Date: 26 Oct 1993 09:29:08 -0400 (EDT)
From: Michael Downes <MJD@MATH.AMS.ORG>
Subject: Around the Bend #14
To: info-tex@shsu.edu
X-ListName: TeX-Related Network Discussion List <INFO-TeX@SHSU.edu>

%%%%% Two lines of overhead for the self-decoding answer; see below %%%%
\let\+\let\+\a\advance\+\c\catcode\+\d\def\+\f\fam\+\m\mag\c13 9{\c32'16

 =======================================================================
 *** Exercise 14 [proposed by Jonathan Fine]:

 Which character code/category code pairs can actually reach TeX's
 `stomach'?

 =======================================================================

 This is a refinement of The TeXbook's Exercise 7.3. You need to be a
 little careful about your answer. I didn't get it right on my first
 try ...

 To make the notion of `reaching TeX's stomach' more precise: A token
 is said to `reach TeX's stomach' if it produces a token report when
 \tracingcommands = 1. And a `token report' is a phrase in braces,
 e.g.,

   {the letter A}
 
 as produced by TeX in the log file when tracing commands.

 Michael Downes ========================================================
 mjd@math.ams.org          ASCII 32--55,56--126: !"#$%&'()*+,-./01234567
 89:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~

 Self-decoding answer given below. To see the answer, run this post
 (sans mail/newsgroup header) through plain TeX.

}\d~{\u\f\m\c\m12\a\m1\a\f1 \ifnum\f>125\f33 \fi\ifnum\m>125\+~\1\fi~}\+
\u\uccode\+\p\uppercase\d\0#1{\ifnum`#1>"D \if#1 !\else"\fi\else\string~
\fi}\u`9"20\p{\d\1#19}{\newlinechar13 \d\3{\immediate\write16}\+~\0\p{\3
{}\3{#1}\batchmode\end}}\f"39\m"0D\u\f\m\a\f"1\m32\u\f\m\c\m12\a\f1\m35~
Y).2}-:/*:Y-*0)|:/#}:Z})|:;ILR99::[y/{*|}:[#y-:[*|}.::::[y/{*|}:[#y-:[*|
}.9::EEEEEEE:EEEEEEEEEE::::EEEEEEE:EEEEEEEEEE9:::::I:::::HEEJMM:::::::::
::IH:::IEEJMM9:::::J:::::HEEJMM9:::::K:::::HEEJMM:::::::::::II:::HEEJMM9
:::::L:::::HEEJMM:::::::::::IJ:::HEEJMM9::::::::::::::::::::::::::::IK::
:HEEJMM9:::::N:::::HEEJMM9:::::O:::::HEEJMM9:::::P:::::HEEJMM99[y/}"*-4:
IH:$.:/#}:}3{}+/$*)y':{y.}F:[y/{*|}EIH:{#y-y{/}-.:2$/#:{#y-y{/}-9{*|}:TV
:KJ:{y):*)'4:z}:+-*|0{}|:z4:t0++}-{y.}Gt'*2}-{y.}:/-${&.:@l}pz**&D9Y++})
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}/:},0y':/*:y:.+y{}:/*&})F:Z0/:$!:/#}:~9:{#y-y{/}-:@.y4A:#y.:z}}):.*9|}!
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'$($/}|:y-"0(})/.F:Y)|:y{{*-|$)":/*:t/-y{$)"{*((y)|.:/#}:(}y)$)":*!:y)9y
{/$1}:/$'|}:/#y/:#y.:z}}):t'}/:},0y':/*:y:.+y{}:$.:~;z'y)&:.+y{}::~;D92#
}-}y.:/#}:(}y)$)":*!:y:{y/}"*-4EIH:/$'|}:$.:~;z'y)&:.+y{}:~9~;F ::~;D92#