Adjust kerning with superscript

[Henri Menke <henrimenke@gmail.com>]

Dear list,

I am unhappy with how some fonts kern particular subscripts.  For example in
the default Latin Modern, subscripts to the Greek capital gamma could use some
negative kerning.  Because I can't (and don't want to) fix the font, I thought
that I could patch the font on-the-fly in ConTeXt.

The usual approach with fonts.handlers.otf.addfeature works well in math mode,
but does not extend into subscripts (MWE below).  Is there a method to adjust
subscript kernings?  Could this be done using mathkerns?  Latin Modern doesn't
have any but maybe they can be added on the fly.

Cheers, Henri

---

\startluacode
fonts.handlers.otf.addfeature {
    type = "kern",
    name = "kern",
    data = { [0x0393] = { [0x0030] = -200 } }
}
\stopluacode

\setupbodyfont[modern]

\starttext
$\Gamma0$ % works fine :)

$\Gamma_0$ % doesn't work :(
\stoptext

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