\begin{tabbing}
(\=(Using [`n',$b$] (BackThruLemma `mul\_cancel\_in\_eq`)) \+
\\[0ex]CollapseTHENA ((Auto\_aux (first\_nat 
\-\\[0ex]1:n) ((first\_nat 1:n),(first\_nat 3:n)) (first\_tok :t) inil\_term)))$\cdot$
\end{tabbing}