[Tex/LaTex] What does \@car do

macros

The title says it all. I'd like to know what \@car does. I came across it reading through the source of the hyperref package, but it doesn't seem it's defined as part of the package. I tried Google, but there is the sub-string "car" …

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In LISP, it returns the first element of a list or a pair. In LaTeX, its definition is \def\@car#1#2\@nil{#1} (defined right before \def\@cdr#1#2\@nil{#2}), and it is defined in LaTeX core (latex.ltx).

The origins of the word come from the architecture of some early IBM computers.

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[Tex/LaTex] How to find a package name by a command name

$ grep -r '\\commandname' $TEXMF/tex/latex

[Tex/LaTex] Redefining sectioning commands

The output of \show cannot be used as you are trying. It says that \subsection is a "long" macro that has no arguments and the replacement text is what follows ->.

The right place where to look is report.cls, where you find

\newcommand\subsubsection{\@startsection{subsubsection}{3}{\z@}%
                                     {-3.25ex\@plus -1ex \@minus -.2ex}%
                                     {1.5ex \@plus .2ex}%
                                     {\normalfont\normalsize\bfseries}}
\newcommand\paragraph{\@startsection{paragraph}{4}{\z@}%
                                    {3.25ex \@plus1ex \@minus.2ex}%
                                    {-1em}%
                                    {\normalfont\normalsize\bfseries}}

The key point that explain the different behavior are: the negative fourth argument of \@startsection for \subsubsection which suppresses indentation of the following paragraph; the positive fifth argument tells TeX that the title will not be "run-in". So

\makeatletter
\renewcommand\subsection{%
  \@startsection{subsubsection}{3}{\z@}{3.25ex \@plus1ex \@minus.2ex}%
  {-1em}{\normalfont\normalsize\bfseries}}
\makeatother

The second argument to \@startsection tells LaTeX the level of the sectioning command for the purposes of secnumdepth and tocdepth and should stay 3.

Important note

The \makeatletter and \makeatother commands are necessary because we are doing stuff involving the internal command of LaTeX.

Every time a command containing @ in its name has to be used, the code must either be in a .sty file or surrounded by that pair. See What do \makeatletter and \makeatother do? for more information.

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[Tex/LaTex] How to find a package name by a command name

[Tex/LaTex] Redefining sectioning commands

Important note

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