Topic Maps
— Compact Syntax
ISO (the International Organization for Standardization) and
IEC (the International Electrotechnical Commission) form the
specialized system for worldwide standardization. National bodies
that are members of ISO or IEC participate in the development of
International Standards through technical committees established by
the respective organization to deal with particular fields of
technical activity. ISO and IEC technical committees collaborate in
fields of mutual interest. Other international organizations,
governmental and non-governmental, in liaison with ISO and IEC, also
take part in the work. In the field of information technology, ISO and
IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the
rules given in the ISO/IEC Directives, Part 2.
ISO/IEC 13250-6
was prepared by Joint Technical Committee
ISO/IEC JTC 1, Information Technology, Subcommittee SC 34, Document
Description and Processing Languages.
ISO/IEC 13250
consists of the following parts, under the
general title Topic Maps:
- Part 1:
Overview and Basic Concepts
- Part 2:
Data Model
- Part 3:
XML Syntax
- Part 4:
Canonicalization
- Part 5:
Reference Model
- Part 6:
Compact Syntax
- Part 7:
Graphical Notation
CTM (Compact Topic Maps) is a text-based notation for representing topic
maps. It provides a simple, lightweight notation that complements the
existing XML-based interchange syntax described in [XTM]
and can be used for
manually authoring topic maps;
providing human-readable examples in documents;
serving as a common syntactic basis for TMCL and TMQL.
The principal design criteria of CTM are compactness, ease of human
authoring, maximum readability, and comprehensiveness rather than
completeness. CTM supports all constructs of the [TMDM],
except item identifiers on constructs that are not topics.
Since CTM is not designed as interchange syntax, care should be taken
when using CTM as a basis for interchanging topic maps.
This part of ISO/IEC13250 should be read in conjunction with
[TMDM] since the interpretation of the CTM syntax is defined
through a mapping from the syntax to the data model there defined.
Topic Maps
— Compact Syntax
This part of ISO/IEC13250 defines a text-based notation for representing
instances of the data model defined in [TMDM]. It also defines
a mapping from this notation to the data model. The syntax is defined
through an EBNF grammar.
The following referenced documents are indispensable for the application
of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document
(including any amendments) applies.
NOTE:
Each of the following documents has a unique identifier that is used to
cite the document in the text. The unique identifier consists of the part
of the reference up to the first comma.
Ed. Note:LH: TODO: Correct citations
Unicode, The Unicode Standard, Version 5.0.0, The Unicode Consortium, Reading, Massachusetts, USA, Addison-Wesley Developer's Press, 2007, ISBN 0-321-48091-0, http://www.unicode.org/versions/Unicode5.0.0/
XML 1.0, Extensible Markup Language (XML) 1.0, W3C, Third Edition, W3C Recommendation, 04 February 2004, http://www.w3.org/TR/REC-xml/
TMDM, ISO 13250-2 Topic Maps — Data Model, ISO, 2006, Lars Marius Garshol, Graham Moore, http://www.isotopicmaps.org/sam/sam-model/
XTM, ISO 13250-3 Topic Maps — XML Syntax, ISO, 2006, http://www.isotopicmaps.org/sam/sam-xtm/
XSDT, XML Schema Part 2: Datatypes Second Edition, W3C, W3C Recommendation, 28 October 2004, http://www.w3.org/TR/xmlschema-2/
RFC3986, RFC 3986 - Uniform Resource Identifiers (URI): Generic Syntax, T. Berners-Lee, R. Fielding, L. Masinter, 2005, http://www.ietf.org/rfc/rfc3986
RFC3987, RFC3987 - Internationalized Resource Identifiers (IRIs), M. Duerst, M. Suignard, 2005, http://www.ietf.org/rfc/rfc3987.txt
The acronym CTM is often used to refer to
the syntax defined in this part of ISO/IEC13250.
Its full name is Compact Topic Maps Syntax.
The namespace for the CTM syntax is http://www.topicmaps.org/ctm/.
A CTM document is a text document that conforms to the CTM syntax.
This clause defines the syntax of CTM documents using an EBNF grammar
based on the notation described in [XML 1.0], and their semantics using
prose describing the mapping from CTM documents to [TMDM].
The full EBNF can be found in Annex A.
