[texhax] defin'd?

[Uwe Lueck]

On Sunday, Dec 27 2015, at 00:27:00 CET, Reinhard Kotucha wrote:
>> >> > \def\@IfUndefined#1{%
>> >> > \begingroup\expandafter\expandafter\expandafter\endgroup
>> >> > \expandafter\ifx\csname #1\endcsname\relax
>> >> > \expandafter\@firstoftwo
>> >> > \else \expandafter\@secondoftwo \fi}
>> Besides resetting the meaning to "undefined" (I believe Bernd Raichle
>> wrote that this was the motivation, and this has been Phil's point,
>> which I addressed), however, it would be nice if the \endgroup /removed/
>> the hash table entry. Can the quotes in Phil's
>
> In order to support plain TeX too, it's necessary to avoid that
> undefined cs names are set to \relax. I'm sure that _this_ was
> Bernd's intention.

Which his

    \begingroup\expandafter\expandafter\expandafter\endgroup
    \expandafter\ifx\csname ProvidesPackage\endcsname\relax\else
 
does not achieve?

(While it resets the meaning of \ProvidesPackage to "undefined" 
 after setting it to \relax, as I wrote.)

 -- Uwe.