The Standard Application Model for Topic Maps

3.1 Locator items

An information resource is a resource that can be represented as a sequence of bytes, and thus could in theory be retrieved over the network. Topic maps can refer to information resources external to themselves in order to make statements about them. These information resources are not part of the topic map, but only referenced from it.

A locator is a string, an addressing expression that references one or more information resources. Locators are always expressed in some notation, which defines their formal syntax and interpretation. The definition of locator notations is outside the scope of this specification.

In this model locator items represent locators and serve as the connection point between the topic map and the information resources it describes. Locator items may have the following properties:

1. [notation]: A non-empty string. The string is the name of the notation used by this locator. If the string is "URI" the notation is that described in [RFC2396]. If not, the two first characters of the string must be "X-"; all other values are reserved.

2. [reference]: A non-empty string. The string is the actual locator, whose interpretation and syntax is governed by the value of the [notation] property.

Locator information items are equal if they have the same values in their [notation] and [address] properties.

3.2 Source locators

The source locators of an information item may be used to refer to the information item, either from other information items, or from outside the topic map. When a topic map is created through deserialization from some topic map syntax source locators are created that point back to the syntactical constructs that gave rise to the information items. Information items may also have source locators assigned to them by means not constrained by this specification, in order that they may be referred to from outside (or within) the topic map.

Source locators are used in this specification to define reification, in the syntax specifications to ensure that information loaded from different information resources is correctly collated, and it is expected that other specifications and technologies will use them to define mechanisms for referencing topic map constructs.

It is an error for two different information items to have locator items that compare as equal in their [source locators] properties.

3.3 The topic map item

A topic map is a collection of topics and associations, which may be represented in many forms. Its purpose is to convey information about subjects through the assignment of characteristics to topics representing those subjects. The topic map itself has no meaning or significance beyond its use as a container for the information about those subjects; in particular, the topic map itself does not represent anything.

There is exactly one topic map item in each information set, and all information in the set is available from the properties of that item. Every topic map item represents a topic map.

Topic map items have the following properties:

1. [source locators]: A possibly empty set of locator items. This is the source locators of the topic map.

2. [topics]: A possibly empty set of topic items. This is the set of all the topics in the topic map.

3. [associations]: A possibly empty set of association items. This is the set of all the associations in the topic map.

4. [reifier]: A topic item, or null. The topic item is the topic that reifies this information item.

   The value of the property can be computed as follows: if there exists a topic item in whose [subject indicators] property can be found a locator item equal to one in the [source locators] property of this information item that topic item is the value of this property. If not, the value is null.

   Issue (prop-reifier-computed):

   Is it acceptable for this property to be computed, or must it be a fundamental property?

There is no defined procedure for comparing topic map items.

3.4 Topic items

A subject is anything that has identity. In the most generic sense, a subject is anything whatsoever, regardless of whether it exists or has any other specific characteristics, about which anything whatsoever may be asserted by any means whatsoever. In particular, it is anything on which the creator of a topic map chooses to discourse.

The standard makes the following assumptions about subjects:

• Subjects are atomic, meaning that a subject is conceptually a single thing. That thing may be divisible into smaller components, but it is still a single thing. That is, to speak of two things is to speak of two subjects. To speak of a group consisting of two things is to speak of a single thing, which is the group—or possibly three: the group, and its two members.

• Subjects are identifiable, meaning that not only do subjects have identity, but their identity can also be established. Two mechanisms are provided for the purposes of establishing subject identity, one of which is formal, while the other is informal.

• Subjects are persistent, meaning that while a physical subject may disappear the subject still has identity in the sense that we may continue to speak of it. For example, the halibut William Cowper ate is long gone [cowper], but it is still possible to make statements about it, and so it is still a subject.

• Subjects are independent of perspective, meaning that while there may be different points of view on what something is each point of view is a subject in its own right. So while Plato speaks of 'the good' his notion of it is clearly a different subject from 'the good' of John Stuart Mill. For this reason topics have no scope; there is no limit to their validity as notions.

A topic is a symbol used within a topic map to represent some subject, about which the creator of the topic map wishes to make statements. Topics only exist in order to act as proxies for the subjects they represent, in order to allow statements to be made about the subjects through the assignment of characteristics to the topics that represent them.

