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Dear TUG,</div>
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<br>
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<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">I'm wri</span><span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">ting to report a bug in The TeXbook
(Computers & Typesetting, Volume A).</span><br>
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<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;"><br>
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<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">This bug appears on page 315, line -3. It concerns the code </span><span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">\hfil\vadjust{\vskip\parskip}\break\indent.</span><br>
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I think there are two issues:</div>
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<ol>
<li>The \vskip\parskip part suggests that a parameter should be used, rather than, say, writing \vskip 0pt plus 1pt \relax. This accommodates a document-wide nonzero \parskip setting from the user. If so, then for the sake of consistency, \hfil would be inappropriate.
Instead, one should write \hskip\parfillskip, so that a different \parfillskip value from "0pt plus 1fil" can be accommodated as well.</li><li>This code cannot be a replacement of \par, as suggested by the answer. A regular \par would perform \unskip, followed by \nobreak\hskip\parfillskip (and finally a \break). This code lacks the "\unskip\nobreak" part. In certain cases, lacking leads to extra
empty lines, which are surely undesired.</li></ol>
<div>To demonstrate the above two issues, consider the following plain TeX example:</div>
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\hsize=155pt
<div>\parindent=20pt</div>
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<div>% \par alone, or simply leaving an empty line</div>
<div><br>
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<div>\TeX\ is a typesetting software written by D.~Knuth in the late 70's.</div>
<div>\par</div>
<div>\TeX\ is a typesetting software written by D.~Knuth in the late 70's.</div>
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<div>\TeX\ is a typesetting software written by D.~Knuth in the late 70's.</div>
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<div>\bigskip</div>
<div><br>
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<div>% \hfil\vadjust{\vskip\parskip}\break\indent</div>
<div>% does not accommodate unusual \parfillskip values</div>
<div>% also creates extra and undesired empty lines</div>
<div><br>
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<div>\TeX\ is a typesetting software written by D.~Knuth in the late 70's.</div>
<div>\hfil\vadjust{\vskip\parskip}\break\indent</div>
<div>\TeX\ is a typesetting software written by D.~Knuth in the late 70's.</div>
<div>\hfil\vadjust{\vskip\parskip}\break\indent</div>
<div>\TeX\ is a typesetting software written by D.~Knuth in the late 70's.</div>
<div><br>
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<div>\bigskip</div>
<div><br>
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<div>% \unskip\nobreak\hskip\parfillskip\vadjust{\vskip\parskip}\break\indent</div>
<div><br>
</div>
<div>\TeX\ is a typesetting software written by D.~Knuth in the late 70's.</div>
<div>\unskip\nobreak\hskip\parfillskip\vadjust{\vskip\parskip}\break\indent</div>
<div>\TeX\ is a typesetting software written by D.~Knuth in the late 70's.</div>
<div>\unskip\nobreak\hskip\parfillskip\vadjust{\vskip\parskip}\break\indent</div>
\TeX\ is a typesetting software written by D.~Knuth in the late 70's.<br>
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\bye</div>
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That is all. Thank you for your consideration.</div>
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<div style="margin-top:0px; margin-bottom:0px" class="elementToProof"><font face="Calibri,Arial,Helvetica,sans-serif">Ruixi Zhang</font></div>
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