Guide to LaTeX (4th Edition) (Tools and Techniques

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D.2. L AT E X programming commands 449
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D.2.10 Useful internal commands
Notwithstanding the guidelines in Section D.1.2 that recommend avoiding internal
! L AT E X commands, there are a number that are fairly fundamental, and indeed form
many of the building blocks of the L AT E X kernel and many standard packages. Since
they are still internal commands, they are not guaranteed for all future updates.
However, if they were to vanish, many of the interesting extension packages
provided by the L AT E X Team itself would have to be drastically overhauled. We
merely present them briefly here for the sake of the bolder user.
\@namedef{cmd}{def }
\@nameuse{cmd}
define and execute a command named \cmd, where the backslash is not included
in the command name. This name may contain any characters, even those
normally forbidden in command names.
\@ifundefined{cmd}{true}{false}
executes true if the command \cmd does not exist, else false. Again, the backslash
is not included in cmd, and any characters may appear in the command name.
This test is often used to define commands conditionally, a task that has been
taken over by \providecommand. It may also be employed to determine whether
the main class is article-like or not: \@ifundefined{chapter}{..}{..} tests
for the existence of the \chapter command.
\@ifnextcharchar{true}{false}
tests if the next character is char, and if so, executes true, else false. This
command is traditionally used to define commands with optional arguments,
where char is [. The extended syntax of \newcommand offers a high-level means
of achieving this.
\@ifstar{true}{false}