engine: assignments and stretch carry no maths - #173
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Three families reached the maths layer as unknown commands, and an unknown command
costs the WHOLE formula:
- \setlength{\len}{dim} is \def\setlength#1#2{#1 #2\relax} (latex.ltx:7347), an
assignment with two arguments — \addtolength beside it (:7348);
- \hfil and its kin are \hskip with a fixed glue:
primitive("hfil",hskip,fil_code) with fil_glue = 0pt plus 1fil (tex.web:20547,
§3318), so they take no argument at all. The maths layer cannot stretch, so what
they contribute here is nothing;
- \leftskip and the other glue PARAMETERS are assignments that read a <glue>
(tex.web §224), with TeX's optional equals before it, which scanMathGlue now
accepts.
Measured over 200 arXiv papers and 200 beamer talks: 11 documents change and 3
repaginate for +1290 glyphs; the page error against the 157 tectonic renders falls
656 -> 655. beamer gains 17 glyphs on one talk.
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Three families reached the maths layer as unknown commands, and an unknown command costs the whole formula.
\setlength{\len}{dim}\def\setlength#1#2{#1 #2\relax}latex.ltx:7347\hfil\hfill\hss\hfilneg(and the v-forms)\hskipwith a fixed glue, so no argumentprimitive("hfil",hskip,fil_code),fil_glue = 0pt plus 1fil— tex.web:20547, §3318\leftskip\parskip\baselineskip…<glue>The maths layer cannot stretch, so what
\hfillcontributes inside a formula is nothing — and the glue parameters take TeX's optional equals, whichscanMathGluenow accepts (\leftskip=0ptas well as\leftskip 0pt).Measured — 200 arXiv papers, 200 beamer talks
11 documents change and 3 repaginate for +1290 glyphs; the page error against the 157 tectonic renders falls 656 → 655. beamer gains 17 glyphs on one talk.
Test
mathskips_test.go: eight shapes —\hfill,\hss,\vfil,\setlength,\addtolength,\leftskip=0pt,\leftskip 0pt,\parskip\medskipamount— must each leave the maths layer with nothing to drop. All eight fail onmain.