I've been looking to see if I can make \atopwithdelims and related commands a little more consistent between the tex engines (specifically in relation to the amsmath \genfrac usage)
Short version: would it be possible to make \atopwithdelims scale \Udelimiter specified delimiters to a size determined by font parameters 20 and 21? Long version: The attached test file xetexatop.tex shows (I think) that xetex obeys fontdimens 20 and 21 to size the delimiters used with "withdelims" but only if the supplied delimiters have \delcode settings and tfm metrics specifying two slots. If the delimiters have \Udelcode settings just specifying a single slot in the OTF file then (I think) they are just set natural size. (I set the delcode and Udelcode to strange values in the test so () typeset as {} just to check the values were being picked up) You can compare the xetex result against pdftex or against xetex \\let\\Ucharcat\\undefined\\input xetexatop which makes it take the "classic tex" branch. The OpenType Math table doesn't have entries corresponding to 20/21 but these values could be set from the tex side while loading the font so it would still be useful if xetex supported this use (luatex has two additional named parameters for the same issue FractionDelimiterDisplayStyleSize and FractionDelimiterSize) For the specific use in latex where the use (should) always use an argument form \binom{a}{b} not {a \choose b} it is I think possible to define things using \left\right and get a compatible definition see the attached genfrac-vialeftright.tex but the definition is a bit of a mess using \mathchoice and magic numbers (2.39, 1.35 etc) designed to make the scaling for computer modern be exactly the same whether this definition or the primitive \atopwithdelims is used. For the primitive syntax used as an infix operator it is much harder to make a working definition just from tex macro redefinitions. hence the question if the underlying engine could help here... David
xetexatop.tex
Description: TeX document
genfrac-vialeftright.tex
Description: TeX document
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