Extensible Hyper Linkage (XHL)

Partial revision of W3C Working Draft 31-January-97

This version: modified by Henry S. Thompson <ht@cogsci.ed.ac.uk>
Previous versions:
Latest version:
Tim Bray (Textuality) <tbray@textuality.com>
Steve DeRose (EBT/Inso) <sjd@ebt.com>

Status of this memo

This is a fragment of a revision of a W3C Working Draft for review by W3C members and other interested parties. It is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress". A list of current W3C working drafts can be found at http://www.w3.org/pub/WWW/TR.
Note: Since working drafts are subject to frequent change, you are advised to reference the above URL, rather than the URLs for working drafts themselves.
This work is part of the W3C SGML Activity (for current status, see http://www.w3.org/pub/WWW/MarkUp/SGML/Activity).


Extensible Hyper Linkage (XHL) is a simple set of constructs that may be inserted into SGML documents to describe links between objects. XHL is designed to use the power of SGML to create a structure that can describe both the simple unidirectional links of today's HTML, as well as more sophisticated multi-ended, typed, self-describing links. The SGML constructs used in XHL are simple enough to be available in the Extensible Markup Language (XML) subset of SGML. XHL is completely described in this document.

Extensible Hyper Linkage

Version 1.0 INITIAL DRAFT, partial redraft

11 February 1997
This draft is intended for public discussion.
It is subject to approval by the W3C SGML Editorial Review Board

Table of Contents

1. Introduction
    1.1 Origin and Goals
    1.2 Relationship to Existing Standards
    1.3 Notation
    1.4 Terminology
    1.5 Types of link types
2. Link Recognition
    2.1 Link Recognition by Attribute
    2.2 Link Recognition by Element Type
    2.3 Link Recognition by Other Means
3. Link Elements
    3.1 Information Associated With Links
    3.2 Multilink
    3.3 Terminus Links
4. Addressing
    4.1 HREFs and Reference Types
    4.2 Location Source
    4.3 SGML Reference Types
    4.4 URL Reference Types
    4.5 TEI Locator Reference Types
    4.6 Query Reference Types
5. Extended Link Groups
    5.1 Identifying Extended Link groups
    5.2 LINKS and LINKSET Elements


A. The TEI extended pointer syntax
            A.1.1.1 Location Ladders
            A.1.1.2 Location Terms
            A.1.1.3 The ROOT Keyword
            A.1.1.4 The HERE Keyword
            A.1.1.5 The ID Keyword
            A.1.1.6 The CHILD Keyword
            A.1.1.7 The DESCENDANT Keyword
            A.1.1.8 The ANCESTOR Keyword
            A.1.1.9 The PREVIOUS Keyword
            A.1.1.10 The NEXT Keyword
            A.1.1.11 The PRECEDING Keyword
            A.1.1.12 The FOLLOWING Keyword
            A.1.1.13 The TOKEN Keyword
            A.1.1.14 The SPACE Keyword
            A.1.1.15 The FOREIGN Keyword
B. Production note
C. W3C SGML Working Group and Editorial Review Board
    C.1 W3C SGML Working Group
    C.2 W3C SGML Editorial Review Board

1. Introduction

Extensible Hyper Linkage (XHL) describes a set of constructs which may be inserted in SGML documents to describe links between objects (the objects may or may not be (in) SGML documents). A link, as the term is used here, is a relationship between two or more data objects or portions of data objects, called its endpoints. Not all relationships are links: the relationship of a chapter to its paragraphs, of one word to the next, or any non-explicit relationship, are not considered links here. The relationship expressed using SGML's id and idref attributes is a link, subsumed within the framework presented here.

Links in XHL are described by elements contained in SGML documents. Such an element is called a link description, or linkd. The link description element itself may, but need not, be one of the endpoints of the link it describes. A link description can be understood as a mapping from document instances and their contexts to links.

Just as SGML Applications consist of a DTD and a specification of the meaning or significance of the structures defined therein, so an XHL Application must include a specification of the meaning or significance of the link names and endpoint names it employs.

1.1 Origin and Goals

XHL is part of the overall XML effort, on which see .... It aims to provide an effective yet compact means for describing links that can have multiple named endpoints, indirection, and flexible yet precise means for locating endpoints in all kinds of data. It also aims to represent the abstract structure and significance of links, leaving rendering and other issues of link-engendered behaviour to stylesheets or other mechanisms as far as practical (it is acknowledged that there is a grey area here).

