\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.