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\title{The Use of \emph{newcommand} in GELLMU}
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\begin{center}\LARGE\bfseries{}
The Use of \emph{newcommand} in GELLMU
\end{center}

\medskip

\section*{1\ \ \label{SU-1}Introduction}
\par{In \textsc{GELLMU} the command \emph{newcommand} is a meta-command that is
handled by macro-like pre-processing in the \textsc{GELLMU} syntactic translator. \   This pre-processing
takes place very early after the processing of the required meta-command
\emph{documenttype} and after the insertion of any (relatively rare)
internal \textsc{SGML} declaration subset. \ 

}

\section*{2\ \ \label{SU-2}First Examples}
\par{The markup for the field \(\mbox{\textbf{Q}}\) of rational numbers is a \emph{newcommand}
invocation \texttt{"}\verb+\+\texttt{Q"} without arguments that expands to
\texttt{"}\verb+\+\texttt{regch}\verb+{+\texttt{}\verb+\+\texttt{bold}\verb+{+\texttt{Q}\verb+}+\texttt{}\verb+}+\texttt{"}.\footnote{The command \emph{regch}
is an alternative form of \emph{mbox} in the didactic \textsc{GELLMU} document
type definition that differs from the latter in that it is permitted as
the content of an ``accent'' tag as in the example \(\bar{\mbox{\textbf{Q}}}\)\@, 
which is written in the \textsc{GELLMU} source here with
\texttt{\textbackslash{}ovbar\{\textbackslash{}Q\}}. \ }
Newcommand names are case-sensitive. \   Here the newcommand invocation
\texttt{"}\verb+\+\texttt{q"} expands to \texttt{"}\verb+\+\texttt{regch}\verb+{+\texttt{}\verb+\+\texttt{slnt}\verb+{+\texttt{Q}\verb+}+\texttt{}\verb+}+\texttt{"},
which yields the ad-hoc math symbol \(\mbox{\textsl{Q}}\)\@. \ 
}
\par{Markup to represent the general linear group may be a \emph{newcommand}
with definition
\begin{center}
\verb+\newcommand{\gl}[2][2]{\mbox{GL}_{#1}(#2)}+ . \ 
\end{center}
When invoked with \texttt{"}\verb+\+\texttt{gl}\verb+{+\texttt{3}\verb+}+\texttt{}\verb+{+\texttt{F}\verb+}+\texttt{"}, it yields
\(\mbox{GL}_{3}(F)\)\@. \ 
}
\par{Mention of the group \(\mbox{GL}_{2}(F)\) is obtained alternatively here with the
minimized markup invocation \texttt{"}\verb+\+\texttt{gl}\verb+{+\texttt{F}\verb+}+\texttt{"} relying on the
default value of the first argument given in the definition of
\texttt{"}\verb+\+\texttt{gl"}. \ 

}

\section*{3\ \ \label{SU-3}Some Details}
\par{With the \emph{newcommand} definition
\begin{center}
\verb+\newcommand{\Q}{\regch{\bold{Q}}}+
\end{center}
the markup \verb+\Q;+ may always be used instead of \verb+\Q+
and must be used if the \(\mbox{\textbf{Q}}\) is followed immediately by characters
that could have the effect of making the \texttt{`Q'} in the markup
look simply like the first character in a larger string. \   However, the
markup \verb+\Q;{}+ is incorrect unless \emph{regch} is a
command that takes two arguments. \   For it expands to
\begin{center}
\verb+\regch{\bold{Q}}{}+ . \ 
\end{center}