1.2 Relationship to Existing Standards

Three standards have been especially influential on XHA:

  1. HTML: Defines several SGML element types that describe links, as well as popularizing a locator type, the URL, mainly focused on locating entire data objects, though with some provision for linking to elements with IDs, regions in graphics, and so on.
  2. HyTime: Defines locator types applicable to all kinds of data, as well as in-line and out-of-line link description elements and some semantic features including traversal control and placement of objects into a display or other space.
  3. Text Encoding Initiative: Provides a formal syntax for locators for structured data, graphics, and other data, and elements incorporating these for describing links and collections of links.

Many other relevant linking systems have also informed this design, including Dexter, MicroCosm, InterMedia, and others.

1.3 Notation

1.4 Terminology

The following basic terms for parts of links and link descriptions apply in this document. They also appear on the following structural diagram of a simple link.

figure of link derivation
  1. link: An n-place relation, n>=2, between data objects. A link has a link name and named endpoints. It is up to XHL Applications to define link meaning in terms of link names and endpoint names.
  2. endpoint: One part of a link. In principle, every piece of data may be an endpoint. In practice most endpoints will be elements or text in SGML or XML documents.
  3. link description: The specification of a link, expressed in XML documents using a prescribed set of XML elements and attributes, which provide means to specify the link name and endpoint names and to locate the endpoints themselves.
  4. locator: The part of a link description that specifies one of the data objects that serve as the link's endpoints. This may be as simple as a URL, ID, or query, or may be a complex chain of relative locators that eventually lead to a collection of discontinuous data portions that together make up the entire endpoint.
  5. gloss: A gloss for an endpoint, suitable for showing users as a means of explaining its significance. If there are many links of the same name, the gloss might or might not be the same for all the endpoints with the same name therein.
  6. in-line link: A link whose description constitutes one of its endpoints. HTML <A>, HyTime clink, and TEI <XREF> are all examples of in-line links.
  7. out-of-line link: A link whose description does not constitute one of its endpoints (except perhaps by chance). Such links only make sense given a notion like link databases, where applications know to look for links. Nevertheless, out-of-line links are useful for post-hoc annotation, and necessary for creating links with endpoints inside read-only data objects.

1.5 Types of link types

Many papers have been written on how to categorize links by type. The XHL effort distinguishes several major axes for organization. XHL provides ways to define a link's type along each axis, but only provides a standard vocabulary for the first kind of typing, namely link relationships. The vocabulary of XHL link types can be extended by creating sub-types at all levels.

  1. link relationships: Links express various kinds of relationships between the data objects or portions they connect, in terms of their role in the defining XML Application. Some links may be criticisms, others add support or background, while others have a very different meaning such as providing access to demographic information about a data object (its author's name, version number, etc), or to navigational tools such as index, glossary, and summary. A link's name provides a simple hierarchical mechanism for specifying link types.
  2. *****REDRAFT ENDS HERE 11/2/97********
  3. link topology: In-line versus out-of-line links differ in their structure, as do links with varying numbers of pointers.
  4. locator language: Links commonly differ in what formal language their pointers use to specify terminus locations. Different pointers of the same link may use different languages.
  5. link formatting: Links commonly differ in how they should be presented. This includes both how the user is informed that a link is present (such as highlighting, perhaps conditioned on history or other factors as in typical Web browsers), but also how the other termini are presented, such as having their title, summary, or explainer shown, or even being completely followed and presented inline with no user action required for traversal.
  6. link behavior: Links may have a wide variety of effects when traversed, such as opening, closing, or scrolling windows or panes; displaying the data from various termini in various ways; testing, authenticating, or logging user and context information; executing various programs. Ideally, link behavior should be determined by a semantic specification based on link types, pointer roles, user circumstances, and other factors; just as element formatting is determined by a stylesheet based on element type, context, and other factors. It is recognized that there is significant overlap between the areas of link formatting and link behavior.

2. Link Recognition

An XHL Link is an element contained in an SGML document. The fact that an element is a link must be recognized reliably by software in order to provide appropriate display and behavior. XHA links may be recognized based on the use of specially reserved attributes, the use of specially reserved element types, or through other means not described in this specification.

2.1 Link Recognition by Attribute

A link may be recognized based on the value of a reserved attribute named XHL-. Possible values are MLINK, TLINK, POINTER, XLG, and XLD, signalling in each case that the element may be treated as an element of the indicated type, as described in this specification.

An example of such a link:

<A XHL-="TLINK" HREF="http://www.w3.org/">The W3C</A>

2.2 Link Recognition by Element Type

If the document is an XML document, a link may be recognized based on the use of the reserved types -XML-MLINK, -XML-TLINK, -XML-POINTER, -XML-XLG, and -XML-XLD.

