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Subject: [xtm-wg] Reification of topic map constructs
Notes regarding the reification of topic map constructs as topic
nodes.
Note 1: "Reification" herein means "The creation of a topic node in a
topic map graph whose subject is what is being reified." The
subject is said to be "reified" by the topic node. Every
topic node reifies a subject. In a topic map graph, no two
topic nodes are supposed to reify the same subject, although
human error may permit this to happen, since machines cannot
intuit subjects. All of the topic characteristics that are
known about a given subject are supposed to be combined in the
one topic node that corresponds to that subject.
Note 2: <topic> elements specify their subjects by pointing at
"subject descriptors". Subject descriptors are information
objects. Every information object potentially describes two
subjects that are usually very different:
(1) One subject is whatever subject is described by the
information object (subject descriptor).
(It is very likely that only a human being can make a
determination as to the exact nature of the subject that
is described by a subject descriptor, and only by
perceiving some rendition of the information object.
However, by means of carefully designed subject
descriptors, it seems that it is possible for useful
"nature of subject" determinations to be made by machine.)
Here's an example of a subject descriptor:
<paragraph id="p1">The domesticated sheep has four legs,
is herbivorous, is not very intelligent, and is usually
exploited for its wool, milk, and flesh.</paragraph>
The above paragraph describes the domesticated sheep. If
the paragraph were referenced as a subject descriptor of a
<topic>, and if it were being used as a "subject
describing" subject descriptor, then it's reasonable to
conclude that the subject of the topic is the domesticated
sheep.
(2) The other subject is the subject descriptor itself,
considered as an information object. If the same
paragraph (id="p1") were used as a "subject is itself"
subject descriptor of a topic, then the subject of the
topic would *not* be domesticated sheep. Instead, the
subject would be the paragraph element shown above, whose
unique identifier is "p1".
The subject descriptors of a topic node are specified in a
topic map document by means of <topicRef> elements in
<resource> elements within <occurrences> elements within the
<topic> element corresponding to that topic node. The
distinction between the two kinds of subject is made by an
attribute of the <topicRef> elements called -referent-. The
-referent- attribute must have one of two possible values:
"describesSubject" (the default) and "isSubject". These two
values correspond to the two cases described above.
<topic> elements
The subject of the given <topic> is what is being reified by the
topic node. There is no need to re-reify the subject of a topic
node; it is already reified by that topic node.
When a <topic> is referenced by a
<topicRef referent="describesSubject">,
what is being referenced is the subject of the <topic>, which is
always also the subject of the <topic>'s corresponding topic node.
For practical purposes, the reference is to that corresponding topic
node.
When a <topic> element is referenced by a
<topicRef referent="isSubject">,
what is being referenced is a topic node *other* than the one that
corresponds to the <topic> element being referenced. This other
topic node's subject is the <topic> element, considered as an
expression of information, syntax and all. (This is probably only
useful when the purpose of the topic map includes providing access
information about topic map documents considered as XML documents,
not as topic map graphs. It's a rarity, but it's important for
completeness. It would not be good if topic maps could discuss
everything but themselves.) If the referenced <topic> element is
not otherwise already reified as a topic node (whose subject is that
<topic> element), then such a topic node is called into existence.
Note: When a <topic> element is referenced by a <resourceRef>, it's
just a resource, like any other resource. <resourceRef>s
don't reify anything; they don't demand the existence of topic
nodes. They simply provide references to resources.
<topicMap> elements
<topicMap> elements do not explicitly have any way to specify their
subjects. However, it is possible, and probably even commonplace,
to regard a <topicMap> element as somehow being all about some
particular subject, which, again, is not specified by the <topicMap>
element itself.
When a <topicMap> is referenced by a
<topicRef referent="describesSubject">,
what is being referenced is the unspecified subject of the
<topicMap>. As always, the use of <topicRef> demands the existence
of a topic node that corresponds to the subject.
If it is desired to explicitly describe the subject of a <topicMap>,
one way to do it is to specify a <topic> with at least two subject
descriptors, one of which is the <topicMap> element (<topicRef
referent="describesSubject">), and one of which is a resource that
describes the subject of the <topicMap> element (again, <topicRef
referent="describesSubject">).
When a <topicMap> element is referenced by a
<topicRef referent="isSubject">,
what is being referenced is a topic node whose subject is the
<topicMap> element, considered as an expression of information,
syntax and all. (This is probably only useful when the purpose of
the topic map is to organize information about topic map documents
as XML documents, not as topic map graphs. It's a rarity, but it's
important for completeness. It would not be good if topic maps
could discuss everything but themselves.) If the referenced
<topicMap> element is not otherwise already reified as a topic node
(whose subject is that <topicMap> element), then such a topic node
is called into existence.
<occurrences> elements
<occurrences> elements are like <topicMap> elements. They do not
reify themselves, but they can be reified, if desired, by regarding
them as the subject descriptors of additional <topic> elements.
(See the entry for <topicMap>, above.
<resource> elements
<resource> elements can only appear within <occurrences> elements.
Each <resource> element specifies an "occurrence" of material which
is regarded as relevant to the subject of the containing <topic>
element. (<resource> is misleadingly named, because what it really
does is to specify an occurrence, which is a resource, to be sure,
but only considered from a certain perspective.)
