XeLaTeX italic/quote kerning problem

Question

I'm using xelatex. I would like, I think, to alter the kerning between certain italic/math characters, and apostrophes and quotation marks. Here's my problem:

\documentclass[a4paper,12pt]{minimal}
\usepackage{mathspec}
\setallmainfonts[Ligatures=TeX]{Adobe Garamond Pro}
\begin{document}
$F$' $F$''
\end{document}

This, with xelatex, gives me:

That clearly seems wrong.

My question is whether it's possible to tell xelatex to alter the kerning here automatically, without inserting extra commands within the document body itself. I have tried to alter the code given here, but to no avail. The following doesn't work:

\documentclass[a4paper,12pt]{minimal}
\usepackage{mathspec}
\setallmainfonts[Ligatures=TeX]{Adobe Garamond Pro}

\makeatletter
\XeTeXinterchartokenstate=1
\newXeTeXintercharclass\LetterClass
\newXeTeXintercharclass\QuoteClass
\@tempcnta=`\A
\loop\unless\ifnum\@tempcnta>`\Z
  \XeTeXcharclass \@tempcnta \LetterClass
  \advance \@tempcnta by 1
\repeat
\@tempcnta=`\a
\loop\unless\ifnum\@tempcnta>`\z
  \XeTeXcharclass \@tempcnta \LetterClass
  \advance \@tempcnta by 1
\repeat
\XeTeXcharclass `\'=\QuoteClass
\XeTeXinterchartoks\LetterClass\QuoteClass={\kern 2pt} % deliberately big
\makeatother

\begin{document}
$F$' $F$'' $F'$ F'
\end{document}

I can see what's going wrong, but I'm not sure what I should do to fix it, or whether this sort of method is wrong-headed from the start.

The solution should let me do this automatically, to whole classes of italic/math letters.

Thanks in advance.

Answer 1

Here are two solutions to your problem.

Use proper italics

The first questions why you would ever want to have a text quote following a math element. Either the math element contains the quote (in which case it should be realized as a prime in math mode) or the math element isn't really math, and you're using $...$ to create text italics, which is incorrect. If the latter is the problem, then it suffices just to use \textit or \emph for your italics and the kerning will come out correctly:

\documentclass[a4paper,12pt]{minimal}
\usepackage{mathspec}
\setallmainfonts[Ligatures=TeX]{Adobe Garamond Pro}
\begin{document}
$F$' $F$''

\textit{F}'
\textit{F}''
\end{document}

Use \XeTeXinterchartoks

If you absolutely need to do exactly what you are doing, it is possible to do it, but comes with some caveats, and I don't really recommend it. We can use \XeTeXinterchartoks to insert space between the end of a group and the quotation marks. Since there's no way to distinguish the end of a math group from the end of any other group in this case, this solution might add extra space in places you don't want. Here's a full example:

% !TEX TS-program = XeLaTeX
\documentclass{article}
\makeatletter
\chardef\CharBound=\e@alloc@intercharclass@top
\makeatother
\XeTeXinterchartokenstate=1 % enable character classes
\newXeTeXintercharclass\mathquote % create a new class
\XeTeXcharclass `’ \mathquote
\XeTeXcharclass `' \mathquote
\XeTeXcharclass `” \mathquote
% between any character of class 0 and \mathquote
\XeTeXinterchartoks \CharBound \mathquote = {\hspace{3pt}} % purposefully large for testing
\begin{document}
$F$’
$A$’
$F$”
F’   % this is ok because no end of group.
{F}’ % fails here because you can't distinguish end-of-math from end-of-group
$F'$ % this is still ok.

\XeTeXinterchartoks \CharBound \mathquote = {\hspace{.2pt}} % Now make it more reasonable
$F$’
$A$’
$F$”
F’   % this is ok because no end of group.

\end{document}