An example of such a link:

<-XML-TLINK HREF="http://www.w3.org/">The W3C</A>

2.3 Link Recognition by Other Means

The fact that a particular element type is to be recognized as an XHL link may be asserted by external means to the software processing the document. For example, such a program might have special sensitivity to the syntax of HTML, HyTime or some other hyperlinking syntax which is compatible with XHL.

An example of such a link:

<A HREF="http://www.w3.org/">The W3C</A>

3. Link Elements

XHL defines two types of Link Elements. First, a general Multilink (MLINK) which is out-of-line and may be used for multi-pointer links, links into read-only data, and so on. Second, a more constrained Terminus Link (TLINK), which is always in-line and one-directional, very like the HTML <A> element.

3.1 Information Associated With Links

This specification describes a variety of information that may be (and in some cases is required to be) associated with a link. As a matter of principle, this information is given entirely as markup rather than as character data; thus, any text contained in the link elements may be assumed to be a valid candidate for display along with the rest of the document in which the link is embedded.

The following information may be provided with link elements:

Every link has a link type, given in its TYPE attribute. A set of pre-defined link types is included in this specification. A sub-type may be created by appending "." and the sub-type name. This process may be repeated in order to create type hierarchies.
Every pointer has a role, given in the ROLE attribute. A starter set is similarly provided and similarly extensible.
Every pointer must identify an terminus in some fashion. This is done with a combination of attributes (HREF, HRTYPE, LOCSRC, IMPLIED-LOCSRC) as described in the next major section, Addressing.
Every pointer may specify a explainer in the TITLE attribute. This specification does not require that applications make any particular use of the explainer.
The BEHAVIOR attribute may be used for an author to communicate intentions concerning the traversal behavior of the link; this specification does not provide any default values, nor does it require that applications make any particular use of this information.
Formatting and Display
The RENDER attribute may be used for an author to communicate intentions concerning how the link and its termini should be rendered; this specification does not provide any default values, nor does it require that applications make any particular use of this information.

3.2 Multilink

A multilink can associate any number of termini, and an application may be expected to provide traversal between all of them (subject to semantic constraints outside the scope of this paper). The key issue with multilinks is how to find them, since they do not necessarily co-occur with any of their termini, and often are located in completely separate documents. This process is discussed under XHA Link Groups below.

A multilink's pointers are expressed as child elements of the multi-link, each with its own set of attributes. Here is the declaration for the XML form of the multilink:

          XHL-           CDATA              #FIXED "MLINK"
          TYPE           CDATA              #REQUIRED
          LOCSRC         CDATA              #IMPLIED
          XHL-     CDATA #FIXED "POINTER"
          ROLE     CDATA #REQUIRED
          HREF     CDATA #REQUIRED
          HRTYPE   CDATA "URL"

3.3 Terminus Links

Terminus links are very much like HTML <A> or TEI <XREF> elements, but with more general reference capabilities. A terminus link may contain only one pointer; thus there is no necessity for a separate child element, and the pointer attributes appear attached directly to the terminus link. The location where the link is placed is not required to act as a terminus; that is to say, there is no requirement that this location can be reached from the explicit terminus, though applications are free to provide that capability if they have a means of knowing the link exists.

For simplicity and compatibility with existing practice, the link TYPE and ROLE may be omitted.

Here is the declaration of the XML form of the terminus link:

          XHL-           CDATA              #FIXED "TLINK"
          TYPE           CDATA              #IMPLIED
          ROLE           CDATA              #IMPLIED
          HREF           CDATA              #REQUIRED
          HRTYPE         CDATA              "URL"
          LOCSRC         CDATA              #IMPLIED
          TITLE          CDATA              #IMPLIED     
          BEHAVIOR       CDATA              #IMPLIED
          RENDER         CDATA              #IMPLIED     

4. Addressing

XHA links can use many kinds of data as termini:

  1. documents: Entire data objects such as files in any representation, can be termini.
  2. nodes: Many documents have a well-defined structure of nodes, such as paragraphs in a flat ASCII file, elements in an SGML document, virtual objects in a VRML or vector-graphic model, and so on. Some such nodes may have unique names, identifiers, or serial numbers; other may be reachable by a scan of the document, such as walking down a tree by child numbers. Any such nodes can be termini.
  3. regions: Many documents have a normative rendered form for two-dimensional display such as on paper or screens. Any point, rectangle, or other region in such a form can be a terminus. For some data such as bitmapped images, the rendered form may be the only one of substantial interest, while for other data such as an online book the rendered form may be completely transient.

Note also that there is no requirement that a terminus be a singular data object; there is sufficient opacity in addressing methods that a single pointer might in fact address multiple data objects, which together would be considered a terminus.

