12.14.3 \CheckCommand

Synopsis:

\CheckCommand{cmd}[num][default]{definition}
\CheckCommand* (same parameters)

Like \newcommand (see \newcommand & \renewcommand) but does not define cmd; instead it checks that the current definition of cmd is exactly as given by definition and is or is not \long as expected. A long command is a command that accepts \par within an argument.

With the unstarred version of \CheckCommand, cmd is expected to be \long; with the starred version, cmd must not be \long

\CheckCommand raises an error when the check fails. This allows you to check before you start redefining cmd yourself that no other package has already redefined this command.