[tex-k] minor errors in TeXbook Appendix G concerning genfrac

Sophie Alpert hi at sophiebits.com
Wed Dec 12 01:06:35 CET 2018

Thank you! These were found while building KaTeX, a TeX-inspired
JavaScript formula typesetting library for the web:


On Tue, Dec 11, 2018 at 4:02 PM Karl Berry <karl at freefriends.org> wrote:
> Hi Sophie,
>     1. TeXbook page A444 line 18 from the bottom mentions "let
>     $(\lambda, \rho)$ be the left and right delimiters". However it
>     never uses those names $\lambda$ and $\rho$ again, instead only
>     mentioning the "left delimiter" and "right delimiter" in step 15e on
>     page A445.
> Well, they are trivially used at the end of item 15:
>   The values of $\lambda$ and $\rho$ are null unless
>   the fraction is ``with delims.''
> But your point is taken nevertheless. I will consult.
>     2. TeXbook page A445 line 13 says to replace "the generalized
>     fraction by an Inner atom...". I believe this text is meant to
>     instruct replacing the genfrac by an *Ord* atom.
>     ...
>     3. Relatedly, TeXbook page A445 line 14 (end of Rule 15e) does not
>     indicate how to proceed after this processing. In contrast, Rules 1
>     through 14 all end with an instruction ...
> Ack, thanks. --karl.
> P.S. Are you reimplementing math typesetting in some new system? Just
> curious how you ended up here.

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