A special case arises when the terminus indicated by a pointer proves to be another link element. When an application traverses or displays such a link, it may be desired to read and traverse that link to whatever its terminus (termini) is (are) and operate on the result of that traversal. This has the benefit of allowing greatly increased indirection and flexibility; on the other hand, it might lead to potentially unbounded delays in traversing simple-looking links. This specification does not constrain the behavior of applications in this situation.

4.1 HREFs and Reference Types

Each pointer is required to address a terminus using an HREF (HREF may have at one point stood for "Hypertext reference"; the name is adopted for compatibility with existing practice). An HREF is a character string containing the information used to address the terminus. An HREF may operate as a locator, as with a URL, a name, as with a URN or SGML FPI, or a query.

An HREF may be accompanied by a Reference Type, a name which identifies the interpretation of the HREF.

The HREF and reference type are provided, respectively, in the HREF and HRTYPE attributes.

4.2 Location Source

Many addressing mechanisms assume the existence of some sort of a base address whose value effects the interpretation of the address. To formalize this, every HREF has a location source, an object or objects which serves as the base address. In most cases the location source can be omitted and is implied to be the document element of the document that contains the link. However, this implied location source can also be specified as being the non-link element that refers to the link element (the referrer).

The location source is specified using the the location-source attribute. The format of the location-source string is not specified; to be useful, clearly it must be in a form that can be processed by the processor that can deal with the HREF and reference type to which it is attached. If the location-source attribute is omitted, the location source is implied according to the rule indicated by the IMPLIED-LOCSRC attribute.

The implied-locsrc attribute takes one of the values REFERRER or DOCELEM. The value REFERRER indicates that the implied location source is the referrer element. The value DOCELEM indicates that the location source is the document element, the document's root element.

The default value for implied-locsrc is DOCELEM.

4.3 SGML Reference Types

SGML HREFs use standard SGML Element and Entity addressing mechanisms. If the reference type is ENTITY, then the HREF must be an entity name which references an external entity. If the reference type is ELEMENT, then the HREF must be the value of a unique identifier attribute within the current document. If the reference type is SGML, then the HREF must contain an entity name interpreted as with ENTITY, followed by the character "#", followed by the value of a unique identifier attribute within the document referenced by the leading entity name.

4.4 URL Reference Types

If the reference type is URL, then the HREF must be a URL.

URLs may be used to refer to data objects of any kind.

4.5 TEI Locator Reference Types

If the reference type is TEI, the HREF must be a TEI locator.

XHA uses a subset of the syntax defined for TEI extended pointers. These operate on groves as defined in DSSSL, using the grove plan (set of structural information) specified in HyTime. Every construct in such locators has a corresponding expression in DSSSL's SDQL query language, and most also have direct equivalents in the HyTime location module. A full description of the syntax is available in the TEI Guidelines; a summary appears as Appendix A of this document.

The basic form of such a locator is a series of location terms, each of which specifies a location, either absolute or (more frequently) relative to the prior one. Each term has a name, such as ID, CHILD, ANCESTOR, and so on, and can be qualified by parameters such as an instance number, element types, or attributes. For example, the locator string "CHILD (2 CHAP) (4 SEC) (3)" refers to the 3rd child of the 4th SEC within the 2nd CHAP within the referenced document.

Such a locator can be considered a query, and the extended pointer syntax a query language. Thus it can be appended to a URL that identifies an appropriate document, in order to locate some portion of that document; or can be declared as a HyTime query, and thus used in a fully HyTime-conforming manner. [need to add an example of the necessary declarations, similar to Eliot's straw proposal].

XHA omits some features of TEI extended pointers:

  1. regular expression matching is not included, either as the "pattern" location term, or within parameters for other location terms.
  2. links to spans which are not elements are not supported, and so the "ditto" location term is not allowed.
  3. the "ref" location term for canonical references are not allowed.
  4. the "str" location term is not allowed, because of the subtleties associated with counting bytes and/or characters in international settings.
  5. the "HyQ" location term is not allowed, since it is obsolete.

[We may want to include a proposed TEI addition that provides for pointing directly to the values of attributes per se]

4.6 Query Reference Types

[Add a way to specify additional query languages]

5. Extended Link Groups

5.1 Identifying Extended Link groups

XHL describes the syntax of link elements embedded in documents. Many applications, when processing a document, may wish to process not only the links embedded in that document, but links in other documents which point into it. For example, it may be desirable to highlight the termini of such links to make the linkage network's existence apparent. In other words, it may be appropriate to process, rather than a single document, a group of interlinked documents.

In these cases, the Extended Link Group (XLG) element may be used to store a list of identifiers of other documents that together constitute an interlinked document group. Each such document, which must be known as an external text entitiy, is identified using the DOC attribute of an Extended Link Document (XLD) element, which is a child element of the XLG.

