[tex-k] D???: comma as part of denominator

[胡亚捷 (Hu Yajie)]

Earlier I reported that page A169, line 13

$${52!\over13!\,13!\,26!}$$

has extra braces that serve no function. But such extra braces are
actually good practice, because you can easily append "+1" or ","
to the fraction. Without the braces, "+1" or "," will appear in the
denominator.

Now here's a example. Lines 6347--8 of mf.web says

$$\beta_k^2\cdot\\{ee}\over\beta_k^2\cdot\\{ee}+\alpha_k^2\cdot
  \\{cc}\cdot\\{dd},$$

and you know what happens -- the \over applies to the whole formula,
including the final comma, in section 289 of Volume D.

(But I don't regret failing to catch this typo. I regret failing to
catch the "character "28 of family 0" one.)

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Previous reports:
https://github.com/dine2014/texbook-mfbook-bugs