\makeatletter
\ProvidesFile{in.tex}[2009/04/25 
                      testing \string\in@\space versions (Uwe Lueck)]
\nofiles

%% \in@ VERSIONS:
% LaTeX so far: 
\expandafter \let \csname in@LaTeX-so-far\endcsname \in@
% \def\in@#1#2{%
%  \def\in@@##1#1##2##3\in@@{%
%   \ifx\in@##2\in@false\else\in@true\fi}%
%  \in@@#2#1\in@\in@@}
% Morton: 
\def\in@morton#1#2{%
 \def\in@@##1#1##2##3\in@@{%
  \ifx\in@##2\in@false\else\in@true\fi}%
 \in@@#2\@nil#1\in@\in@@}
% Heiko: 
\def\in@heiko#1#2{%
 \def\in@@##1#1##2\in@@{%
  \def\in@@{##2}%
   \ifx\in@@\@empty \in@false
   \else            \in@true
   \fi}
 \in@@#2\@nil#1\in@@}
% Uwe: 
\newcommand*{\in@uwe}[2]{%
 \def\in@@##1#1##2\in@@{%
  \ifx\in@@##2\in@@\in@false\else\in@true\fi}%
 \in@@#2\in@#1\in@@}

%% MACROS FOR TESTING:
\newcommand*{\UseInVersion}[1]{%
  \expandafter \let \expandafter \in@tested \csname in@#1\endcsname
  \typeout{Using \string\in@-version `#1'%
  \space which is:^^J\expandafter \meaning \csname in@#1\endcsname
  ^^J...:%
}}
\newcommand*{\InTest}[2]{%
  \in@tested{#1}{#2}\typeout{`#1' \ifin@\else not \fi in `#2'!}}

%% ACTUAL TESTING: 
\UseInVersion{LaTeX-so-far}
% \UseInVersion{morton}
% \UseInVersion{heiko}
% \UseInVersion{uwe}
\InTest{bonbon}{bon}
\InTest{bon}{bonbon}
\InTest{bona}{bon}
\InTest{bon}{bona}
\InTest{msam}{msa}
\InTest{ionization}{ionizat}
\stop