XLG and XLD elements, just as with MLINK, TLINK, or POINTER elements, may be recogized by the use of the XHL- attribute with the values XLG or XLD, in XML documents by the use of the reserved element types -XML-XLG and -XML-XLD, or through other external mechanisms.

Here is the declaration for the XML form of the XLG and XLD elements:

          XHL-     CDATA  #FIXED "XLG"
          XHL-     CDATA  #FIXED "XLD"
          DOC      ENTITY #REQUIRED

5.2 LINKS and LINKSET Elements

Multilinks may only occur in restricted contexts, in order to facilitate software finding them (their related termini, of course, may be anywhere at all).

First, multilinks may occur in the LINKS element of a document, which is part of its header. So long as any part of the document remains open, all termini of all multilinks in the header shall be known to the software and available to the user. Applications may apply their own conventions for when a document is no longer considered "open", except that any document any part of which remains in view, is open by definition.

Second, multilinks may occur in separate documents that contain only multilinks, called LINKSETS. Any document may specify any number of other documents and/or LINKSETS that should be opened. All termini specified in a LINKSET shall remain active at least as long as any of documents that points to that linkset remains open. [There is an argument to be made for keeping them open longer, such as for a whole session, or while any documents that have termini in the linkset are open; or for letting the referencing document(s) state how long they want it open; etc]. Software shall also provide a way for users to specify global or other LINKSETS, which should be kept open more persistently, such as throughout an entire user session, or automatically for all user sessions.


A. The TEI extended pointer syntax

A.1.1.1 Location Ladders

Each location pointer specification consists of a sequence of location term s, each of which consists of a keyword specifying a location type followed by one or more parenthesized parameter lists, each of which specifies a location value via a list of parameters. Location types and values, and the parameters within a location value, must be separated by white space characters.

Using terms borrowed from HyTime , we say that each TEI location term in a specification provides the location source for the next, and the entire specification is equivalent to a location ladder . By specifying the entire ladder in a single attribute value, the TEI extended pointer mechanism greatly reduces the syntactic and processing complexity of hypertextual pointers.

In formal terms:

ladder   ::=  locterm          |    ladder locterm 

A.1.1.2 Location Terms

The keywords used in location terms are these; references to the tree mean the tree representing the SGML document hierarchy.

points at the root of the target document
points at the location of the pointer
points at an ID within the target document
gives a canonical reference to a location in the target document
indicates an element found by descending one level in the tree
indicates an element found by descending one or more levels in the tree
indicates an element found by ascending one or more levels in the tree
indicates an element found by traversing the older siblings of the current location source
indicates an element found by traversing the younger siblings of the current location source
indicates an element found by traversing the entire portion of the document preceding the current location source
indicates an element found by traversing the entire portion of the document which follows the current location source
specifies a regular expression to be located within the existing location source
points at one or more tokens in the character content of the location source
points at one or more characters in the character content of the location source
points at a location using coordinates in some (application-defined) n-dimensional space
points at a location using some non-SGML method, and gives the name of the method

In formal terms:

   locterm ::=   'ROOT'                        // default first location
|    'HERE'                        // location of the xptr          |    'ID'
'(' NAME ')'             // only one ID allowed.          |    'REF' '('
characters ')'      // only one ref allowed          |    'CHILD' steps
|    'DESCENDANT' steps          |    'ANCESTOR' steps          |
'PREVIOUS' steps          |    'NEXT' steps          |    'PRECEDING' steps
|    'FOLLOWING' steps          |    'PATTERN' regs                // mult
patterns allowed          |    'TOKEN' '(' range ')'          |    'STR' '('
range ')'          |    'SPACE' '(' NAME ')' pointpair          |    'FOREIGN'
parms          |    'HYQ' parms          |    'DITTO'                       //
valid only in TO att. 

The keywords are not case sensitive.

Each location term specifies a location in the target document; this location may be a single point, more often a span of text (often the span of a single element) within the target document. The location ladder as a whole is interpreted from left to right, and each location term specifies a location relative to the location specified by the sequence prior to that point (i.e. to its location source). Unless here or id is specified as the first location term, the beginning location source is always root . An empty location sequence thus is the same as root and specifies the entire destination entity.

Some of the location terms make sense only in documents that have tree-like representations; these are id , child , ancestor , descendant , previous , next , preceding , and following . The latter six involve traversing the tree representing the SGML document hierarchy and are most easily understood when their location source is a single SGML element. If the location source is not a single SGML element, the tree-traversal keywords operate upon its beginning end-point, its front end (in English, this will be the leftmost point of the location source; in Arabic or Hebrew it will be the rightmost point). In this case child and descendant have no meaning, since character data has no descendants in the document tree; the first ancestor of such a location source is the element immediately containing the character data in question, and the siblings referred to by next and previous are the other children of that immediately containing element.

