GNU-Darwin Web
"http://www.w3.org/TR/REC-html40/loose.dtd"> How to detect and correct errors
 4 How to detect and correct errors

Most of the problems that occur during the translation of a given LATEX file (say trouble.tex) can be detected and solved at the macro-level. That is, most problems induce a macro-related warning and can be solved by writing a few macros. The best place for these macros is an user style file (say trouble.hva) given as argument to HEVEA.
# hevea trouble.hva trouble.tex

By doing so, the macros written specially for HEVEA are not seen by LATEX. Even better, trouble.tex is not changed at all.

Of course, this will be easier if the LATEX source is written in a generic style, using macros. Note that this style is recommended anyway, since it eases the changing and tuning of documents.

 4.1 HEVEA does not know a macro
Consider the following LATEX source excerpt:
You can \raisebox{.6ex}{\em raise} text.

LATEX typesets this as follows:

Since HEVEA does not know about \raisebox, it incorrectly processes this input. More precisely, it first prints a warning message:
trouble.tex:34: Unknown macro: \raisebox

Then, it goes on by translating the arguments of \raisebox as if they were normal text. As a consequence some .6ex is finally found in the HTML output:

You can .6exraise text.
To correct this, you should provide a macro that has more or less the effect of \raisebox. It is impossible to write a generic \raisebox macro for HEVEA, because of HTML limitations. However, in this case, the effect of \raisebox is to raise the box a little. Thus, the first, numerical, argument to \raisebox can be ignored in a private \raisebox macro defined in trouble.hva:
\newcommand{\raisebox}[2]{$^{\mbox{#2}}$}

Now, translating the document yields:

You can raise text a little.
Of course, this will work only when all \raisebox commands in the document raise text a little. Consider, the following example, where text is both raised a lowered a little:
You can \raisebox{.6ex}{\em raise}
or \raisebox{-.6ex}{\em lower} text.

Which LATEX renders as follows:

Whereas, with the above definition of \raisebox, HEVEA produces:

You can raise or lower text.
A solution is to add a new macro definition in the trouble.hva file:
\newcommand{\lowerbox}[2]{$_{\mbox{#2}}$}

Then, trouble.tex itself has to be modified a little.
You can \raisebox{.6ex}{\em raise}
or \lowerbox{-.6ex}{\em lower} text.

HEVEA now produces a satisfying output:

You can raise or lower text.
Note that, for the document to remain LATEX-processable, it should also contain the following definition for \lowerbox:
\newcommand{\lowerbox}[2]{\raisebox{#1}{#2}}

This definition can safely be placed anywhere in trouble.tex, since by HEVEA semantics for \newcommand (see section B.8.1) the new definition will not overwrite the old one.

 4.2 HEVEA incorrectly interprets a macro

Sometimes HEVEA knows about a macro, but the produced HTML does not look good when seen through a browser. This kind of errors is detected while visually checking the output. However, HEVEA does its best to issue warnings when such situations are likely to occur.

Consider, for instance, this definition of \blob as a small black square.
\newcommand{\blob}{\rule[.2ex]{1ex}{1ex}}
\blob\ Blob \blob

Which LATEX typesets as follows:

HEVEA always translates \rule as <HR>, ignoring size arguments. Hence, it produces the following, wrong, output:

Blob

There is not small square in the symbol font used by HEVEA. However there are other small symbols that would perfectly do the job of \blob, such as a bullet (\bullet). Thus, you may choose to give \blob a definition in trouble.hva:
\newcommand{\blob}{\bullet}

This new definition yields the following, more satisfying output:

· Blob ·
Now, if you insist on having a square blob'', you can. It suffices to have LATEX typeset some subparts of the document and then to include them as images, section 6 explain how to proceed.

 4.3 HEVEA crashes

HEVEA failure may have many causes, including a bug. However, it may also stem from a wrong LATEX input. Thus, this section is to be read before reporting a bug...

 4.3.1 Simple cases: LATEX also crashes
In the following source, environments are not properly balanced:
\begin{flushright}
\begin{quote}
This is right-flushed quoted text.
\end{flushright}
\end{quote}

Such a source will make both LATEX and HEVEA choke. HEVEA issues the following error message that shows the LATEX environment that is not closed properly:
trouble.tex:7: hml: DIV closes BLOCKQUOTE
trouble.tex:5: Latex environment quote'' is pending

Thus, when HEVEA crashes, it is a good idea to check that the input is correct by running LATEX on it.

 4.3.2 Complicated cases

Unfortunately, HEVEA may crash on input that does not affect LATEX. Such errors usually relate to environment or group nesting.

Consider for instance the following optimized'' version of a quoteright environment:
\newenvironment{quoteright}{\quote\flushright}{\endquote}

\begin{quoteright}
This a right-flushed quotation
\end{quoteright}

The \quote and \flushright constructs are intended to replace \begin{quote} and \begin{flushright}, while \endquote stands for \end{quote}. Note that the closing \endflushright is omitted, since it does nothing. LATEX accepts such an input and produces a right-flushed quotation.

However, HEVEA usually translates LATEX environments to HTML block-level elements and it requires those elements to be nested properly. Here, \quote translates to <BLOCKQUOTE>, \flushright translates to <DIV ALIGN=right> and \endquote translates to </BLOCKQUOTE>. At that point, HEVEA refuses to generate obviously non-correct HTML and it crashes:
trouble.tex:9: hml: BLOCKQUOTE closes DIV
trouble.tex:7: Latex environment quoteright'' is pending

In this case, the solution is easy: environments must be opened and closed consistently. LATEX style being recommended, one should write:
\newenvironment{quoteright}
{\begin{quote}\begin{flushright}}
{\end{flushright}\end{quote}}

And we get:

This is a right-flushed quotation

Unclosed LATEX groups ({...) are another source of nuisance to HEVEA. Consider the following horreur.tex file:
\documentclass{article}

\begin{document}
In this sentence, a group is opened now {\em and never closed.
\end{document}

LATEX accepts this file, although it produces a warning:
# latex horreur.tex
This is TeX, Version 3.14159 (Web2C 7.2)
...
(\end occurred inside a group at level 1)
Output written on horreur.dvi (1 page, 280 bytes).


By contrast, running HEVEA on horreur.tex yields a fatal error:
# hevea horreur.tex
horreur.tex:5: Latex env error: document'' closes ''
horreur.tex:4: Latex environment '' is pending

Thus, users should close opening braces where it belongs. Note that HEVEA error message Latex environment env'' is pending'' helps a lot in locating the brace that hurts.