# The Linear Topic Map Notation

## Definition and introduction, version 1.2

By: | Lars Marius Garshol
Affiliation: | Ontopia A/S
Date: | $Date: 2002/05/15 18:25:18 $
Version: | Version 1.2 ($Revision: 1.16 $)

## Abstract

This technical report defines version 1.2 of the Linear Topic Map Notation, also known as LTM. It provides both an introduction and a formal definition, the latter in the form of a complete EBNF specification given at the end of the document.

Please note that this document is not a formal specification from any recognized standards body, but a Technical Report published by Ontopia, a commercial company, for the convenience of all interested parties. The specification may be taken up by some standards body at some point, but nothing suggests that this will happen soon.

Version 1.2 extends version 1.1 to provide support for reifying the topic map itself, specifying a base URI for the topic map, merging in external topic maps, and specifying scope on occurrences and associations. Some other minor simplifications and improvements to the syntax have also been made.

Version 1.2 is not completely backwards compatible with versions 1.1 and 1.0. One change has been made: scope must now be specified after the base name rather than in front of it, as before. This change was made to make scope on base names consistent with that on associations and occurrences. Colons are also no longer allowed in topic IDs, in order to avoid problems when users write `related-to(a: b, c: d)`, since the colon here is ambiguous.

## Table of contents

- 1. Linear topic map notation? What's that?

- 2. An introduction to the notation

- 2.1. Defining topics

- 2.2. Defining associations

- 2.3. Defining occurrences

- 2.4. A complete example

- 2.5. Directives

- 2.5.1. The TOPICMAP directive

- 2.5.2. The MERGEMAP directive

- 2.5.3. The BASEURI directive

- 3. Formal syntax definition

- 3.1. Character encoding handling

- 3.2. Processing and references

- 3.3. Completeness

## 1. Linear topic map notation? What's that?

The linear topic map notation is a simple textual format for topic maps. Just like the XML interchange format it represents the constructs in the topic map standard as text, but unlike the XML format it is compact and simple. The notation can be written in any text editor and processed by topic map software that supports it, or converted into the XML format supported by such software.

The XML-based topic map interchange format is defined in such a way as to make it easy to comprehend for humans and to develop software for, and these purposes it fulfills very well. However, this benefit has been realized at the cost of making it awkward to read and write for humans. Now, humans were not really intended to do this, they were intended to use specialized topic map editors, which would insulate their users from the syntactical details of the interchange format.

However, there is still a need for a simple textual format that can be used to concisely and clearly express topic map constructs in emails, discussions and similar contexts. Such a format also makes it easy to quickly create and maintain small topic maps for demonstration and personal purposes. This is useful while we wait for good topic map editors to be developed.

While you may find that this syntax provides you with a convenient and easy way to maintain your topic maps, please note that the only standardized forms for interchangeable topic maps remain the XTM 1.0 and HyTM syntaxes.

This notation has been developed by Ontopia. Steve Pepper came up with the original idea, based on the linear notation for conceptual graphs. The notation has since been refined by Lars Marius Garshol, with input from Geir Ove Grønmo and Steve Pepper. Useful contributions from Murray Altheim and Akio Yamamoto are also gratefully acknowledged.

While the copyright to both this description and the format itself is held by Ontopia Ontopia reserves only the right to be recognized as the originator of the notation. Permission to use it in any way for any purpose whatsoever is hereby granted in perpetuity to all potential users.

## 2. An introduction to the notation

### 2.1. Defining topics

The basis of the notation is the ability to define topics, which is done by writing the ID of the topic in square brackets. An example is shown below.

    [ltm]

This represents a topic map consisting of a single topic that has the ID 'ltm', but no other characteristics. If you want, you can provide it with a base name and a sort name as well, as in the example below. Note that the sort name is optional.

    [ltm = "The linear topic map notation";
           "linear topic map notation, the"]

You can even add a display name, if you want. If you have a display name the sort name is optional, but you need two semicolons to tell the parser that the second name is a display name and not a sort name. The example below shows a topic with all three name types.