The details of each keyword are given below, along with definitions of their syntax and semantics of their results. Examples are also provided. It is strongly recommended that when IDs are available, they should be used in preference to the other methods for pointing defined here.

For all keywords, the description assumes that the target document does in fact contain a span or element which matches the description; otherwise, the location term has no referent and is said to fail . If any location term fails, the entire pointer fails. No backtracking or retrying is performed.

A.1.1.3 The ROOT Keyword

The location term root selects the document element. Since root is assumed as the implicit first term in any ladder, the following two location ladders have the same meaning:


A.1.1.4 The HERE Keyword

The keyword here designates the location at which the pointer element itself is situated; it allows extended pointers to select items like the paragraph immediately preceding the one within which this pointer occurs . Since it ignores any existing location source, this keyword typically makes sense only as the first location term in a location specification.

To designate the paragraph preceding the current one , the following location ladder could be used:


(See below for descriptions of the keywords ancestor and previous .)

A.1.1.5 The ID Keyword

The resulting location is the element within the destination entity whose ID attribute has the value specified as the location value.

For example, the location specification

ID (a27)

chooses the necessarily unique element of the destination entity which has an attribute of declared value of type ID, whose value is a27 .

A.1.1.6 The CHILD Keyword

The child location type specifies an element or span of character data in the document hierarchy using a location value which functions as a domain-style address. The value is a series of parenthesized steps, separated by white space. Each such step represents one level of the hierarchy within the location source. Each step may contain one or more parameters separated by white space and interpreted in order as follows:

  1. an instance indicator, which is a signed or unsigned integer or the special value ALL
  2. optionally, an SGML generic identifier
  3. optionally, one or more pairs, the first matching an SGML attribute name and the second matching an SGML attribute value

In formal terms, the location value of child is a series of steps :

steps    ::=  '(' step ')'          |    steps '(' step ')'  step     ::=
instance          |    instance element          |    instance element avspecs
avspecs  ::=  attribute value          |    avspecs attribute value  

Location values of the same form are also used by the keywords descendant , ancestor , previous , and next.

If an instance indicator alone is specified, as a number n , it selects the n th child of the location source. If the special value ALL is given, then all the children of the location source are selected. If the instance indicator is specified with following parameters, it selects all, or the n th, among those children of the location source which satisfy the other parameters. Negative numbers count from the last child of the location source to the first. The location source must contain at least n children; if it does not, the child term fails.

In formal terms, the first parameter of a step is an instance indicator, which in turn is either the special value ALL or a signed integer:

instance ::=  'ALL'          |    signed  signed   ::=  NUMBER
// default sign is +          |    '+' NUMBER          |    '-' NUMBER  

If a second parameter is given, it is interpreted as an SGML generic identifier, and only elements of the type indicated will be selected. For example, the location specification

CHILD (3 DIV1) (4 DIV2) (29 P)

chooses the 29th paragraph of the fourth sub-division of the third major division of the initial location source.

Constraint by generic identifier is strongly recommended, because it makes links more perspicuous and more robust. It is perspicuous because humans typically refer to things by type: as the second section , the third paragraph , etc. It is robust because it increases the chance of detecting breakage if (due to document editing) the target originally pointed at no longer exists.

The generic identifier may be specified as a normal SGML name or using the reserved values #CDATA or * . If the generic identifier is specified as * , any generic identifier is matched; this means that CHILD (2 *) is synonymous with CHILD (2) . If the second parameter is #CDATA , the location term selects only untagged sub-portions of an element having SGML mixed content.

The location ladder


thus chooses the third span of character data directly contained by the current location source. If the location source is a paragraph containing
  1. a sentence (A)
  2. an embedded quotation, marked as a q
  3. another sentence (B)
  4. an embedded note, marked as a note
  5. another sentence (C)
  6. a second embedded quotation, marked as a q

where the three sentences A, B, and C are character data enclosed by no element smaller than the paragraph itself, then CHILD (3 #CDATA) selects sentence C, while CHILD (3) selects sentence B.

If specified as a name, the generic identifier is case sensitive if and only if the SGML declaration specifies that generic identifiers are case sensitive (by default they are not).

In formal terms the second parameter of a step is defined thus:

element  ::=  NAME          |    '#CDATA'          |    '*'          |
'(' regular ')' 

The third and fourth parameters, if given, are interpreted as an attribute-value pair, and only elements which match that pair in the way described below will be selected; the fourth and fifth parameters, and all following pairs of parameters, are interpreted in the same way. When more than one pair is given, all must be matched.

