[tex-k] Exercise 8.6 of the TeXbook

[Hanson Char]

Here is a git diff of the proposed fix to the latest texbook.tex:

$ git diff
diff --git a/tex/texbook/texbook.tex b/tex/texbook/texbook.tex
index a1e5078..54975e2 100644
--- a/tex/texbook/texbook.tex
+++ b/tex/texbook/texbook.tex
@@ -2956,7 +2956,7 @@ category~9 when |INITEX| begins.)
 characters, since the first |^^B| is converted to code~2 which doesn't
 equal the following character |^|. Hence
 the result is seven tokens: |^^B|$_7$ |^^B|$_7$
-|M|$_{11}$ \cstok{\^{}\^{}B} \]$_{10}$ |^^M|$_{11}$ \cstok{M\^{}\^{}M}.
+|M|$_{11}$ \cstok{\^{}\^{}B} |^^C|$_{10}$ |^^M|$_{11}$ \cstok{M\^{}\^{}M}.
 The last of these is a control word whose name has two letters.
 The \<space> after |\M| is deleted before \TeX\ inserts the \<return> token.


> On Jul 22, 2024, at 2:25 PM, Hanson Char <hanson.char at gmail.com> wrote:
> 
> Hi,
> 
> I think I may have found an error in the answer of exercise 8.6 on page 305 of the TeXbook. 
> 
>> EXERCISE 8.6
>> Assume that the category codes of plain TEX are in force, except that the characters
>> ^^A, ^^B, ^^C, ^^M belong respectively to categories 0, 7, 10, and 11. What tokens are produced from the (rather ridiculous) input line ‘^^B^^BM^^A^^B^^C^^M^^@\M␣’? 
>> (Remember that this line is followed by ⟨return⟩, which is ^^M; and recall that ^^@ denotes the ⟨null⟩ character, which has category 9 when INITEX begins.)
> 
> The current answer says the result is seven tokens as follows: 
> 
>    ^^B_7 ^^B_7 M_11 ^^B ␣_10 ^^M_11 M^^M
> 
> But shouldn't it rather be:
> 
>    ^^B_7 ^^B_7 M_11 ^^B ^^C_10 ^^M_11 M^^M
> 
> ?  Only the category code of ^^C is changed to 10, but the character code remains the same (ie 3).
> 
> Thoughts?
> 
> Regards,
> Hanson


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