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Standard Generalized Markup Language (SGML) marks up document logical structure in a layout-independent manner [Gol90][HPR92][Org90][SGM86]. A Document Type Definition (DTD) is used to encapsulate the logical structure of specific classes of documents. Thus, SGML provides a notation for describing classes of structured documents and for coding documents belonging to described classes. An advantage of SGML and other grammar-based document representations is the ability to perform multiple applications on a single document source file. The International Committee on Accessible Documents (ICAD) has been working on defining an accessible DTD[+], but at present their work does not encompass mathematical content.

Though SGML is now used to markup a variety of documents by many
government agencies, it still has very little support for marking up
technical content, *e.g.,* mathematics. There is ongoing work to remedy
this situation. In the last year, the SGML-Math committee has been
working on a math DTD for SGML. This work is not yet complete, but it
has raised a few interesting issues. The main point of discussion has
been whether it is possible to design a math DTD that captures
semantic information about the mathematical constructs being marked
up. Though it would be nice to have all of a mathematical
construct's semantic content when processing the document, *e.g.,* in our
case producing audio renderings, this seems almost unattainable.
There is as yet no firm agreement on this point, but the trend seems
to be to move towards a math DTD that captures the layout as embodied
by TeX. Defining a DTD that captures full mathematical semantics
would make it difficult to invent new notation. TeX, by only
capturing the layout constructs used to build up written mathematics,
side-steps this issue, and the resulting system makes it easy to
invent new notation. However, this also makes recognition more
difficult. Some of the problems present in La)TeX are being
addressed by ongoing work on the project.

Significant work has been carried out in the context of
structure-sensitive editors for documents. This work has focused on
the design of appropriate document encodings that capture high-level
structure unambiguously. Another topic of interest has been the
capture of hypertext links within the context of structured
documents. The logical structure of documents is typically captured
using a tree-like representation consisting of hierarchical units.
The challenge of integrating this model with the notion of hypertext
links has been successfully addressed by the design of HyperText
Markup Language (HTML), an SGML-based markup system for encoding
structured hypertext documents. Finally, the aim of achieving the
best of two worlds, *i.e.,* the power afforded by a grammar-based markup
system and the user-interface provided by WYSIWYG systems (what you
see is what you get) has led to work on providing multiple
synchronized views of a document [Har88].
See [Bro88][QNA90][PS88][Ver90][BG90][CJ90][KS84][Ass86][Kat87][FS89][SFR92][Lev88][FBN<6287>>+90][SF90][SF88][PI88][KLMN90][BB90][LG90][QV92]
for details on relevant work in this area.

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Thu Mar 9 20:10:41 EST 1995