Every topic represents one, and only one, subject. The process of merging ensures that whenever two topics are known to represent the same subject they are merged automatically. It may well be, however, that two topics may represent the same subject without this being detected by the rules of this standard. The merging of topics is therefore disallowed only when it is clear that they must represent different subjects.

3.5 Base name items

A base name is a name or label for a subject, expressed as a string. That is, it is something that identifies the subject (though not necessarily uniquely) and can be used as a label for the subject in user interfaces. The notion of a base name corresponds closely to the common sense notion of a name. Suitable base names for people, countries, and organizations are their names, while base names for documents, musical works, and movies might be their titles. Base names may have variant names, which are alternative forms of the name that may be more appropriate in some context.

Base names always have a scope, which defines in what contexts the base name is an appropriate label for the subject. A subject may have any number of base names, and the only basis for choosing which base name(s) to use in any given situation is their scope.

Base names items represent base names, and have the following properties:

1. [source locators]: A possibly empty set of locator items. This is the source locators of the base name.

2. [value]: A string, which may be empty, but not null. This string is the base name.

   Issue (prop-value):

   Is [label] a better name?

3. [scope]: A possibly empty set of topic items. This set is the scope that describes the validity of this base name as a label for the subject.

4. [variants]: A possibly empty set of variant items. This set is the variant names that are alternative forms of the base name.

5. [reifier]: A topic item, or null. The topic item is the topic that reifies this information item.

   The value of the property can be computed as follows: if there exists a topic item in whose [subject indicators] property can be found a locator item equal to one in the [source locators] property of this information item that topic item is the value of this property. If not, the value is null.

Base name information items are considered equal if the values of their [value] and [scope] properties are equal.

3.6 Variant items

A variant name is an alternative form of a base name that may be more suitable in certain contexts than the base name itself. The scope of the variant name is the only basis for establishing what variant name is most suitable in any given situation. A variant name may be a string, but it may also be any other kind of information resource.

When choosing a label for a topic applications are expected to select the base name they consider most appropriate, and then evaluate which of the forms of that base name is best suited for display in that particular context, which may be the base name or one of its variants. This standard does not constrain the process by which this is done.

Variant items represent variant names, and have the following properties:

1. [source locators]: A possibly empty set of locator items. This is the source locators of the variant name.

2. [value]: : A string, which may be empty, or it may be null. The string, if set, is the variant name. If it is not null, the [resource] property must be null.

3. [resource]: A locator item, or null. The locator, if set, refers to the information resource that is the variant name. If it is not null, the [value] property must be null.

4. [scope]: A non-empty set of topic items. This set is the scope that describes in what context(s) the variant name may be preferred as a label for the topic.

   The value of the [scope] property of each variant item must be a true superset of the value of the [scope] property the base name item that is its parent.

5. [reifier]: A topic item, or null. The topic item is the topic that reifies this information item.

   The value of the property can be computed as follows: if there exists a topic item in whose [subject indicators] property can be found a locator item equal to one in the [source locators] property of this information item that topic item is the value of this property. If not, the value is null.

Variant information items are considered equal if the values of their [value], [resource], and [scope] properties are equal.

3.7 Occurrence items

An occurrence is a relationship between an information resource and a subject. The relationship between the information resource and the subject is described by the occurrence type, itself a subject, that is attached to the occurrence. Occurrences are generally used to connnect information resources with the subjects they are relevant to. Note that the occurrence is properly not the resource, but the relationship between it and the subject. The information resource may either be a string inside the topic map or an external resource.

All occurrences have a scope, which defines the contexts in which the occurrence relationship between the information resource and the subject is valid.

An occurrence item represents an occurrence and can have the following properties:

1. [source locators]: A possibly empty set of locator items. This is the source locators of the occurrence.

2. [value]: A string, which may be empty, or it may be null. The string, if set, is the information resource the occurrence connects with the subject. If it is not null, the [resource] property must be null.

3. [resource]: A locator item, or null. The locator, if set, is the information resource the occurrence connects with the subject. If it is not null, the [value] property must be null.

4. [scope]: A possibly empty set of topic items. This set is the scope that describes in what context the occurrence relationship may be considered valid.