The process of exporting a topic map from an implementation's internal
representation of the data model to an instance of a Topic Maps syntax is
known as serialization. The opposite process, deserialization, is the
process of building an instance of an implementation's internal
representation of the data model from an instance of a Topic Maps syntax.
This clause defines how instances of the CTM syntax are deserialized into
instances of the data model defined in [TMDM]. Serialization is
only implicitly defined, but implementations should guarantee that for
any data model instance the CTM serialization produced by the
implementation should when deserialized to a new data model instance
produce one that has the same canonicalization as the original data model
instance, according to [ISO 13250-4].
The input to the deserialization process is:
A CTM document.
-
An absolute IRI. This is the IRI from which the CTM document was
retrieved, known as the document IRI. This IRI shall always be
provided, as it is necessary in order to assign the item identifiers
of the topic items created during deserialization.
If the CTM document was not read from any particular IRI the
application is responsible for providing an IRI considered suitable.
Deserialization is performed by processing each component of the document
in document order. Components are defined in terms of text that matches a
syntactic variable of the EBNF. For each component encountered the
operations specified in the clause for the corresponding syntactic
variable are performed.
Whenever a new information item is created, those of its properties which
have set values are initialized to the empty set; all other properties are
initialized to null.
Each CTM processor must be aware of the following prefixes
- xs
http://www.w3.org/2001/XMLSchema#
This is the namespace for the XML Schema Datatypes.
- ctm
http://www.topicmaps.org/ctm/
This is the namespace for this part of ISO/IEC13250
Ed. Note:
LH: Which clause-ordering makes sense?
Comments are fragments of the character stream which are
ignored by any CTM processor. Comments are allowed where whitespace
characters are allowed. Comments are introduced by a hash
(#) and reach until the end of the current line, or until
the end of the text stream, whatever comes first.
Two or more physical lines may be joined into logical lines using backslash
characters (\): When a physical line ends in a backslash,
it is joined with the following forming a single logical line.
The backslash character and the end-of-line character are not part of the
resulting line.
Neither comments nor tokens (except for strings) can be continued.
To create an IRI from a string unescape the string by replacing
%HH escape sequences with the characters they represent,
and decode the resulting character sequence from UTF-8 to
a sequence of abstract Unicode characters. The resulting string is
turned into an absolute IRI by resolving it against the document IRI.
QNames are used to abbreviate IRIs. They are declared as follows:
A QName a causes a locator to be created. The IRI to which the prefix
is bound to, and the local part are concatenated. The result of
such process is always an absolute IRI.
The prefix should have been bound to an IRI as specified in
3.12.3; if the prefix is unbound, an error is flagged.
NOTE:
The local part can neither end with a forward slash (/) nor
with a colon (:) nor with a hash sign (#), although
these characters are allowed inside it.
IRI references are either QNames or absolute IRIs. They are interchangeable:
Everywhere where an IRI is used a QName can be used (if the prefix was
previously defined).
Topics are referenced by an item identifier, a subject identifier,
or a subject locator.
During deserialization, one topic item is created for each
topic-ref.
If the topic-ref is an identifier, a locator
is created by concatenating the document IRI, a #
character, and the value of the identifier.
The locator is added to the [item identifiers] property of the topic.
If the topic-ref is specified by a subject identifier, a
locator is created and added to the [subject identifiers] property of
the topic.
If the topic-ref is specified by a subject locator, a
locator is created (the introducing = is not part of the locator)
and added to the [subject locators] property of the topic.
If the topic-ref is specified by a wildcard, a locator is
created. The locator should use the document locator as base with a concanated
# and a random identifier. That locator should not be equal to any other
subject identifier, subject locator or item identifier contained in the topic map.
The unique, newly created locator is added to the [item identifiers] property of
the topic. This operation should not cause any merging operation.
If the topic-ref is specified by a named-wildcard, at the
first occurrence of such construct a unique locator is created which shall use
the document locator concatenated with # and a random identifier.
This locator is added to the [item identifiers] property of the newly created topic.
This operation should not cause any merging operation.
If the named-wildcard is used again, the same topic is reused.
NOTE:
For the named-wildcard, no whitespaces are allowed between the
* and the identifier.
The identifier part shall not be used to create a locator, to
avoid merging situations. The locator shall be unique in the topic map.
