[Function] string -> char array

debug.setmetatable("", {
    __index = function(a, b)
        if tonumber(b) then
           return string.sub(a, b, b)
        else return a end
    end,
    __add = function(a, b)
        return a .. b
    end
})

str = "abc"
print(str[1] + str[3] + str[2])