\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