The difference between an ordinary identifier and a named-wildcard is, that
an identifier causes the creation of the same item identifier if the
topic map is read a second time. A named-wildcard (and an ordinary wildcard)
causes the generation of a newly, random item identifier every time, the CTM
document is read.
If the created topic item is equal to another topic item
(c.f. [TMDM] 5.3); the two topic items are merged
according to the procedure given in [TMDM].
EXAMPLE:
# A topic referenced by the subject locator "http://www.isotopicmaps.org/"
= http://www.isotopicmaps.org/
# A topic referenced by a subject identifier
http://psi.example.org/the-beatles/John
# A topic with a unique, randomly created item identifier, it is not possible to refer to
# this topic inside the CTM document.
* - "A topic"
# A topic with a unique, randomly created item identifier with the possibility to refer
# to it again
*foo - "A new, unique topic"
*foo isa subject # *foo refers to the same topic which was created by the first "*foo" occurrence
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The scope construct is used to assign a scope to an information item.
Each topic-ref element is processed according to the
procedure described in 3.3.6. These topic items
are gathered into a set that is assigned as the value of the [scope]
property of the Topic Maps construct being processed.
The reifier construct is used to refer from the Topic Maps construct on
which it appears to the topic reifying that construct.
The reference is a topic-ref as described in
3.3.6
During deserialization the topic-ref is resolved into a
topic item following the procedure in 3.3.6.
The topic item is set as the value of the [reifier] property of the
Topic Maps construct being processed.
The type construct is used to assign a type to the Topic Maps construct
where this construct occurs.
The type is always a topic, indicated by the topic-ref.
During deserialization the topic-ref produces a topic
item following the procedure in 3.3.6, which is
set as the value of the [type] property of the information item
being processed.
The topicmap component acts as a container for the topic map
but has no further significance. It is declared as follows:
During deserialization the topic is resolved following the
procedure in 3.5 and the topic item is set as the value of the
[reifier] property of the topic map.
NOTE:
If the topicmap-reifier production occurrs more than once in a
CTM document, the produced topic item gets merged with the existing topic map reifier
according to the procedure given in [TMDM].
EXAMPLE:
The following CTM fragments produce exactly the same topic map.
# Fragment A
~ tm-reifier - "My topic map"
# Fragment B
~ tm-reifier
tm-reifier
- "My topic map"
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If additional subject identifiers or subject locators are provided, a
locator is created for each of them according to the procedure described in
3.3.6 and added to the [subject identifiers] (
respectively [subject locators] property of the topic.
If this procedure makes the topic equal to another topic item
(c.f. [TMDM] 5.3); the two topic items are merged
according to the procedure given in [TMDM].
If the topic declaration contains template invocations
3.11, the first argument is automatically
bound to the the produced topic item.
EXAMPLE:
Valid topic definitions
john-lennon
lennon . mccartney . harrison . starr . # Four topics in one line
# A topic defined by its subject locator
= http://www.isotopicmaps.org/ - "The ISO Topic Maps website"
# A topic defined by a subject identifier
http://psi.example.org/the-beatles/John
- "John Lennon"
# A topic with a local identifier and a subject identifier
john http://psi.example.org/the-beatles/John
- "John Lennon"
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During deserialization an association item is created for each
association and added to the [associations] property of
the topic map item. The [type] property of the association is set to
the topic produced by the assoc-type as decribed in
3.3.9.
During deserialization an association role item is created for each
role-type / player pair. The role item is added
to the [roles] property of the association item.
NOTE:
If the assoc-type is an IRI, at least one whitespace shall be
between the IRI and the (, otherwise the parenthesis
will be part of the IRI.
NOTE:
If the association declaration ends with a scope declaration which is
not followed by a refier, there should be a end-of-definition
marker (this part of ISO/IEC13250 recommends an empty line) before any topic declaration
to delimit the scope.
EXAMPLE:
is-member-of(group: The-Beatles, member: john)
# Declaration of an association which refers to topics by their subject identifiers
is-member-of(http://psi.example.org/group : http://psi.example.org/the-beatles ,
http://psi.example.org/member : http://psi.example.org/john )
%prefix tm http://psi.topicmaps.org/iso13250/model/
tm:supertype-subtype(tm:subtype: woman, tm:supertype: human)
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During deserialization the occurrence construct causes an occurrence
item to be created, and added to the [occurrences] property of the topic
item created by the procedure decribed in 3.5.