    [foo = "basename"; "sortname"; "dispname"]

The topic can also be typed. The example below adds the type 'format' to the `ltm` topic. Multiple type IDs can be listed after the colon, separated by whitespace, if the topic has more than just one type.

    [ltm : format = "The linear topic map notation";
                    "linear topic map notation, the"]

Note that even if no topic with the ID 'format' is defined anywhere in the LTM file this reference will cause the topic to be created by the LTM processor. The 'format' topic will have an ID, but no other characteristics. Note also that LTM is oblivious to whitespace. You can add as much whitespace as you want anywhere (except inside strings) without having any effect on the resulting topic map.

LTM also supports providing subject indicators for topics, as shown below. The URL of the subject indicator is quoted and preceded by an '@' character. Any number of subject indicators can be given.

    [ltm : format = "The linear topic map notation";
                    "linear topic map notation, the"
         @"http://www.ontopia.net/download/ltm.html"]

For topics which represent network-retrievable resources it is not necessary to use a proxy resource (a subject indicator) to indicate the identity of the subject; it can instead be referred to directly. LTM supports this, by using a '%' character followed by the quoted URL of the resource. An example is shown below.

    [xmlspec : specification = "The XML 1.0 specification"
         %"http://www.w3.org/TR/REC-xml"]

The final construct supported by LTM for topics is scoping of names. This can be done for the base name, sortname, dispname-trinity as a whole, by appending a topic ID preceded by a slash after the name, as shown below. Multiple topic IDs are allowed, separated by whitespace.

    [ltm : format = "Den lineære topic map-notasjonen";
                    "lineære topic map-notasjonen, den"
                    / norwegian
         @"http://www.ontopia.net/download/ltm.html"]

Note that if this example and the previous `[ltm]` example were to appear in the same LTM file it would cause a single topic to be created with the union of the characteristics of these two definitions. That means that the topic would have the 'ltm' ID, the format type, the two different name sets and the given subject indicator.

Note also that there are no requirements on the order in which constructs appear in LTM files. A topic type can be used before it is defined, for example.

### 2.2. Defining associations

The LTM notation also supports defining associations. In the example below the LTM topic defined above is associated with a topic with the ID 'topic maps' by an association that has the `format-for` type. ('format-for' is of course the ID of the topic that types that association.)

    format-for(ltm, topic-maps)

The meaning of this example is that LTM is a serialization format for topic maps. This should perhaps be made clearer by adding association role types. The example below does this.

    format-for(ltm : format, topic-maps : standard)

Note that if the association role type is omitted the role type will default to the type of the topic. The rationale for this is that it is a useful shorthand for a commonly occurring construction.

As a shorthand it is allowed to specify a topic in the role player position, instead of just referencing it. All the constructs used when defining topics can be used here, which means that it is possible to define topics with their characteristics in the associations they participate in without defining them anywhere else. The example could therefore also have been written as follows.

    format-for(ltm, [topic-maps : standard = "Topic maps"])

Associations can also be scoped, as with base names, by appending a slash followed by the IDs of the scoping topics, separated by whitespace. The example below illustrates this.

    [lmg : person = "Lars Marius Garshol"

    format-for([ltm] : format, [topic-maps] : standard) / lmg

### 2.3. Defining occurrences

LTM also supports defining occurrences. This is done using the notation shown below, where the occurrence information is given in curly braces. Three pieces of information, all of which are required, appear inside the braces, separated by commas. The first is the ID of the topic which has the occurrence, the second is the ID of the occurrence role type and the third is the locator of the occurrence in double quotes.

    {ltm, specification, "http://www.ontopia.net/download/ltm.html"}

You can also specify thr resource data of an occurrence inline in the LTM file, as shown below.

    {ltm, description, [[A simple text-based format for topic maps.]]}

Occurrences are scoped in the same way as associations:

    {ltm, specification, "http://www.ontopia.net/download/ltm.html"} / english

### 2.4. A complete example

Below is given a more complete example of an LTM topic map. Note that text appearing between '/*' and '*/' is comments.