5. [occurrence type]: A topic item, or null. The topic item represents the subject that defines the nature of the occurrence relationship.

6. [reifier]: A topic item, or null. The topic item is the topic that reifies this information item.

   The value of the property can be computed as follows: if there exists a topic item in whose [subject indicators] property can be found a locator item equal to one in the [source locators] property of this information item that topic item is the value of this property. If not, the value is null.

Occurrence information items are considered equal if the values of their [value], [resource], [scope], and [occurrence type] properties are equal.

3.8 Association items

An association is a relationship between one or more subjects. Associations have an association type, a subject which describes the nature of the relationship. The involvement of each subject in the relationship is called its association role.

All associations have a scope, which defines the context in which the statement represented by the association can be considered valid. The scope applies to the association roles as characteristics of the topics they connect, but all association roles in an association have the same scope, and so the scope is considered to apply to the association as a whole as well.

Association items represent associations, and may have the following properties:

1. [source locators]: A possibly empty set of locator items. This is the source locators of the association.

2. [scope]: A possibly empty set of topic items. This set is the scope that describes in what context the association may be considered valid.

3. [association type]: A topic item, or null. The topic item represents the subject that defines the nature of the relationship.

4. [roles]: A non-empty set of association role items. The association role items represent the association roles that make up this association.

5. [reifier]: A topic item, or null. The topic item is the topic that reifies this information item.

   The value of the property can be computed as follows: if there exists a topic item in whose [subject indicators] property can be found a locator item equal to one in the [source locators] property of this information item that topic item is the value of this property. If not, the value is null.

Association information items are considered equal if the values of their [scope], [association type], and [roles] properties are equal.

3.9 Association role items

An association role connects two pieces of information within an association: the subject participating in the association, known as the association role player, and the subject defining the nature of its participation, known as the association role type.

Ed. Note:(larsga)
Should we include examples to make this clear? Here, or under association?

Association role items represent association roles, and may have the following properties:

1. [source locators]: A possibly empty set of locator items. This is the source locators of the association role.

2. [role playing topic]: A topic item, or null. This is the topic item that represents the association role player.

3. [role type]: A topic item, or null. This is the topic item that represents the association role type.

4. [reifier]: A topic item, or null. The topic item is the topic that reifies this information item.

   The value of the property can be computed as follows: if there exists a topic item in whose [subject indicators] property can be found a locator item equal to one in the [source locators] property of this information item that topic item is the value of this property. If not, the value is null.

Association role information items are considered equal if the values of their [role type] and [role player] properties are equal.

4.1 Merging topics

Whenever two different topic items have

• at least one equal locator item in their [subject indicators] properties,

• at least one equal locator item in their [source locators] properties,

• an equal locator item in their [subject address] property, or

• an equal locator in the [subject indicators] property of the one topic item and the [source locators] property of the other.

they must be merged.

Merging of two topic items, A and B by following this procedure:

1. Create a new topic item C.

2. Replace A by C wherever it appears in one of the following properties of some information item: [topics], [classes], [scope], [occurrence type], [association type], [role player], and [role type].

3. Repeat for B.

4. Set C's [source locators] property to the union of the values of A and B's [source locators] properties.

5. Set C's [subject indicators] property to the union of the values of A and B's [subject indicators] properties.

6. If A or B has a value for the [subject address] property, set this as the value of C's [subject address] property.

7. Set C's [classes] property to the union of the values of A and B's [classes] properties.

8. Set C's [base names] property to the union of the values of A and B's [base names] properties, merging equal base name items in C's [base names] property according to the rules of 4.2 Merging base names.

9. Set C's [occurrences] property to the union of the values of A and B's [occurrences] properties.

10. Set C's [roles played] property to the union of the values of A and B's [roles played] properties.

If both A and B have a value in their [subject address] property and those values are different A and B cannot be merged. If A and B are required to be merged by the rules of this specification the topic map is invalid.

Ed. Note:(larsga)
We need a defined term for topic maps that contain errors. Also, we need a consistent policy for dealing with them. Obviously, the errors must be reported, but then what? Should there be a defined recovery procedure? It must work both for import, generation, human editing, and API modifications.