EXAMPLE:
paul-mccartney
birthday: 1942-02-18 # Occurrence of type "birthday" with the value "1942-02-18" and datatype "xs:date"
# Now a difficult to read (but valid) example
# Topic with identifier "paul-mccartney", occurrence of type "birthday", default name in
# scope "formal" and an occurrence of type "website"
paul-mccartney birthday: 1942-02-18 - "Sir Paul McCartney" @formal website: http://www.paulmccartney.com/
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The name construct is used to add a topic name to a topic. It is
declared as follows:
NOTE:
The colon (:) may be omitted, but for readablitity this part of ISO/IEC13250
recommends the usage of the colon for names where an explicit type is
provided.
During deserialization the name construct causes a topic name item to be
created, and added to the [topic names] property of the topic item created
by the procedure decribed in 3.5.
If the type is not specified, the [type] property of the
topic name item is set to the topic item whose [subject identifiers]
property contains http://psi.topicmaps.org/iso13250/model/topic-name; if no such topic item
exists, one is created.
EXAMPLE:
paul-mccartney - "Paul McCartney" # Name with the default name type, colon omitted
paul-mccartney -: "Paul McCartney" # Name with the default name type, colon provided
john-lennon - "John Lennon" # Default name
- fullname: "John Winston Lennon" # Name of type "fullname"
- surname "Lennon" # Name of type "surname", colon omitted
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The variant construct is used to add a variant name to a topic name. It is
declared as follows:
During deserialization the variant construct causes a variant item to be
created and added to the [variants] property of the topic name item
created by the procedure described in 3.8.
After the scope has been processed, the topics in the [scope]
property of the topic name item created by the parent name construct are
added to the [scope] property of the variant name item.
EXAMPLE:
%prefix tm http://psi.topicmaps.org/iso13250/model/
# Topic with name "John Lennon" which has a variant which represents a sort name
john - "John Lennon" ("lennon, john" @tm:sort)
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Templates are containers for arbitrary Topic Maps constructs and
a subset of directives specified by this part of ISO/IEC13250.
The template body consists of ordinary topics and associations with the
additional possibility to replace topic references
(3.3.6) and literals (3.13) with
variables.
Template declarations are defined as follows:
Everywhere, where a topic-ref or a literal
is allowed, a variable is allowed.
The declaration of a template does not change the topic map content until it has been
used within a template invocation (3.11).
If a template with the same identifier exists, an error is flagged.
If a template contains a prefix directive (3.12.3), the
declared prefix is only valid within the template body. The prefixes, which
were declared outside the template are accessible in the template body.
If a named-wildcard is used inside the template body, this
named-wildcard is accessible only in the template body.
A named-wildcard causes the creation of a topic each
time the template is invoked.
EXAMPLE:
def is-a($instance, $type)
%prefix tm http://psi.topicmaps.org/iso13250/model/
tm:type-instance(tm:instance: $instance, tm:type: $type)
end
# An error is flagged, because the prefix "tm" is not specified.
tm:type-instance(tm:instance: mccartney, tm:type: person)
%prefix wiki http://en.wikipedia.org/wiki/
def is-beatle($member)
# Accessing the prefix "wiki" works, because it was defined in the document's scope
is-member-of(member: $member, group: wiki:The-Beatles)
end
is-beatle(wiki:John_Lennon)
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EXAMPLE:
# Declaration of a template
def born($left, $right, $in)
$left isa person
birthday: $right
born-in(person : $left, birthplace : $in)
end
# Invocation of the template
mccartney born(1942-06-12, Liverpool)
# The same effect would have been the following declaration:
mccartney isa person
birthday: 1942-06-12
born-in(person: mccartney, birthplace: Liverpool)
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The usage of templates make the topic maps much more compact
especially in maps in which one can find a lot of these kind of repetitive
facts around persons like in this example.
If the template name is already used by another template declaration,
an error is flagged.
Template names and topic identifiers do not share the same
namespace. If a template with name A exists, it is still possible to
declare a topic with the identifier A.
