parse: \genfrac, and the two triangle binary operators

[tannevaled]

Three of the commands the go-tex/engine corpus reports as dropped formulas live here, not in the engine.

\triangleleft and \triangleright — 98 dropped formulas

Missing from the symbol table. They are binary operators at U+25C1 and U+25B7:

\UnicodeMathSymbol{"025B7"}{\triangleright}{\mathbin}{(large) right triangle, open}
\UnicodeMathSymbol{"025C1"}{\triangleleft }{\mathbin}{(large) left triangle, open}
                                          % unicode-math-table.tex:694/704
\DeclareMathSymbol{\triangleleft}{\mathbin}{letters}{"2F}   % fontmath.ltx:264-265

They are not the ⊲ ⊳ of \vartriangleleft/\vartriangleright already in the table, which amssymb makes relations: the two pairs differ in glyph and in spacing.

\genfrac — 88 dropped formulas

\genfrac{left}{right}{thickness}{style}{num}{den}, amsmath's generalised fraction:

\edef\@tempb{\@nx\@genfrac\@mathstyle{#4}%
  \csname @@\ifx @#3@over\else above\fi
  \ifx\@tempa\@empty \else withdelims\fi\endcsname}   % amsmath.sty:245-250

An empty thickness means \over — the font's own rule — and any other means \above that thickness, 0pt being no rule at all. That is how \binom itself is defined: \genfrac()\z@{} (amsmath.sty:240). The style argument is TeX's digit — 0 display, 1 text, 2 script, 3 scriptscript — and empty keeps the style in force.

Both real uses in the corpus are the delimited no-rule form: a q-binomial written \genfrac[]{0pt}{}{n}{k} and \genfrac{\{}{\}}{0pt}{}. Note the delimiters arrive unbraced there, as TeX takes single tokens for arguments, which readOptionalDelim handles alongside {} for "no delimiter on this side".

Documented reduction: a non-zero explicit thickness draws the font's rule rather than that exact width — fraction() takes its thickness from the font's MATH table.

Test

genfrac_test.go: both triangles render as operators; the five \genfrac shapes render; and the rule is what separates the two families — an empty thickness draws a <rect>, 0pt does not.