Models for Hypertext syntax

A syntax defines an abstract structure, and it could perhaps be an interesting art form to design hypertexts on purely abstract structural grounds, by analogy with abstract painting or sculpture, or phonetic poetry. In order to assign content to a syntactic structure, it must be interpreted in terms of a model. Mathematically, a model is a structure of some kind, but for the interpretation of signs we require a different notion of a model, one which is an approximation to our experience of reality.

Our experience of reality may be approximated to varying degrees of granularity, and on the basis of a various abstractions of properties from what we experience. For example, we may consider the following features of the syntax of texts (or sentence, or words, ..).

The simplest text syntax model for hypertext is linear arrangement of parts of the text in time. This model is in fact the basic model for simple traditional texts like straightforward chapter structure of a novel: the parts of the text can be regarded as hypertext documents linked end-to-beginning. This a hypertext built on this syntaax model is a string hypertext.

A more interesting text syntax model is that of a text with a table of contents and footnotes, which may be modelled by a tree structure model superimposed on the basic string model. A hypertext which corresponds to this model is a tree hypertext.

More interesting still is the text syntax model of a text with table of contents and footnotes, and with bibliographical references. Since the items in the bibliography may be referenced from more than one position, the model is no longer a tree, but a graph with re-entrancy (known in computational linguistics as structure sharing). A hypertext built on this model is a DAG hypertext or `directed acyclic graph hypertext'. The simpler models described above are special cases of DAG hypertexts.

Finally, a text syntax model which does not apparently occur in traditional texts is that in which a position back to a previous position in the DAG hypertext, resulting in a DCG hypertext, a `directed cyclic graph hypertext'.

But text syntax has two dimensions which are not yet covered by these distinctions:

1. The text syntax of the hypertext, on the basis of a DAG or DCG text syntax model for a given genre of hypertext. This is known as ID ordering or `immediate dominance' ordering in linguistics. This level is not identical to, but maps closely to the conceptual interpretation of the hypertext.
2. The relative linear ordering imposed on individual documents by the syntax of natural language, which may continue to the relation between individual documents, in the case of the string hypertext. This ordering is sometimes known as LP ordering or QLP ordering, for `linear precedence ordering' or `quasi-linear precedence ordering' in computational linguistics. This ordering is imposed by certain properties of the medium, and includes line-break, page-break, and document break specifications. This level corresponds to the abstract layout which can be defined in a language such as HTML.
3. The absolute linear (or higher dimensional) ordering imposed by the viewing situation, in terms of absolute position in space and time coordinates. These coordinate-based descriptions are also abstractions from our specific experiences, though they are intended to mirror these experiences. Coordinate-based ordering will be called CM ordering, for `coordinate matrix ordering'. This ordering corresponds, for example, to the structure imposed by the browser environment on a specific computer.

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The specification of the syntax of a hypertext document, for example in HTML, will be interpretable in terms of our own viewing experience by using a model of this kind. The model is very general, and applies to linguistic `microstructures' (where it has been known as the ILEX, or `Integrated Lexicon') model, as well as to hypertext `macrostructures'.

 
Figure 2: A three-layer model of text syntax.