The third, fifth, seventh, etc., parameters are interpreted, if specified, as attribute names. Like generic identifiers, attribute names may be specified as * in location ladders in the (unlikely) event that an attribute value constitutes a constraint regardless of what attribute name it is a value for.

For example, the location term


selects the first child of the location source for which the attribute target has a value.

As with generic identifiers, attribute names are case sensitive if and only if the SGML declaration says they are.

In formal terms, the attribute-name parameter of a tree-traversal step is defined thus:

attribute ::= NAME          |    '*'    

If a fourth, sixth, eighth, etc., parameter is specified, it is interpreted as an attribute value, and only elements satisfying the other constraints and also bearing an attribute of the specified name and value will be selected. The attribute value may be specified exactly as in an SGML document; as a consequence, if the attribute value to be specified contains white space characters, it must be enclosed in quotation marks. The attribute value may also be specified using the two special values #IMPLIED and * .

For example, the location specification

CHILD (1 * N 2) (1 * N 1)

chooses an element using the global n attribute. Beginning at the location source, the first child (whatever kind of element it is) with an n attribute having the value 2 is chosen; then that element's first direct sub-element having the value 1 for the same attribute is chosen.

If specified with quotation marks or as a regular expression, the attribute-value parameter is case-sensitive; otherwise not.

The location specification


selects the first child of the location source which is an fs element for which the resp attribute has been left unspecified.

In formal terms, the attribute-value parameter of a tree-traversal step is defined thus:

value    ::=  LITERAL                  // i.e. quoted string.          |
NAME                     // As for attribute values in          |    NUMBER
// document, NMTOKENs need not          |    NUMTOKEN                 // be
quoted          |    '#IMPLIED'               // No value specified, no
default          |    '*'                      // Any value matches. 

A.1.1.7 The DESCENDANT Keyword

If the descendant keyword is used, the location term selects an element or character-data string which is a descendant of the current location source. The parameters are the same as for child. The set of elements and strings which may be selected, however, is the set of all descendants of the location source (i.e. the set of all elements contained by it), rather than only the set of immediate children.

The location specification


thus selects the second term element with a lang of de occurring within the element with an id of a23 . The search for matching elements occurs in the same order as the SGML data stream; in terms of the document tree, this amounts to a depth-first left-to-right search.

If the instance number is negative, the search is a depth-first right-to-left search, in which the right-most, deepest matching element is numbered -1, etc. The location specification


thus chooses the last note element in the document, that is, the one with the rightmost start-tag.

A.1.1.8 The ANCESTOR Keyword

The ancestor location term selects an element from among the direct ancestors of the location source in the document hierarchy. The location value is of the same form as defined for the child. However, the ancestor keyword selects elements from the list of containing elements or ancestors of the location source, counting upwards from the parent of the location source (which is ancestor number 1) to the root of the document instance (which is ancestor number -1).

For example, the location term

ANCESTOR (1 * N 1) (1 DIV)

first chooses the smallest element properly containing the location source and having attribute n with value 1 ; and then the smallest div element properly containing it.

A.1.1.9 The PREVIOUS Keyword

The previous keyword selects an element or character-data string from among those which precede the location source within the same containing element. We speak of the elements and character-data strings contained by the same parent element as siblings ; those which precede a given element or string in the document are its elder siblings ; those which follow it are its younger siblings .

The instance number in the location value of a previous term designates the nth elder sibling of the location source, counting from most recent to less recent. The location ladder

ID (a23) PREVIOUS (1)

thus designates the element immediately preceding the element with an id of a23 . Negative instance numbers also designate elder siblings, counting from the eldest sibling to the youngest. The location source must have at least as many elder siblings as the absolute value of the instance number. If the location source has at least one elder sibling, then the location term


designates its eldest sibling and is thus synonymous with the ladder


The value ALL may be used to select the entire range of elder siblings of an element: the location ladder


thus designates the set of elements which precede the element with an id of a23 and are contained by the same parent.

A.1.1.10 The NEXT Keyword

The keyword next behaves like previous , but selects from the younger siblings of the location source, not the elder siblings. The location ladder

ID (a23) NEXT (1)

thus designates the element or string immediately following the element which has an id of a23 . Negative instance numbers also designate younger siblings, counting from the youngest sibling to the location source. The location source must have at least as many younger siblings as the absolute value of the instance number. If the location source has at least one younger sibling, then the location term

NEXT (-1)

designates its youngest sibling and is thus synonymous with the ladder


A.1.1.11 The PRECEDING Keyword

The preceding keyword selects an element or character-data string from among those which precede the location source, without being limited to the same containing element. The set of elements and strings which may be selected is the set of all elements and strings in the entire document which occur or begin before the location source. (For purposes of the keywords PRECEDING and FOLLOWING , elements are interpreted as occurring where their start-tag occurs.) The PRECEDING keyword thus resembles PREVIOUS but differs in searching a larger set of strings and elements; its result is not guaranteed to be a subset of its location source.

