[tex-eplain] optional arguments

[Karl Berry]

    \catcode`@=\letter
    \def\residx{\@getoptionalarg\finresidx}
    \def\finresidx#1{\ifx\@optionalarg\empty{\sidx{#1}}
                 \else{\sidx{#1}[\@optionalarg]}\fi}

    And a result later. \residx[sub]{result}
    [...]
    However, what is produced in the index is the entry "result" with the
    subentry "optionalarg ". (Note it also has an extra space.)

Looking at the .idx file, I see:

\indexentry{result!\@optionalarg }{1}

So evidently the \@optionalarg macro is not expanded in the call to
\sidx.  I don't see any particularly easy workaround.  Might have to
construct the second call \sidx in a macro and \edef it :(.  Oleg?

The spurious space is a side effect of the non-expansion.

Sorry,
Karl