Two-level Hypertext Models as an Underpinning for AHSs
Baltimore MD 21228-5398 USA
In recent years there has been an explosion of interest in models of
hypertext that go beyond the explicit hard-wired links found in
traditional hypertext models. In these new models, text is augmented
with a secondary structure through which all links are routed. A
variety of secondary structures have been proposed, including semantic
nets [trigg86, collier87, rada90, kheirbek93, mayfield93], Petri nets
[furuta89], and connectionist nets [hu93]. The purpose of the
secondary structure also varies, although in each case it adds
knowledge about the nodes and links of the hypertext.
I contend that two-level models form a natural underpinning for AHSs,
because they provide a level of abstraction to hypertext links that is
required if a hypertext system is to be adaptive. This abstraction
allows the system to provide the appearance of an extremely malleable
hyperspace which is nonetheless drawn out of a primarily static set of
nodes and links.
SNITCH [mayfield93] is an example of a two-level hypertext model that
provides this link abstraction capability. In the SNITCH model, a
semantic net representation of a text is connected to the phrases of
that text. A hypertext link in this model is a virtual connection
between two pieces of text called anchors. Each anchor comprises one
or more words in the text that express a single concept. A semantic
net is a graph whose vertices correspond to concepts (e.g. "person" or
"house"), and whose edges correspond to relationships between those
concepts (e.g. "owns"). Semantic nets are used to represent knowledge
about the world and to express the meaning of phrases and sentences of
languages such as English. The underlying representation of a SNITCH
hypertext link is a path that starts at an anchor in the text, moves
across to the semantic net, traverses a sub-path through the semantic
net, and then returns to another anchor in the text.
The original objectives of the SNITCH project were to support the
automatic construction of hypertext links, and to endow those links
with rich semantics. The augmentation of a set of documents with a
semantic net has six significant advantages over existing hypertext
Of these advantages, the first and last are particularly important
from the standpoint of AHSs. While many adaptive systems have a
long-term component, it seems to me crucial in most contexts for the
system to adapt to a user within a single session. By altering the
shapes of the allowable paths through the semantic net, the SNITCH
model can provide this kind of intrasession adaptation without
requiring modification to the node/anchor/link structure of the
hypertext. Furthermore, automatic inference processes can be tailored
to particular users' needs, producing novel paths through the semantic
- Links between documents are discovered dynamically. Links that
connect different parts of the corpus can be discovered dynamically,
as long as the components of the desired relationship have been
expressed in the semantic net. This dynamic discovery of links means
that the system designer need not anticipate every link type that a
user might ultimately request.
- Knowledge can be reused. Because a single edge in the semantic net
can be shared by many different paths, the knowledge implicit in a
single semantic net edge is available for use in many distinct
contexts. Furthermore, portions of the net can be developed
independently, allowing system designers to use existing semantic nets
(such as CYC [guha91] or WordNet [miller90]) that express facts about
the domain of interest. The ongoing development of large, sharable
ontologies and knowledge bases [neches91] makes this advantage
- Texts can be incorporated individually. Using the SNITCH approach,
input texts can be processed separately. Since each input text is
connected to the same semantic net, paths between two related texts
will be generated even if the texts are incorporated at separate
- Links derive meaning from the semantic net. Since each link
between anchors is composed of edges, and since each edge has an
associated semantics, the composition of these edge semantics give
meaning to that link. Thus, the use of even a small number of semantic
net edge types can result in great expressive power. This contrasts
with the approach taken in many existing hypertext systems, whereby
hypertext links indicate only an anonymous association between two
pieces of text.
- Links between documents can be created automatically. The SNITCH
approach facilitates the automatic creation of meaningful links, by
using natural language processing techniques to translate a text into
a semantic net representation of that text.
- New links can be created by automatic inference processes. Once an
input text has been connected to its semantic net representation,
automatic inference processes can manipulate and update the semantic
net. New semantic net edges created by such a process can be
interpreted as components of new hypertext links.
SNITCH is not the only two-level hypertext model; each such model
presents slightly different advantages from the AHS viewpoint. The
basic feature provided by all of them though is a level of abstraction
between the appearance of a link and its underlying structure. Such
abstraction is crucial for AHSs. I hope that thinking of AHSs as
two-level models will help to clarify the nature of the requirements
that we place on the hypertext half of AHSs.
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