This is TeX, WAITS Version 2.1 (preloaded format=plain 87.2.17)  28 APR 1987 14:10
**a
(A.TEX[TEX,DEK]
! Undefined control sequence.
<recently read> How did this happen? 
                                     
l.4 \answer
           
? 
)
*r tex

*\def\a{\a}

*\a
! Interruption.
<*> \a
      
? i\tracingall
! Interruption.
<insert>   
           \tracingall
<*> \a
      
? 2
Type <return> to proceed, S to scroll future error messages,
R to run without stopping, Q to run quietly,
I to insert something, 
H for help, X to quit.
? x
No pages of output.