\begin{isabelle}%
%
\begin{isamarkuptext}%
\noindent
Define the datatype of binary trees%
\end{isamarkuptext}%
\isacommand{datatype}~'a~tree~=~Tip~|~Node~{"}'a~tree{"}~'a~{"}'a~tree{"}%
\begin{isamarkuptext}%
\noindent
and a function \isa{mirror} that mirrors a binary tree
by swapping subtrees (recursively). Prove%
\end{isamarkuptext}%
\isacommand{lemma}~mirror\_mirror:~{"}mirror(mirror~t)~=~t{"}\end{isabelle}%
