We do not know whether the PorgyAndBess in the
  first reference is the same as the PorgyAndBess
  in the same reference

31
We Need An Authority Figure

• Let us imagine that there is some authority that
  keeps track of al CDs that are released
• This is similar to books and their ISBN numbers
  which are unique
• We will call the fictitious authority
  MuzicBiz.org
• MuzicBiz.org maintains a central database of CDs
  that have been released

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Tables Now ...
Key http//purl.org/dc/elements/1.1/Title http//purl.org/dc/elements/1.1/Creator
http//MuzicBiz.org/Album/1011234 Eine Kleine Nacht Muzik Wolfgang Amadeus Mozart
http//MuzicBiz.org/Album/7655432 The Ecleftic Wyclef Jean
http//MuzicBiz.org/Album/8997654 Kind of Blue Miles Davis
Key http//ebiz.org/Stock http//ebiz.org/Cost
http//MuzicBiz.org/Album/1011234 5 16.00
http//MuzicBiz.org/Album/7655432 4 19.00
http//MuzicBiz.org/Album/8997654 10 12.00
33
Unique Identifiers

• Since we are guaranteed that these identifiers
  ALWAYS refer to the same CD any table row having
  a specific key will ALWAYS refer to the same CD -
  there is NO reason to doubt this
• Data validity is enforced

34
Meta-Data

• Meta-Data
• Data that describes data
• Creator, Type, Date are all kinds of meta-data
• So far the meta-data we have described consists
  of two values - an attribute name and an
  attribute value

35
Meta-Data

• To be precise we need to add one more piece of
  meta-data to complete any meta-data we might have
• Since it is entirely possible to have as Creator,
  the value Mozart, we need to identify what/where
  Mozart is the creator of - the so-called DOCUMENT

36
Triples

• The combination of Source, Attribute name, and
  Value makes what is called in the RDF-biz a
  TRIPLE and that constitutes a fundamental element
  in RDF

37
Transporting Triples

• We will assume the following -
• Meta-data can be expressed as a set of triples
• Key to sharing meta-data is the URI
• Now given that we accept this representation, the
  next challenge is to decide how we will share
  this information (transport)

38
Sharing Meta-Data and Data
Key http//ebiz.org/Stock http//ebiz.org/Cost
http//MuzicBiz.org/Album/1011234 5 16.00
http//MuzicBiz.org/Album/7655432 4 19.00
http//MuzicBiz.org/Album/8997654 10 12.00

• The database contains the information as
  organized in the table above
• We need to transform this data into the accepted
  form, i.e., triples

39
Sharing Data and Meta-Data
Document Name Value
http//MuzicBiz.org/Album/1011234 http//ebiz.org/Stock 5
http//MuzicBiz.org/Album/1011234 http//ebiz.org/Cost 16.00
http//MuzicBiz.org/Album/7655432 http//ebiz.org/Stock 4
http//MuzicBiz.org/Album/7655432 http//ebiz.org/Cost 19.00
http//MuzicBiz.org/Album/8997654 http//ebiz.org/Stock 10
http//MuzicBiz.org/Album/8997654 http//ebiz.org/Cost 12.00
40
Sharing Data and Meta-Data
Document Name Value
http//MuzicBiz.org/Album/1011234 http//ebiz.org/Stock 5
http//MuzicBiz.org/Album/1011234 http//ebiz.org/Cost 16.00
http//MuzicBiz.org/Album/7655432 http//ebiz.org/Stock 4
http//MuzicBiz.org/Album/7655432 http//ebiz.org/Cost 19.00
http//MuzicBiz.org/Album/8997654 http//ebiz.org/Stock 10
http//MuzicBiz.org/Album/8997654 http//ebiz.org/Cost 12.00

• We have adequately represented the meta-data and
  it is ready for transport via XML
• But this table only represents the meta-data and
  does not relate to any data described by it

41
Sharing Data and Meta-Data
Document Name Value
http//MuzicBiz.org/Album/1011234 http//ebiz.org/Stock 5
http//MuzicBiz.org/Album/1011234 http//ebiz.org/Cost 16.00
http//MuzicBiz.org/Album/7655432 http//ebiz.org/Stock 4
http//MuzicBiz.org/Album/7655432 http//ebiz.org/Cost 19.00
http//MuzicBiz.org/Album/8997654 http//ebiz.org/Stock 10
http//MuzicBiz.org/Album/8997654 http//ebiz.org/Cost 12.00

• We need a way to identify the document that the
  meta-data describes
• For this purpose we add a name/value pair that
  names the URL of the document

