\documenttype{article}
\title{Why \emph{eos}\eoq;}
\author{William F. Hammond}
\begin{document}

The \abbr{GELLMU} \emph{article} document type has an end-of-sentence
mark \emph{eos}, which is a defined-empty \abbr{XML} element,
corresponding to the concept of sentence in languages such as English
and French.  But there is no provision for regarding a western
sentence itself as an \abbr{XML} element.  Why?

There are two reasons.

Sometimes one wants to begin a \emph{display} in the middle of a
sentence.  Then it can happen that the display is the last part of
the sentence.  It is a formal rule of \abbr{XML} that if an element
begins inside another element, then the second element must be closed
before the first element is closed.  Following this rule, when a
display is the last part of a sentence, the display must be ended
before the sentence is ended.  As a consequence an \abbr{XML} processor
must usually work very hard to place the sentence-ending punctuation
mark correctly.

Is this just a technical \abbr{XML} issue?  Not really.

The second reason for modeling an end-of-sentence mark but not a
sentence is that some literary use of a language such as English
does not actually resolve into clean sentence units even though
end-of-sentence punctuation is used.

One could argue that when a sentence is used, it could be marked up
with a \emph{sentence} element\footnote{The model would then likely
permit each of \emph{sentence} and \emph{display} to contain the
other}.  In that event it is unlikely that authors would want to be
required to insert end-of-sentence marks explicitly.  Moreover, there
would be something of a dilemma for the \abbr{XML} processor if it
happens to notice an item of \abbr{CDATA} at the end of a sentence
that appears to be an end-of-sentence mark.  There would still be
the vexation caused by a display that ends a sentence.  And would
authors use the \emph{sentence} element?

Will authors want to use the explicit \emph{eos} rather than the
simple \abbr{CDATA} punctuation mark \quochar{.}?  If so, how is the
sequence \qquostr{.<eos/>} to be handled by a processor?

Authors are the end users, and authors need convenience.  Reasonable
convenience lies in the convention that began with the dawn of the
mechanical typewriter:
\quote{A sentence is ended with a period followed either by a
newline or by two or more blank spaces\eos;}

Handling this convention is not a reasonably efficient task for
an \abbr{XML} processor.  But it works very well with a \latex;-like
markup interface for \abbr{XML}, i.e., when there is pre-processing
from \latex;-like markup to \abbr{XML} markup.

\end{document}