[texhax] Eplain TeX: problem with \@optionalarg

[Rodolfo Medina]

Hi to all TeX users.

I posted the following problem to eplain mailing list but couldn't solve it.
Anybody there accustomed with eplain commands?

At:

 http://tug.org/eplain/doc/eplain/Macro-arguments.html

it is suggested the following code:

     \makeatletter
     % \mo{m}[o]
     \def\mo#1{\def\mo at arg{#1}\@getoptionalarg\fin at mo}

     \def\fin at mo{\vskip1pc
      \message{^^JArg: \mo at arg}%
      \message{^^JOptional arg: \@optionalarg}%
     }

Now, I wish to put a condition there:

   \ifx\@optionalarg\empty
        \fin at mo
   \else
        \@getoptionalarg\fin at mo
   \fi

, but it does not seem to work.  To test, edit a file as follows:

    \input eplain
     \makeatletter
     % \mo{m}[o]
     \def\mo#1{\def\mo at arg{#1}%
   \ifx\@optionalarg\empty
        \fin at mo
   \else
        \@getoptionalarg\fin at mo
   \fi
}

     \def\fin at mo{\vskip1pc
      \message{^^JArg: \mo at arg}%
      \message{^^JOptional arg: \@optionalarg}%
     }

\mo{test}[optional text]
\mo{test}

\bye


Thanks in advance for any help!
Rodolfo