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Back to the "Quotes and punctuation" problem:  So the main problem in
TeX which makes pattern matching/lookahead/parsing very difficult in
TeX is that the parsing routine cannot know into what kind of stuff
future tokens expand without literally expanding the whole document.

Would a two-tier expansion mechanism (to be implemented as an
extension to TeX-the program) help? I am thinking along these lines:
Both macro names and each of their arguments would get one of the
attributes TRANSPARENT (T) or OBLIQUE (O). One could then have a
first-tier expansion (T-expansion, say) in which only those macros and
arguments which have the T feature are expanded. The result could then
be processed with traditional methods (regular expression matching,
tansformation patterns etc.) before final expansion of the O-macros
takes place.

Below I indicate the expansion categories for a few standard LaTeX
macros in a sort of Pascal like fashion, i.e., the first category
refers to the macro itself, the others to the arguments:

TRANSPARENT \ref{TRANSPARENT}
OBLIQUE \section[OBLIQUE]{TRANSPARENT}
OBLIQUE \label{OBLIQUE}
OBLIQUE \newcounter{OBLIQUE}[OBLIQUE]
OBLIQUE \flushbottom
OBLIQUE \begin{tabular}[OBLIQUE] TRANSPARENT \end{tabular}
OBLIQUE \makebox(OBLIQUE,OBLIQUE)[OBLIQUE]{TRANSPARENT}
TRANSPARENT \cite{TRANSPARENT}
OBLIQUE \bibitem[OBLIQUE,OBLIQUE]
OBLIQUE \emph{TRANSPARENT}
OBLIQUE \itshape
TRANSPARENT \newcommand{OBLIQUE}[OBLIQUE][OBLIQUE]{OBLIQUE}

Assume, e.g., that the \ref command is defined to typset boldface,
and a command \last has been defined which expands into "last".
Let's T-expand the line

Theorem \ref{fermat} was his \emph{\last}.

Since \ref is TRANSPARENT, it expands, while \emph is OBLIQUE, but has
a TRANSPARENT argument, which expands.  So the result may look like

Theorem \textbf{1.3} was his \emph{last}.

A pattern matcher could now e.g. ignore all OBLIQUE tokens in this
text, and thus match on "his last".

How will this help with the quote problem?  The text

\newTcommand{\story}[T]{There was a man, who said, <I'll tell you a
story>, and he began: <#1>}

\story{\story{\story{\ldots}}}}

will T-expand into

There was a man, who said, <I'll tell you a story>, and he began:
<There was a man, who said, <I'll tell you a story>, and he began:
<There was a man, who said, <I'll tell you a story>, and he began:
<\ldots>>>

I assume that < and > are active characters of category OBLIQUE, so
they do not expand at this stage. When full expansion takes place, the
> can do a lookahead and, e.g., detect the following > from which it
can determine optimal spacing (keeping track of the nesting level
should be easy, I guess). The problem with braces (e.g. when a font
change takes place) does seem linked to the particular way that
\futurelet works, but I don't see how this poses a fundamental problem

Two comments regarding other points that came up in the discussion:

While I think that this quote problem might be useful to illustrate
general parsing problems, I don't think it is reasonable to expect any
author to write \quote{Quoted text}, or to make super smart decisions
about the placement of punctuation.  Any author who cares will do it
right, and those who don't care won't want to write \quote, anyway.

Related to this: Yes, I do believe that the primary goal of LaTeX
should be to provide a human readable direct input typesetting
language. If the abstract structure of the document can be completely
specified, the better, but it seems that these two goals may be
ultimately incompatible. I really know SGML/XML (Sebastian, what do
you hope to get out of the LaTeX3 project that you think you cannot
get out of SGML?), but I seem to understand that SGML is well suited
to provide complete logical markup. So why re-do something that
already exists and supposedly works well? Moreover, especially when
typing Mathematics, complete logical mark-up is way beyond what most
authors need in practice. If I need a tool which is optimized w.r.t.
typsetting, I use LaTeX. I don't care whether the mark-up is sloppy in
certain places---it is not important to me whether I can re-use the
input in more structured applications. An example: for humans it makes
perfect sense to use \ldots in a formula, but I could not even expect
a symbolic system like Mathematica to understand what I mean. If my
requirements go beyond publishing, my primary tools are different
(although it would help if they could export to LaTeX when it comes to
publishing), but then I don't mind the extra effort required.

Marcel