private int lower(LuaCallFrame callFrame, int nArguments) {
        BaseLib.luaAssert(nArguments >= 1, "not enough arguments");
        String s = getStringArg(callFrame,1,names[LOWER]);

        callFrame.push(s.toLowerCase());
        return 1;
    }
Only works with non-negative numbers
main/src/main/java/cgeo/geocaching/wherigo/kahlua/stdlib/StringLib.java