The instance number in the location value of a preceding term designates the nth element or character-data string preceding the location source, counting from most recent to less recent. The location ladder

ID (a23) PRECEDING (5)

thus designates the fifth element or string before the element with an id of a23 . Negative instance numbers also designate preceding elements or strings, counting from the eldest to the youngest; the ladder The location source must have at least as many elder siblings as the absolute value of the instance number; otherwise, the preceding term fails. The value ALL may be used to select the entire portion of the document preceding the beginning of the location source.

A.1.1.12 The FOLLOWING Keyword

The keyword following behaves like preceding , but selects from the portion of the document following the location source, not that preceding it.

A.1.1.13 The TOKEN Keyword

The token keyword selects a sequence of one or more tokens chosen from within the character content of the location source, where tokens are counted exactly as for the corresponding HyTime dataloc form with quantum=word. The location value must be either a single positive integer, or a pair of positive integers separated by white space, representing the first and the last token numbers to be included in the resulting location. If two integers are specified, the second must not be less than the first. The location source must contain at least as many tokens as are specified in the location value.

This location type must not be used to count across element boundaries.

For example, the location specification

ID (a27) TOKEN (3 5)

chooses the 3rd, 4th, and 5th tokens from the content of the element whose identifier is a27 . If this element contained the string This is _not_ a very good idea , the target selected would be not_ a very .

In formal terms the location value of the token and str keywords is defined as a range:

range    ::=  NUMBER          |    NUMBER NUMBER 

A.1.1.14 The SPACE Keyword

The space location term applies to entities which represent graphical or spatio-temporal data; typically such entities are not encoded in SGML, but in one of many specialized graphical formats. SGML provides standard mechanisms (the NOTATION declaration and related constructs) for specifying what format such an entity uses.

The location value for space consists of two or three parenthesized parameter lists. The first contains the name of the co-ordinate space in use. The second and third each consist of any number of signed integers. The numbers in a parameter list represent locations along each dimension of a Cartesian co-ordinate space with all axes orthogonal; the length of the list equals the number of dimensions/axes of the space (usually, but not inevitably, 2, 3, or 4).

If the third parameter list is not specified, the location is the single point in the co-ordinate space specified by the second parameter list. If all three parameter lists are specified, the location is the rectangular prism defined by treating corresponding items of the second and third lists as inclusive bounds along each dimension in turn.

The mapping from co-ordinates to physical or display space, and the meaning and ordering of the axes, are not defined by these guidelines. They should be specified in the TEI header unless they can be determined by definition from the format in which the referenced entity is known to be encoded (for example, many graphics formats can only encode locations in units of pixels, counted in a 3 dimensional left-handed co-ordinate space).

Time may be construed as an axis in addition to any others; when it is, it is TEI recommended practice that it be positioned last. The units used must be defined in the TEI header; it is acceptable in certain media (such as videodiscs) to use frame numbers as a surrogate axis for time.

For example,

SPACE (2D) (0 0) (1 1)

specifies the location of the unit square tangent to the origin in quadrant 1 of a common graph.

The location value for a space location term is a NAME enclosed in parentheses, followed by a point pair:

pointpair ::= '(' numbers ')'          |    '(' numbers ')' '(' numbers
')'  numbers  ::=  signed          |    numbers signed 

A.1.1.15 The FOREIGN Keyword

The foreign keyword takes any number of parenthesized parameter lists, and is terminated by the end of the attribute value, or by the next non-parenthesized token, whichever comes first.

The meaning of the foreign location term is not defined by these Guidelines. It is intended for use in pointing to special kinds of non-SGML, non-coordinate space data. That is, it should be used for making links to data which cannot be specified using the other mechanisms. The meaning of any foreign location types must be specified in the TEI header, as a series of paragraphs at the end of the encodingDesc element defined in section * . If more than one such type is used, it is TEI recommended practice that the first parameter list to foreign be a name associated with the particular type by documentation in the TEI header.

For example, assume that some program uses a proprietary data format called XFORM, and that the program has supplied an identifier 06286208998 for some piece of data it owns. Then the location specification

FOREIGN (XFORM) (06286208998)

would be one way of expressing a link to that piece of data.

B. Production note

The HTML copy of this draft specification was generated automatically from an XML source file using a custom formatter written in perl.

C. W3C SGML Working Group and Editorial Review Board

C.1 W3C SGML Working Group

C.2 W3C SGML Editorial Review Board