    /* topic types */

    [format       = "Format"]
    [standard     = "Standard"]
    [organization = "Organization"]

    /* association types */

    [format-for = "Format for"]
    [defined-by = "Defined by"]

    /* occurrence types */

    [specification = "Specification"]
    [homepage      = "Home page"]

    /* topics, associations and occurrences */

    [topic-maps : standard  = "Topic maps"
                            = "ISO/IEC 13250 Topic Maps" / fullname]
    {topic-maps, specification,
       "http://www.y12.doe.gov/sgml/sc34/document/0129.pdf"}

    [xtm : format = "XTM Syntax"]

    [ltm : format = "The linear topic map notation";
                    "linear topic map notation, the"
         @"http://www.ontopia.net/topicmaps/ltm-tech-report.html"]
    {ltm, specification, "http://www.ontopia.net/topicmaps/ltm-tech-report.html"}

    format-for(ltm, topic-maps)
    format-for(xtm, topic-maps)

    defined-by(ltm, ontopia)
    defined-by(xtm, topicmaps.org)

    [ontopia : organization = "Ontopia AS"]
    {ontopia, homepage, "http://www.ontopia.net"}

    [topicmaps.org  : organization = "TopicMaps.Org"]
    {topicmaps.org, homepage, "http://www.topicmaps.org"}

### 2.5. Directives

LTM has a concept of so-called "syntax directives", which are used not to represent topic map constructs, but to provide information related to processing. There are three different directives, each covered in a separate section below.

#### 2.5.1. The TOPICMAP directive

The `TOPICMAP` directive is used to make it possible to reify the topic map itself. This is useful, since it makes it possible to attach metadata to the topic map using topic map constructs. Below is shown an example of the directive in use.

    #TOPICMAP example

    [tm-topic = "An example topic map"
     @"#example"]

To phrase it in XTM terms, this is like having a `topicMap` element with its `id` set to `example`, and a topic with a subject indicator that refers to that element.

Note that it is an error for a topic to be given the same ID as the topic map itself.

#### 2.5.2. The MERGEMAP directive

The `MERGEMAP` directive is used to merge external topic maps into the LTM topic map. The external topic maps can be in any syntax, but if this syntax is not LTM it must be declared what syntax it is. An example is shown below.

    #MERGEMAP "geography.xtm" "xtm"

This directive causes the topic map at the given URI to be loaded according to the rules of the syntax it is written in and merged with the current topic map once the loading is complete. Note that the URI is allowed to use any URI scheme, although there is no guarantee that an LTM processor will understand any URI schemes beyond 'file'.

LTM processors are required to recognize the syntaxes listed below, but not necessarily to support them. XTM and LTM must be supported, while the other syntaxes are optional. It is an error if the LTM processor is asked to merge in a topic map in a syntax it does not understand. Note that the syntax names are case-insensitive. If no syntax is specified, the default is LTM.

xtm
  The XTM 1.0 XML topic maps syntax.
hytm
  The HyTime-based architectural form syntax defined in the original ISO 13250 standard.
ltm
  The Linear Topic Map Notation.
astma
  The textual syntax for topic maps known as AsTMa=.

#### 2.5.3. The BASEURI directive

This directive is used to change the base URI against which relative URIs in the document are resolved. It works exactly like the `xml:base` attribute in XML Base, or the `BASE` element in HTML. Below is shown an example.

    #BASEURI "http://www.ontopia.net/"

All URIs occurring after the directive will resolve against the given URI, which must be absolute, rather than against the URI of the LTM document itself. This applies to URIs in `MERGEMAP` directives, subject addresses, subject indicators, and the URIs of occurrences. (More formally, it applies to all instances of the grammar symbol `uri`.) Note that the `BASEURI` directive does not apply inside any files included with `MERGEMAP`.

Note that having more than one `BASEURI` directive in the same file is an error.