The following templates are predefined:
- isa
This template creates a type-instance relationship between two
topics (c.f. [TMDM] 7.2).
def isa($instance, $type)
%prefix tm http://psi.topicmaps.org/iso13250/model/
tm:type-instance(tm:instance: $instance, tm:type: $type)
end
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- iko
The is kind of template creates a supertype-subtype relationship between two
topics (c.f. [TMDM] 7.3).
def iko($sub, $super)
%prefix tm http://psi.topicmaps.org/iso13250/model/
tm:supertype-subtype(tm:subtype: $sub, tm:supertype: $super)
end
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A template invocation causes the statements of the template to be added
to the topic map where the variables in the statements are replaced by the
specified arguments.
If the template was not previously defined, an error is flagged.
If the template-reference is a qname and the prefix was
not created by the procedure described in 3.12.6, an error
is flagged.
If not all variables are bound, an error is flagged.
EXAMPLE:
# Valid template invocations
mccartney isa person
plays-for(The-Beatles, piano)
mccartney has-shoesize 42
# Template invocations outside of topic declarations
plays-for(john, The-Beatles, guitar)
has-shoesize(john, 45)
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Directives are used to define the environment for a CTM processor and
are divided into normative directives and non-normative directives (the
user directives).
Each directive shall occur in a line of its own. Only a comment is allowed in the
the same line.
The encoding directive specifies the character encoding used by the
document.
If the encoding declaration is omitted, UTF-8 encoding is
assumed. The name of the encoding should be given as a string in the
form recommended by [XML 1.0].
If the encoding is provided, it shall occur in the first line of the
document.
The version directive states the version number of the CTM syntax,
which is currently "1.0". It is declared as follows:
The version directive tells the parser which version of the CTM syntax
to use during deserialization. Currently the only legal version is
1.0, as defined by this part of ISO/IEC13250.
While the version directive may be omitted, this part of ISO/IEC13250 recommends its
usage for future compatibility.
If the version directive is provided, it should occur after the encoding directive (second
line of the document). If the encoding directive is omitted, the version directive shall
occur in the first line of the document.
The prefix directive is used to associate an IRI with an identifier.
It is declared as follows:
During deserialization the prefix component binds the identifier to
the IRI (fragment).
If the identifier is already bound, an error is flagged unless the identifier
is bound to one and the same IRI.
The iri-fragment may not be a valid IRI but the concatenation
of the iri-fragment and the local part of
a QName should produce a valid IRI, otherwise an error should be flagged.
EXAMPLE:
The typical use of a prefix is shown in the example
%prefix wiki http://en.wikipedia.org/wiki/
# QNames can be used for subject identifiers / subject locators and as value for occurrences / variants
wiki:John_Lennon
- "John Lennon"
description: wiki:John_Lennon # Generates an occurrence of type "description" with the
# value "http://www.wikipedia.org/wiki/John_Lennon" with
# the datatype "http://www.w3.org/2001/XMLSchema#anyURI"
# Here an IRI fragment is bound to "isbn"
%prefix isbn urn:isbn:
isbn:370264850X # Creates a topic with the subject identifier urn:isbn:370264850X
- "Das kleine Ich bin Ich". # A worthwhile book to read :)
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The include directive is used to include another CTM
document into the CTM file. The other document is referenced by an IRI.
The referenced document should use CTM syntax, otherwise an error is flagged.
During deserialization the include component causes the
referenced topic map to be immediately deserialized into a data model
instance. The new data model instance (B) is then merged into the current
one (A) by
Iterating through all topic item in B's [topics] property and identifying
those topics which contain a locator in their [item identifiers] property
which starts with the referenced document locator.
For each of the identified topics a locator is created by subtracting
the referenced document locator from the item identifier which start
with the referenced document locator.
The result of that subtraction is concatenated with the document
locator of A. The result is an absolute locator which is added
to the [item identifiers] property of the topic.
Adding all topic items in B's [topics] property to A's [topics] property.
Adding all association items in B's [associations] property to A's [associations] property.
NOTE:
Adding topics and associations to A may trigger further merges, as
described in [TMDM].
The mergemap directive is used to merge an external
topic map into the topic map produced by deserializing the CTM topic map.
This directive is declared as follows:
The topic map to be merged can be in any syntax,
which must be declared (using the notation component) if
it is not CTM.
This part of ISO/IEC13250 uses the following identifiers for Topic Maps syntaxes:
- http://www.topicmaps.org/ctm/
This is the namespace for CTM.
- http://www.topicmaps.org/xtm/
This is the namespace for XML Topic Maps.
NOTE:
A CTM processor should be capable to support the above mentioned syntaxes.
