\begin{tabbing}
(\=(Using [`A',$P$] (BackThruLemma `iff\_preserves\_decidability`)) \+
\\[0ex]CollapseTHEN (
\-\\[0ex](Auto\_aux (first\_nat 1:n) ((first\_nat 1:n),(first\_nat 4:n)) (first\_tok :t) inil\_term)))$\cdot$
\end{tabbing}