%
\begin{isabellebody}%
\def\isabellecontext{Nested{\isadigit{1}}}%
%
\begin{isamarkuptext}%
\noindent
Although the definition of \isa{trev} below is quite natural, we will have
to overcome a minor difficulty in convincing Isabelle of its termination.
It is precisely this difficulty that is the \textit{raison d'\^etre} of
this subsection.
Defining \isa{trev} by \isacommand{recdef} rather than \isacommand{primrec}
simplifies matters because we are now free to use the recursion equation
suggested at the end of \S\ref{sec:nested-datatype}:%
\end{isamarkuptext}%
\isacommand{recdef}\ \end{isabellebody}%
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