<resource> elements (or, more properly, the occurrences that they
specify) are self-reifying. This means that each <resource> element
has two kinds of impact on the topic map graph:
(1) (Of course) each <resource> element specifies occurrence
characteristics that will become properties of the topic node
that corresponds to the <topic> element that contains the
<resource> element. The characteristics are within the scope
specified by the <scope> element contained by the <resource>
element.
(2) In addition, for each <resource> element, a topic node is
created in the topic map graph that reifies the intent of the
<resource> element.
(a) This topic node has the name characteristics specified by
the <basename> elements contained in the <resource>,
(b) its subject descriptor is the <resource> element itself,
considered as if referent="describesSubject" (it's a
"subject-describing" descriptor, rather than a
"subject-itself" descriptor), and
(c) it plays the role of "instance" in a "class-instance"
relationship with each of the topics referenced by the
<instanceOf> element contained in the same <occurrences>
that contains the <resource>.
<resource> elements can also have a third impact on the topic map
graph: they can specify the subject descriptors of the topic node
that corresponds to the containing <topic> element. (See Note 2 at
the beginning of these remarks for a discussion of this.)
Therefore, if it is desired to add more characteristics to the topic
node that reifies the intent of a <resource> element, this can be
accomplished by creating an additional <topic> element whose
characteristics will be merged into the same topic node as the one
that reifies the intent of the <resource> element, because it refers
to that <resource> element as an occurrence by means of a <topicRef
referent="describesSubject">. In other words, they have exactly the
same subject, so they will be merged automatically.
<association> elements
<association> elements (or, more properly, the relationships that
they specify) are, like the occurrences specified by <resource>
elements, self-reifying. This means that each <association> element
has two kinds of impact on the topic map graph:
(1) (Of course) each <association> element specifies
roles-played-in-relationships characteristics that will become
properties of the topic nodes that correspond to the <topic>
elements among which the <association> element expresses the
relationships. The characteristics are within the scopes
specified by the <scope> elements contained in the
<associateTopic> elements contained in the <association>
element.
(2) In addition, for each <association> element, a topic node is
created in the topic map graph that reifies the relationship
expressed by the <association> element.
(a) This topic node has the name characteristics specified by
the <basename> elements contained in the <association>,
(b) its subject descriptor is the <association> element itself,
considered as if referent="describesSubject" (it's a
"subject-describing" descriptor, rather than a
"subject-itself" descriptor), and
(c) it plays the role of "instance" in a "class-instance"
relationship with each of the topics referenced by the
<instanceOf> element contained in the <association>.
If it is desired to add more characteristics to the topic node
that reifies the relationship specified by an <association>
element, this can be accomplished by creating an additional
<topic> element whose characteristics will be merged into the
same topic node as the one that reifies the relationship
specified by the <association> element, because it refers to
that <association> element as an occurrence by means of a
<topicRef referent="describesSubject">. In other words, they
have exactly the same subject, so they will be merged
automatically.
<associateTopic> elements
<associateTopic> elements (or, more properly, the relationships that
they specify) are, like the occurrences specified by <resource>
elements, and like the relationships specified by <association>
elements, self-reifying. This means that each <associateTopic> element
has two kinds of impact on the topic map graph:
(1) (Of course) each <associateTopic> element specifies
roles-played-in-relationships characteristics that will become
properties of the topic nodes that correspond to the <topic>
elements among which the containing <association> element expresses the
relationships. The characteristics are within the scopes
specified by the <scope> elements contained in the
<associateTopic> elements.
(2) In addition, for each <associateTopic> element, a topic node is
created in the topic map graph that reifies the set of topics
playing the role, and the role that they play, in the relationship
expressed by the containing <association> element.
(a) This topic node has the name characteristics specified by
the <basename> elements contained in the <associateTopic>,
(b) its subject descriptor is the <associateTopic> element itself,
considered as if referent="describesSubject" (it's a
"subject-describing" descriptor, rather than a
"subject-itself" descriptor), and
(c) it plays the role of "instance" in a "class-instance"
relationship with each of the topics referenced by the
<instanceOf> element contained in the <associateTopic>.
(The subjects of these topics are aspects of the role
being played.)
If it is desired to add more characteristics to the topic node
that reifies the set of topics (and the role that they play)
specified by the <associateTopic> element, this can be
accomplished by creating an additional <topic> element whose
characteristics will be merged into the same topic node as the
one that reifies the set of topics (and the role that they
play) specified by the <associateTopic> element, because it
refers to that <associateTopic> element as an occurrence by
means of a <topicRef referent="describesSubject">. In other
words, they have exactly the same subject, so they will be
merged automatically.
<baseName> elements
<baseNameString> elements
<variant> elements
<variantName> elements
<parameters> elements
<instanceOf> elements
<scope> elements
<topicRef> elements
<mergeMap> elements
All these elements are like <topicMap> elements. They do not
reify themselves, but they can be reified, if desired, by regarding
them as the subject descriptors of additional <topic> elements.
(See the entry for <topicMap>, above.
-Steve
--
Steven R. Newcomb, Consultant
srn@coolheads.com
voice: +1 972 359 8160
fax: +1 972 359 0270
405 Flagler Court
Allen, Texas 75013-2821 USA
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