4.2 Merging base names

Base names are merged when the [base names] property of a topic item contains two equal base name items. The procedure for merging two base name items A and B is given below.

• Create a new base name item C.

• Set C's [source locators] value to the union of the value of the [source locators] properties of A and B.

• Set C's [value] value to the value of the [value] property of A or B. If merging was triggered by the rules of this specification A and B should have the same rule. If not, applications may choose either value.

• Set C's [scope] value to the value of the [scope] property of A or B. If merging was triggered by the rules of this specification A and B should have the same rule. If not, applications may choose either value.

• Set C's [variant] value to the union of the [value] properties of A and B.

Ed. Note:(larsga)
Do we really need to bother about when merging is not triggered by the rules given here?

4.3 Merging topic maps

Topic map merging is an operation that given two separate topic maps produces a single topic map which contains all the statements made by the two source topic maps. This specification is not concerned with when topic map merging occurs, but specifies a minimal procedure for merging topic maps. Topic map processors are free to make additional merges based on extra information available to them.

Merging two topic maps A and B is done as follows:

1. Create a new topic map item C.

2. Set the [source locators] property of C to the empty set.

   Issue (merge-prop-srclocs):

   Should source locators be preserved? If they are, it is implied that C is the same topic map as A and B, which is not true. Also, topics reifying A or B will then also reify C.

3. Set the [topics] property of C to the union of the [topics] properties of A and B.

4. Set the [associations] property of C to the union of the [associations] properties of A and B.

5. Apply merging to the [topics] property of C until no duplicates remain.

6. Apply merging to the [associations] property of C until no duplicates remain.

4.4 Merging other information items

Other information items, except locator items, are merged whenever two equal items A and B are found in the value of some property. In all cases, the procedure is as follows:

• Create a new item C of the same type as A and B.

• Set C's [source locators] property to the union of the [source locators] properties of A and B.

• For all other properties, give C the same value as A and B had.

5.1 The class-instance relationship

Ed. Note:(larsga)
Specify the PSI here, but consider relation between this and the section above. Resolve what to do about the [classes] property before moving on here.

5.2 The superclass-subclass relationship

A class may be a subclass of another, which is then considered the superclass of the other. If B is the subclass of A, it follows that every instance of B is also an instance of A. The opposite it not necessarily true. The relationship between a superclass and its subclasses is known as the superclass-subclass relationship. A class may have any number of subclasses and superclasses. Scope applies to this association type in just the same way as it does to any other.

The superclass-subclass relationship is transitive, which means that if B is a subclass of A, and C a subclass of B, C is also a subclass of A.

The superclass-subclass relationship between two classes can be asserted in topic maps using an association item that conforms to the following rules:

• The [association type] property must be set to a topic item that has in its [subject indicators] property a locator item with [notation] set to "URI" and [reference] set to "http://www.topicmaps.org/xtm/1.0/core.xtm#superclass-subclass".

• The [roles] property must contain exactly two association role items.

• One of the association items in the [roles] property must have its [role type] property set to a topic whose [subject indicators] property is set to a locator item with [notation] set to "URI" and [reference] set to "http://www.topicmaps.org/xtm/1.0/core.xtm#superclass". The [role player] property will then contain the topic item representing the superclass.

• One of the association items in the [roles] property must have its [role type] property set to a topic whose [subject indicators] property is set to a locator item with [notation] set to "URI" and [reference] set to "http://www.topicmaps.org/xtm/1.0/core.xtm#subclass". The [role player] property will then contain the topic item representing the subclass.

5.3 Variant name scopes

The subject identifier http://www.topicmaps.org/xtm/1.0/core.xtm#sort (notation "URI"), identifies the notion of suitability of a variant name for use as a sort key for a subject. A variant item that has a topic with this subject identifier in its [scope] property represents a variant name intended to be used as a sort key for the topic item it belongs to.

The subject identifier http://www.topicmaps.org/xtm/1.0/core.xtm#sort (notation "URI"), identifies the notion of suitability of a variant name for use as a display name for a subject. A variant item that has a topic with this subject identifier in its [scope] property represents a variant name intended to be used as a label for the topic item it belongs to. This published subject is included only for backwards compatibility with ISO 13250:2000.