restricted system commands enabled.
**tracing.tex
(./tracing.tex
{vertical mode: \tracingonline}
{\def}
{blank space }
{\message}
\a #1#2->[#1|#2]
#1<-x
#2<-yz
[x|yz]
{blank space }
{\count}
{undefined}
! Undefined control sequence.
l.5 \count1=\numexpr
1+2\relax
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
{the character +}
{horizontal mode: the character +}
{the character 2}
{\relax}
{\tracingmacros}
{\tracingcommands}
[0.1])
Output written on tracing.dvi (1 page, 132 bytes).