## 3. Formal syntax definition

This section defines the LTM syntax using a formal extended BNF grammar. Lexical tokens are given either as single-quoted strings directly in the grammar, or as upper-case names of token types. The token types are defined separately further below.

      topic-map  ::= encoding? directive* (topic | assoc | occur) +

      encoding   ::= '@' STRING

      directive  ::= topicmapid | mergemap | baseuri

      topicmapid ::= '#' 'TOPICMAP' WS NAME

      mergemap   ::= '#' 'MERGEMAP WS uri WS STRING

      baseuri    ::= '#' 'BASEURI' WS uri

      topic      ::= '[' NAME (WS ':' NAME+)? (topname)* subject? indicator* ']'

      subject    ::= '%' uri

      indicator  ::= '@' uri

      topname    ::= '=' basename ((';' sortname) |
                                   (';' sortname? ';' dispname))?
                         scope?

      scope      ::= '/' NAME+

      basename   ::= STRING

      sortname   ::= STRING

      dispname   ::= STRING

      assoc      ::= NAME '(' assoc-role (',' assoc-role)*  ')' scope?

      assoc-role ::= (topic | NAME) (':' NAME )?

      occur      ::= '{' occ-topic ',' occ-type ',' resource '}' scope?

      resource   ::= uri | DATA

      occ-topic  ::= NAME

      occ-type   ::= NAME

      uri        ::= STRING

The lexical token types defined below use Perl-style regular expressions for their definitions. Note that while whitespace (represented by the `WS` token type) is implicitly allowed between any two tokens, it is explicitly required in the 'topic' production in the above grammar. This is to avoid problems caused by the fact that a colon is allowed in topic IDs.

      NAME       = [A-Za-z_][-A-Za-z_0-9.]*

      COMMENT    = /\*([^*]|\*[^/])*\*/

      STRING     = "[^"]*"

      DATA       = \[\[(([^>]+>)*|\])\]

      WS         = [\r\n\t ]+

The `NAME` token type is slightly modified compared to the definition in the XML recommendation. The colon is no longer allowed as a name start character, since otherwise a single colon could be both a name and a separator.

All tokens are case-sensitive.

### 3.1. Character encoding handling

All LTM files are to be processed as if they were composed of Unicode characters. Files may be in any encoding, but if that encoding is not ISO 8859-1 it should be declared using the `encoding` production. If the encoding declaration appears in the file it must appear at the very beginning. Support for this construct is optional, but all processors must allow it to be present and at least ignore it.

The encoding names used are those defined by IANA, which are the same as those used by XML. The IANA character encoding identifier registry can be found at http://www.isi.edu/in-notes/iana/assignments/character-sets.

Below is shown a simple example of an LTM file that uses the UTF-8 character encoding.

    @"utf-8"

    [grove : person = "Geir Ove GrÃ¸nmo"]

(The name is of course Geir Ove Grønmo, encoded in UTF-8, but viewed as if it were ISO 8859-1.)

### 3.2. Processing and references

Any topic referred to by its ID in an LTM file is considered to exist, even if it is never defined anywhere by an explicit occurrence of the `topic` production with that topic. All occurrences of the same topic ID are considered to be references to the same topic.

When an instance of the `topic` production is found, and a topic with the same ID has already been found, the two topic definitions are merged as follows:

- The types of the resulting topic are considered to be the union of the types found in each definition.

- The names of the resulting topic are considered to be the union of the names found in each definition.

- The subject indicators of the resulting topic are considered to be the union of the subject indicators found in each definition.

- More than one subject address for the topic is found, the one occurring last in the file is used.

If two topic definitions are found which have different topic IDs, but in which the same name occurs in the same scope, no specific behaviour is guaranteed. Possible results are that the topics may be merged, that they may remain distinct and that an error may be signalled. The behaviour for topics with equal subject addresses or subject indicators but different IDs is subject to the same unpredictability.

### 3.3. Completeness

The following topic map constructs from ISO 13250 and XTM 1.0 are not supported:

- Base names cannot have other variant names that display and sort names, nor can any of these be anything but strings.

- Facets. Facets are in any case a HyTM feature only, and given the concepts of subject adresses and subject indicators introduced in XTM 1.0 they are now obsolete. They will therefore not be supported by LTM.

- The mnemonic string alternatives to types on various constructs. These only exist in the HyTM syntax, and are really only a HyTime legacy, with no model impact. They are therefore not really part of topic maps per se.

- Reification of base names, occurrences, associations, and association roles is not possible with the current syntax. This may be supported in a later version.