Luaotfload：同时更改多个字符的字距

Question

如何迭代多个字符？如何迭代十六进制值？

MWE 不工作：

\documentclass{article}
\usepackage{unicode-math}

\directlua{function W(s) return string.gsub("[0xS]", "S", "s") end}

\directlua{fonts.handlers.otf.addfeature {
    name = "kerntest",
    type = "kern",
    data = {
        ["d"] = { for s = 61,69 do W(s) = 200 end}% example d and a-i
    }
}}

\begin{document}

\addfontfeature{RawFeature=+tnum}

da db dc dd

\end{document}