42
Sharing Data and Meta-Data
ltdocument type"News Item" url"http//www.ePoli
tix.com/Articles/0000005a4787.htm" xmlnsdc"http
//purl.org/dc/elements/1.1/"gt ltdcTitlegtI will
stand says Portillolt/dcTitlegt ltdcCreatorgtCraig
Hoiylt/dcCreatorgt ltdcSubjectgtTory leadership
contestlt/dcSubjectgt lt/documentgt
43
RDF Model and Syntax

• RDF Model
• In this case the model we are speaking of are the
  triples
• The definition of RDF is representation
  independent
• This means that XML is only one way of writing RDF

44
RDF Terminology

• In RDF terminology a STATEMENT is used to
  describe a triple
• This term arises from using a triple to make a
  statement about a document

45
RDF Terminology

• Triples
• Resources and Properties
• In the RDF specification the name part of the
  name/value pair is regarded as a PROPERTY
• The subject of the meta data is regarded as a
  RESOURCE

46
RDF Terminology

• Triples
• A triple is the combination of the three parts -
  a resource with a property and a value

47
RDF Terminology

• A triple can express a relationship between
  resources

Resource Property Value
http//MuzicBiz.org/Albums/7655432 http//MuzicBiz.org/Prop/Track http//MuzicBiz.org/Tracks/1667653
Track
http//MuzicBiz.org/Albums/7655432
http//MuzicBiz.org/Tracks/1667653
48
RDF Terminology
Track
http//MuzicBiz.org/Albums/7655432
http//MuzicBiz.org/Tracks/1667653

• The terminology for this model is the SUBJECT of
  our statement is the album and the track is the
  OBJECT
• The two resources are joined by a PREDICATE
• The predicate specifies the nature of the
  relationship between the two resources

49
RDF Terminology

• Notation
• When writing about RDF it is useful to be able to
  show statements or sets of triples for discussion

50
Notation

• English
• English is simplist
• Craig Hoy is the author of http//www.ePolitix.com
  /Articles/0000005a4787.htm

51
Notation

• SUBJECT has a PREDICATE of OBJECT
• Example
• http//www.ePolitix.com/Articles/0000005a4787.htm
  has an author of Criag Hoy

52
Notation

• Directed labeled graphs

.../Articles/000000005a4787.htm
Craig Hoy
author
53
Notation

• Three parts of a triple

http//MuzicBiz.org/Review, http//MuzicBiz.o
rg/Albums/101234, A relaxing album to prune
to.
http//MuzicBiz.org/Review, http//MuzicBiz.o
rg/Albums/7655432, Lively! Perfect when mowing
the lawn.
http//MuzicBiz.org/Review, http//MuzicBiz.o
rg/Albums/8997654, Very moody. Great when
planning your next planting.
54
Notation

• Complex sets of data can most compactly be
  represented in a graph

.../Articles/000000005a4787.htm
ltdcCreatorgt
.../Authors/Craig20Hoy
ltdcPublishergt
ltxyzJobTitlegt
.../companynumber/3935644
Editor
ltdcCreatorgt
55
RDF Syntax

• So far weve seen how RDF models meta data
• Now we need to look at how these models are
  expressed in XML

56
RDF/XML
57
How is a statement formed?

• Statement begins -
• Reference to the resource that the statement is
  about (SUBJECT)
• This is in the rdfabout attribute of the
  ltrdfDescriptiongt element

58
How is a statement formed?

• The statement is located inside the
  ltrdfDescriptiongt element
• Says there is a property of this resource -
  dcCreator that has a value of Craig Hoy

59
Many Namespaces

• When there are many namespaces to be defined in
  an RDF document grouping them in one place makes
  them stand out

60
RDF Elements

• ltrdfDescriptiongt Element
• Contains the URI for the resource being described
• The ltrdfDescriptiongt element identifies the
  subject
• A child element defines a predicate/object pair

61
ltrdfDescriptiongt

• More detail about this element -
• Multiple properties for the same resource
• String literals and resource URIs
• Nesting statements
• rdfabout attribute

62
ltrdfDescriptiongt

• More detail about this element -
• The rdfID attribute
• Anonymous resources
• The rdftype attribute

63
ltrdfDescriptiongt

• The ltrdfDescriptiongt element is actually a
  container for as many predicate/object pairs are
  you want

.../Articles/000000005a4787.htm
Craig Hoy
dcCreator
dcPublisher
ePolitix
64
ltrdfDescriptiongt

• One or more properties may be specified for the
  same resource

65
ltrdfDescriptiongt

• An alternative syntax
• Attributes take the place of child elements

66
ltrdfDescriptiongt

• In order that a resource not be confused with a
  string literal, there is an RDF attribute