For any other syntax a CTM processor may flag an error.
During deserialization the mergemap component causes the
referenced topic map to be immediately deserialized into a data model
instance. The new data model instance (B) is then merged into the current
one (A) by
NOTE:
Adding topics and associations to A may trigger further merges, as
described in [TMDM].
The template.import directive is used to make templates from
another CTM document usable in the current document.
A selective-import causes the import of the specified
templates into the local namespace (if the wildcard (*) is
used, all templates are imported).
After the import the templates can be invoked if they have been specified
in the current CTM document.
A namespace-import causes a prefix to be created. After the
creation of the prefix, the templates can be accessed with the QName notation
(identifier:template-name). If the prefix is already in use,
an error is flagged.
NOTE:
Both directives import the template definitions, only. Topics and
associations are not imported.
EXAMPLE:
%from http://example.org/my-template-lib.ctm import is-member-of, \
plays
mccartney is-member-of The-Beatles
plays piano
# The same as above, using the prefix
%import http://example.org/my-template-lib.ctm as my-lib
mccartney my-lib:is-member-of The-Beatles
my-lib:plays piano
|
For debugging purposes, the stop directive is introduced as follows:
This directive causes the CTM processor to stop processing the document any further.
After the stop directive arbitrary text may occur which is not processed.
EXAMPLE:
mccartney is-member-of The-Beatles
plays piano
%stop
# The following topic declaration is ignored by the CTM processor
john isa person
- "John Lennon"
This text is also ignored
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User directives are directives which may be supported by a CTM processor. In
constrast to the normative directives a CTM processor may ignore them.
Every user directive is introduced by %x- and reach until
the end of the line.
NOTE:
Between the %x- and the identifier no whitespaces
are allowed.
NOTE:
User directives are CTM processor dependent.
EXAMPLE:
The following (imaginary) user directive enables / disables logging
%x-log on
mccartney isa person
%x-log off
|
EXAMPLE:
The following (imaginary) user directive executes a query on a RDBMS and inserts the result
into the topic map.
%x-db-query "SELECT ...." | mysql | awk ’...’ |
|
A compatible CTM processor may issue a warning if it finds an unsupported
user directive but should not stop to read the CTM document.
This part of ISO/IEC13250 does neither specify how the user-directive is executed nor how the
command is further processed (additional split ups may be necessary).
A literal is a string value with an optional datatype.
For convenience, this part of ISO/IEC13250 provides the identification of integers,
decimals, IRIs, dates and dateTime values natively.
This part of ISO/IEC13250 associates the following implicit datatypes to the above mentioned
literals
- iri-ref
-
http://www.w3.org/2001/XMLSchema#anyURI
If the iri-ref is a QName, the
QName converted into an IRI by the procedure described in
3.3.4
- decimal
http://www.w3.org/2001/XMLSchema#decimal
- integer
http://www.w3.org/2001/XMLSchema#integer
- null
http://www.topicmaps.org/ctm/null
- date
http://www.w3.org/2001/XMLSchema#date
- date-time
http://www.w3.org/2001/XMLSchema#dateTime
- string
http://www.w3.org/2001/XMLSchema#string
Any literal can be expressed by representing the value as a string and
appending the datatype qualifier (^^) and a iri-ref
which indicates the datatype.
If a literal occurs inside of an occurrence 3.7
declaration, the occurrence [value] property is set to the value of the literal
and the occurrence [datatype] property is set to the datatype of the literal.
If a literal occurs inside of a variant 3.9
declaration, the variant [value] property is set to the value of the literal
and the variant [datatype] property is set to the datatype of the literal.
EXAMPLE:
"42"^^xs:integer # This declaration has the same effect as just writing 42
"12-22"^^xs:gMonthDay
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The following escape sequences can be used inside strings:
- \u HEX HEX HEX HEX
- @@@TODO: Escape syntax for unicode characters
- \"
- @@@TODO: Escape syntax for double quotes
- \\
- @@@TODO: Escape syntax for backslash
Core Syntax
ISO/IEC 13250:2003, Topic Maps, 2003, http://www.y12.doe.gov/sgml/sc34/document/0322_files/iso13250-2nd-ed-v2.pdf
ISO 13250-4, Topic Maps — Canonicalization, http://www.isotopicmaps.org/sam/cxtm/
