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In addition to linearizing the underlying tree structure,
mathematical notation uses *visual attributes* such as
superscripts and subscripts. We extend the prefix form to
capture such visual attributes -hence the name
*quasi*-prefix.

The key feature of the quasi-prefix form is that it delays
the assignment of semantic interpretation to instances of
ambiguous written mathematics. For example, the superscripts in
an expression are represented not as exponents but as attribute
*superscript*. This is because the meaning of these
visual attributes is context dependent. Assigning one of the
several possible interpretations at the recognition step is
unduly restrictive in a fully flexible rendering system. For
example, interpreting the superscript as an exponent would
result in [tex2html_wrap5262] being recognized correctly, but
[tex2html_wrap5264] being incorrectly recognized. Further, it
would be impossible to later distinguish between the correct
and incorrect interpretations. The quasi-prefix form captures
the mathematical notation itself, leaving the assignment of
semantic interpretation to a later step. By doing so, we can
represent content where we do not have sufficient semantic
information. Thus, [tex2html_wrap5266] might denote the first
derivative of [tex2html_wrap5268] with respect to
[tex2html_wrap5270] in a specific context. The superscript and
subscript might mean something entirely different in another
context, *e.g.,* as in [tex2html_wrap5272]. If more
contextual information is available at the rendering step,
AsTeR can speak [tex2html_wrap5274] as ``cap a transpose''. In
the absence of such contextual information, the system can
still produce an audio notation that maps different features of
the written notation to unique audio dimensions.

At the same time, the quasi-prefix form is sufficiently rich
to permit renderings that are independent of the order in which
the written symbols appear on paper. Linear renderings with the
rendering-order hard-coded into the system can be produced with
a simpler representation, *e.g.,* a linear list, or even
the TeX encoding itself. This was shown by , a
string-substitution based program that directly transformed TeX
source to produce linear renderings [Ram92][Ram91].

As an example, assume for the present that
`\kronecker`

[+] is defined as an infix binary
operator. Given the expression [tex2html_wrap5276] encoded as
`$a\kronecker b$`

, we can write a rendering rule for
object *kronecker* represented in the quasi-prefix form
to produce ``a kronecker product b''. This rendering can be
produced by as well, but a simpler list-like representation
restricts the system to this one form of rendering. Using the
quasi-prefix form, AsTeR can also produce ``the kronecker
product of a and b''.

Thus, even though the quasi-prefix form captures only the information present in the TeX encoding, it is still flexible enough to permit more sophisticated processing.

This power is necessary in overcoming the passive nature of listening. In producing printed output, it is sufficient to produce one view; once the information has been presented visually, a person reading the material can access it in any desired order. TeX itself therefore never builds up an internal representation like the quasi-prefix form; its purpose is to typeset the input according to a fixed set of rules, and the TeX encoding directly reflects the linear order[+] in which expressions appear on paper. Thus, here, the displayed information is passive while the person reading it is active. The situation in presenting information orally is exactly the opposite; the information flows past a passive listener. In order to achieve effective oral communication, it is therefore important to be able to present multiple views of the information.

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TV Raman

Thu Mar 9 20:10:41 EST 1995