67
ltrdfDescriptiongt

• Supposing we wanted to add some information to
  the description

ltdcCreatorgt
../Articles/000000005a4787.htm
../Authors/Craig20Hoy
ltdcPublishergt
ltgt
../companynumber/3935644.htm
Editor
68
ltrdfDescriptiongt

• One way to code this in RDF is to simply add a
  statement that contains the new information

69
ltrdfDescriptiongt

• RDF allows for the ltrdfDescriptiongt element to
  be nested

70
ltrdfDescriptiongt

• Both representations are correct and the
  underlying model is the same in both cases
• Which to use depends on context?
• If there are many articles, the nested
  information would be repeated
• Therefore the first representation would be
  preferable in this case

71
ltrdfDescriptiongt

• Attributes
• We know about the rdfabout attribute
• The contents of the rdfabout attribute are a URI

72
ltrdfDescriptiongt

• rdfID
• This attribute allows a resource in a document to
  be named and then referred to with this name
• The ID attribute and the about attribute ARE
  EXCLUSIVE - only one or the other can be used

73
rdfID
74
Anonymous Resources

• An option for the ltrdfDescriptiongt element would
  be to NOT specify an rdfabout or rdfID
  attribute
• This would be the way to introduce anonymous
  resources as part of an RDF description
• The description element would exist for no other
  reason then to be given properties

75
Anonymous Resources
76
Anonymous Resources
77
Anonymous Resources

• Back to Mozart
• Assume that some authority has given the piece
  Eine Kleine Nacht Muzik the URL
• http//MuzikBiz.org/233456
• We can also give this piece of music an assigned
  code from the Dewey Decimal Classification code
• 781.68

78
Anonymous Resources

• The resulting statement describing this would be
• http//MuzikBiz.org/233456 has a dcSubject of
  781.68
• The RDF is shown following

79
Anonymous Resources
80
Anonymous Resources

• If we want now to identify the source of this
  classification we can do so with the RDF value
  tag (shown following)

81
Anonymous Resources
82
Anonymous Resources

• When representing an anonymous resource like this
  one, we know that there is some resource we are
  representing, we just dont know how to name it
• This is why we introduce an ltrdfDescriptiongt tag
  into the RDF without an rdfabout tag
• The result is a graph with an anonymous node

83
Anonymous Resources
84
rdftype Attribute

• Applies to ltrdfDescriptiongt
• Powerful
• Links worlds of knowledge representation to
  object orientation (ooh ... aah)
• Allows us to specify that the resource being
  referred to is of a particular class
• Allows parsers to understand more about the meta
  data

85
rdfType Attribute

• Assume that an organization named the
  International Press Telecommunications Council
  (IPTC) is responsible for the XML format used in
  the ePolitix articles we have been using

86
rdfType Attribute

• IPTC has defined a URI that allows us to indicate
  that the article being referred to is in their
  NITF format
• NITF refers to News Industry Text Format
• This format is used widely to transfer news
  between organizations

87
rdftype Attribute

• The URL for all object types that belong to the
  NITF group of objects is something like -
• http//www.iptc.org/schema/NITF

88
rdftype Attribute

• This information could be used to enhance the RDF
  used in the ePolitix XML

89
rdfType Attribute

• Now the rdftype attribute gives us a very
  powerful capability akin to one that we would
  find in object-oriented programming
• Once we know that a particular resource is of a
  particular type then we can use that information
  to check its meta-tags to insure that the correct
  meta-tags are used

90
rdftype Attribute

• For example if we are referring to a person
  resource AND we have said that a person has a
  FORMAT then this is probably incorrect
• (The dcFormat property is used to specify the
  type of MIME documet)

91
rdftype Attribute

• But we know nothing more at this point about the
  resource
• http//www.ePolitix.com/Authors/Craig20Hoy
• By specifying an rdftype we can give the RDF
  processor more information

92
rdftype Attribute
93
Typed Elements

• An alternate syntax to use to express the same
  type of information are known as TYPED ELEMENTS
• In this notation the resource that would be used
  in the rdftype attribute would be turned into a
  namespace qualified element

94
Typed Elements

• We assumed a namespace prefix for objects created
  by the IPTC for their NITF stnadard
• The namespace prefix was
• http//www.iptc.org/schema/NITF
• It is now possible to create object types or
  references to schemas by specifying a URI as in
• http//www.iptc.org/schema/NITFNewsArticle

