I'm trying to create a LaTeX command with two arguments, one of them optional. Normally I'd do this as
\newcommand{\whatever}[2][default]{first #1 second #2}
where default
is the default value for the first argument. But for this command I want the value of the second argument to be used as the default value for the first argument - that is, I want
\whatever{blah}
\whatever{foo}
\whatever[lol]{rofl}
to be equivalent to
\whatever[blah]{blah}
\whatever[foo]{foo}
\whatever[lol]{rofl}
Does anyone know how to do this? I can drop down to plain TeX if necessary.