\let\mwe@begin\begin
\def\begin{\mwe@some@mark\mwe@begin}
\let\mwe@end\end
\def\end{\mwe@some@m@rk\mwe@end}
\newcommandx\mwe@some@mark[7][1=true,2=bels%%%%%%%%
,3=that,4=some@,5=mwe,6=@,7=false]{\@expandtwoargs%
\mwe@lst@add{\if@mwe@some@that@#1\else#7\fi}{#4#2}%
\expandafter\global\csname#6#5#6#4#3#6#7\endcsname}
\newcommandx\mwe@some@m@rk[6]%%%%%%%%%%%%%%%
[1=bebl,2=bels,3=that,4=some@,5=mwe,6=@]{%%%
\mwe@lst@pop{#4#1}{#4#2}\expandafter\global%
\csname#6#5#6#4#3#6\mwe@some@bebl\endcsname}