\pausing=1
\output{\botmark}
\def\a{\error}
\mark{
 \everyvbox{
  \everypar{
   \everydisplay{
    \everyhbox{
     \everymath{\noexpand\a}
     $\relax}
    \hbox\bgroup\relax}
   $$\relax}
  \noindent\relax}
 \vbox\bgroup\relax}
\hbox{}\vfill\penalty-10000