% Exercise and solution environments for the exercise unit. Solutions are
% typeset only in the solutions mode (target solutions); every other target
% drops them, so one source yields both the exercise sheet and its key.
\newcounter{exercise}[chapter]
\renewcommand\theexercise{\thechapter.\arabic{exercise}}
\NewDocumentEnvironment{exercise}{}
  {%
    \par\addvspace{\medskipamount}%
    \refstepcounter{exercise}%
    \noindent\textbf{\lende{Exercise}{Aufgabe}~\theexercise.}\ %
  }
  {\par}
\NewDocumentEnvironment{solution}{+b}
  {%
    \mode<solutions>{%
      \par\addvspace{\smallskipamount}%
      \noindent\textit{\lende{Solution}{Lösung}.}\ #1\par
    }%
  }
  {}