This is TeX, WAITS Version 2.1 (preloaded format=plain 87.2.17)  21 APR 1987 13:50
**\tracingall

*\mag=0
{\mag}

*\showthe\mag
{\showthe}
> 0.
<*> \showthe\mag
                
? 

*a
{the letter a}
{horizontal mode: the letter a}
{blank space  }

*\bye

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{\par}
@firstpass
[]\tenrm a 
@\par via @@0 b=0 p=-10000 d=100
@@1: line 1.2- t=100 -> @@0

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\line ->\hbox to\hsize 
{\hbox}
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{internal vertical mode: end-group character }}
{restricted horizontal mode: \the}
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{internal vertical mode: \vss}
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\nointerlineskip ->\prevdepth -1000\p@ 
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\pagebody ->\vbox to\vsize {\boxmaxdepth \maxdepth \pagecontents }
{\vbox}
{\boxmaxdepth}

\pagecontents ->\ifvoid \topins \else \unvbox \topins \fi \dimen@ =\dp \@cclv \u
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\footins \fi \ifr@ggedbottom \kern -\dimen@ \vfil \fi 
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\makefootline ->\baselineskip 24\p@ \line {\the \footline }
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Completed box being shipped out [1]
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! Illegal magnification has been changed to 1000 (0).
\plainoutput ...headline \pagebody \makefootline }
                                                  \advancepageno \ifnum \outp...
<output> {\plainoutput 
                       }
\supereject ->\par \penalty -\@MM 
                                  
\bye ->\par \vfill \supereject 
                               \end 
<*> \bye
        
? x
 
Here is how much of TeX's memory you used:
 2 strings out of 1572
 38 string characters out of 12498
 22816 words of memory out of 48028
 999 multiletter control sequences out of 2500
 20136 words of font info for 68 fonts, out of 27000 for 100
 14 hyphenation exceptions out of 307
 7i,4n,0p,15b,22s stack positions out of 200i,40n,60p,500b,600s
No pages of output.