\begin{isabelle}%
%
\begin{isamarkuptext}%
\noindent In Exercise~\ref{ex:Tree} we defined a function
\isa{flatten} from trees to lists. The straightforward version of
\isa{flatten} is based on \isa{\at} and is thus, like \isa{rev}, quadratic.
A linear time version of \isa{flatten} again reqires an extra
argument, the accumulator:%
\end{isamarkuptext}%
\isacommand{consts}\ flatten2\ ::\ {"}'a\ tree\ =>\ 'a\ list\ =>\ 'a\ list{"}%
\begin{isamarkuptext}%
\noindent Define \isa{flatten2} and prove%
\end{isamarkuptext}%
\isacommand{lemma}\ {"}flatten2\ t\ []\ =\ flatten\ t{"}\end{isabelle}%
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