Put in minimal simpset to avoid excessive simplification,
just as in revision 1.9 of HOL/indrule.ML
\documentstyle[11pt,a4,latexsym,amssymb,isa2latex]{article}
\begin{document}
{\isamode
\begin{tabbing}
xxxxxxxx\=xxxxxxxx\=xxxxxxxx\=xxxxxxxx\=xxxxxxxx\=xxxxxxxx\=xxxxxxxx\=xxxxxxxx\=xxxxxxxx\=xxxxxxxx\=xxxxxxxx\=xxxxxxxx\=\kill{}\hspace{-1ex}
\end{tabbing}}
\end{document}