}

\subsection*{3.1\ \ \label{SU-3.1}What Markup Can Follow a \emph{newcommand}? \ }
\par{What characters can safely follow the name of a \emph{newcommand}? \ 
Of course, a following \texttt{`}\verb+{+\texttt{'} can only mean the beginning of
an argument to the command. \   On the other hand the \texttt{`}\verb+{+\texttt{'} can
be escaped. \ 
}
\par{Some examples:
\begin{center}
\begin{tabular}{ll}
{}\textbf{Markup}~~~~ & \textbf{Yield}
\\ {}\verb+$\Q$+ & \(\mbox{\textbf{Q}}\)
\\ {}\verb+$\Q;xy$+ & \(\mbox{\textbf{Q}}xy\)
\\ {}\verb+$\Q;$xy+ & \(\mbox{\textbf{Q}}\)xy
\\ {}\verb+$\Q;;$+ & \(\mbox{\textbf{Q}};\)
\\ {}\verb+$\Q;$;+ & \(\mbox{\textbf{Q}}\);
\\ {}\verb+$\Q\bsl;$+ & \(\mbox{\textbf{Q}}\backslash{}\)
\\ {}\verb+$\Q\{$+ & \(\mbox{\textbf{Q}}\{\)
\\ {}\verb+$\Q\}$+ & \(\mbox{\textbf{Q}}\}\)
\\ {}\verb+$\Q\lsb;$+ & \(\mbox{\textbf{Q}}[\)
\\ {}\verb+$\Q\rsb;$+ & \(\mbox{\textbf{Q}}]\)
\\ {}\verb+$\Q!$+ & \(\mbox{\textbf{Q}}!\)
\\ {}\verb+$\Q@$+ & \(\mbox{\textbf{Q}}@\)
\\ {}\verb+$\Q\#$+ & \(\mbox{\textbf{Q}}\#\)
\\ {}\verb+$\Q\%$+ & \(\mbox{\textbf{Q}}\%\)
\\ {}\verb+$\Q^5$+ & \(\mbox{\textbf{Q}}^{5}\)
\\ {}\verb+$\Q_p$+ & \(\mbox{\textbf{Q}}_{p}\)
\\ {}\verb+$\Q\und;$+ & \(\mbox{\textbf{Q}}\_\)
\\ {}\verb+$\Q\crt;$+ & \(\mbox{\textbf{Q}}\wedge\)
\\ {}\verb+$\Q*$+ & \(\mbox{\textbf{Q}}*\)
\\ {}\verb+$\Q-$+ & \(\mbox{\textbf{Q}}-\)
\\ {}\verb+$\Q(t)$+ & \(\mbox{\textbf{Q}}(t)\)
\\ {}\verb+$\Q+\verb-+-\verb+$+ & \(\mbox{\textbf{Q}}+\)
\\ {}\verb+$\Q=$+ & \(\mbox{\textbf{Q}}\, = \,\)
\\ {}\verb+$\Q$+= & \(\mbox{\textbf{Q}}\)=
\\ {}\verb+$\Q,$+ & \(\mbox{\textbf{Q}},\)
\\ {}\verb+$\Q.$+ & \(\mbox{\textbf{Q}}.\)
\\ {}\verb+$\Q?$+ & \(\mbox{\textbf{Q}}?\)
\end{tabular}
\end{center}

}

\subsection*{3.2\ \ \label{SU-3.2}Comments in newcommand definitions}
\par{It is useful here to comment on \textsc{GELLMU}, which is a bridge from
\LaTeX{} to \textsc{SGML}, as it compares to \LaTeX{}. \ 
\LaTeX{} usage of \emph{newcommand} sometimes takes the form
%nul
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\begin{center}
\begin{tabular}{l}
{}\verb+\newcommand{\foo}[2]{%+
\\ {}\verb+This is an instance of the \emph{newcommand} ``foo'' %+
\\ {}\verb+with first argument `#1' and%  Space following a newline is swallowed+
\\ {}\verb+ second argument `#2'.}+
\end{tabular}
\end{center}
}
\par{This usage is useful in that it gives the author a way, especially with
empty comments, to make the source document more readable. \ 
}
\par{With \textsc{GELLMU} as a general matter such use of comments in regular
command arguments may have side effects if the author is not careful. \ 
Because \emph{newcommand} is a meta-command handled by the \textsc{GELLMU} syntactic translator, this
use of comments will not have side effects. \ 
}
\par{Since \emph{newcommand} in \textsc{GELLMU} is a meta-command, it is \textbf{not}
necessary to restrict an instance of \emph{newcommand} to a document
preamble. \   However, any invocation of a command defined with
\emph{newcommand} must be preceded by the definition. \ 
}
\par{The definition of \texttt{}\verb+\+\texttt{foo} appears in this
document just before the display of its definition, and the next
sentence is the invocation \texttt{"}\verb+\+\texttt{foo}\verb+{+\texttt{cat}\verb+}+\texttt{}\verb+{+\texttt{dog}\verb+}+\texttt{"}. \ 
This is an instance of the \emph{newcommand} ``foo'' with first argument `cat' andsecond argument `dog'. \ 
}
\par{Note that the comment ``Space following \ldots{}'' on the second
line of the value string absorbs the space at the beginning of the
third line with the result that the words \emph{and} and \emph{second}
are run together. \   This phenomenon mimics \LaTeX{}; however, such
comment ``overrun'' is the behavior of the \textsc{GELLMU} syntactic translator
only inside a \emph{newcommand} value string. \   In all other contexts
there is no comment overrun in \textsc{GELLMU}, and space at the beginning of
a line following a comment is significant. \ 

}

\subsection*{3.3\ \ \label{SU-3.3}Newlines in newcommand definitions}
\par{It is important to note, however, that \emph{anything} inside the
value argument of a \emph{newcommand} definition is propogated by the
\textsc{GELLMU} syntactic translator to every invocation. \   With basic \textsc{GELLMU} newlines inside a
newcommand value string are removed since \textsc{SGML} does not acknowledge a
newline as a special form of white space and since newlines in a value
string, when propogated to all invocations cause extra lines in the
\textsc{GELLMU} syntactic translator output, thereby blocking the goal of line number alignment
between \textsc{GELLMU} source and the \textsc{SGML} output, which is important for
the interpretation of \textsc{SGML} parser error messages. \ 
}
\par{In regular \textsc{GELLMU}, newlines can be significant. \   For this reason
newlines in newcommand definitions are not removed. \   However, the use
of newlines inside value strings is deprecated since they break line
number alignment between the \textsc{GELLMU} source and the output of the
\textsc{GELLMU} syntactic translator. \   There are other ways to achieve the effect of a newline inside
a newcommand value string. \ 

}
\end{document}