95
Typed Elements

• By assigning the prefix that was just defined to
  a namespace paceholder, and use the classname as
  the name of an element the ltrdfDescriptiongt
  element can be replaced
• ltrdftype rdfresourcehttp//www.itpc.org/schema
  /NITFNewsArticle gt
• ltrdfRDF xmlnsnitfhttp//www.iptc.org/schema/NI
  TF

96
Typed Elements

• becomes

Namespace definitions
97
Typed Elements

• This feature is very important to RDF
• Anything which can appear in an RDF description
  tag,
• is valid when used as a typed element

98
Typed Elements
Observe the change to attributes
99
Typed Elements

• Being able to do this allows you to extract data
  from existing XML documents in the form of
  triplese

100
Property Elements

• Property Information can be expressed through -
• String literals
• value for a predicate defined by the name of the
  element containing the literal

101
Property Elements

• Said a lot about the ltrdfDescriptiongt elements
  so far
• Recap
• String Literals
• Value for a predicate that is defined by the name
  of the element containing the literal

102
Property Elements

• Example

103
Property Elements

• Resources
• Express properties of a resources
• The value of the predicate is actually another
  resource
• Use a URI to specify which resource it is

104
Property Elements

• Example

105
Property Elements

• Yet another way to accomplish this is to nest RDF
  statements one within another
• This says that the value of the property
  ltdcCreatorgt is itself a resource

106
Property Elements

• Type information can also be specified in the
  content of a property element

107
Property Elements

• Taking a type resource and turning it into a
  namespace-qualified element name could abbreviate
  this

108
parseTypeLiteral

• Sometimes it is necessary to tell the parser that
  it should NOT parse a particular part of the RDF
• The RDF should be stored as is
• Consider the following example

109
parseTypeLiteral

• We are writing a mathematical paper entitled
  Ramifications of (ab)2 to World Peace
• We would like to create a MathML to specify the
  title since it can help us format the various
  symbols properly
• If we place the MathML inside the ltdcTitlegt tag
  we need a way to tell the RDF parser that the
  MathML is not RDF

110
Ramifications of ...

• The contents of this element are not simply a
  string
• The text must be well-formed XML otherwise the
  parser will fail

111
parseTypeResource

• There are times when the parser cannot tell the
  difference between a property value and a
  resource
• Property values are usually inside an
  rdfDescription element

112
parseTypeResource

• If this were all there is to it then all would be
  well. Unfortunately RDF allows us to make
  statements about the author as follows

113
parseTyperesource

• What is all we wanted to do was to provide the
  email of the author?
• We really dont care about identifying the author

114
parseTypeResource

• This still seems too elaborate
• We could simply express this information as
  follows

115
parseTypeResource

• So if we were to interpret this we would come up
  with two different interpretations making its
  meaning ambiguous
• On the one hand if we evaluated the
  representation from the inside out we would have
  an anonymous ltdcCreatorgt element which has a
  ltvEmailgt property

116
parseTypeResource
First Interpretation Inside Out
117
parseTypeResource

• If you interpret the RDF representation from the
  outside in you would say you had a resource of a
  web page that had a ltdcCreatorgt property and
  that this ltdcCreatorgt property refers to an
  anonymous resource of rdftype vEmail

118
parseTypeResource
119
parseType Resource

• This second interpretation of the RDF/XML is the
  one that we would prefer but the parser cannot
  distinguish which of these two models it should
  create
• The problem is we need the ltdcCreatorgt element
  to be interpreted as both a web page and also as
  an anonymous resource so properties can attach to
  it

120
parseType Resource

• RDF/XML does allow us to force the ltdcCreatorgt
  to be interpreted as
• a predicate
• an anonymous resource

121
parseType Resource

• which is exactly the same as specifying the
  anonymous resource explicitly

122
Containers

• Containers
• list of resources
• collection of resources
• Example
• List of articles that make up a web site
• List of authors who have contributed to an article

123
Containers

• RDF
• Three types of containers
• bag
• sequence
• alternative
• can be used anywhere the ltrdfdescriptiongt
  element can be used

124
ltrdfBaggt

• simplest container
• used to contain multiple values for a property
• no significance to the order of the values

125
ltrdfBaggt

• Example
• The elements in a bag may also be literals

126
ltrdfSeqgt

• Whereas a bag does not impose any order on the
  elements in the list that is associated with the
  element, ltrdfSeqgt does require that the list
  attached to it will be in a specific order

127
ltrdfSeqgt

• Example

128
ltrdfAltgt

• ltrdfAltgt provides us with a way to select from a
  list of resources, a specific resource
• In other words ltrdfAltgt provides a way of
  specifying alternative options
• An rdf processor could choose a resource based on
  some desirable property

129
ltrdfAltgt

• Example