Lua function to convert thousands-separated numbers into plain numbers

cnum.lua

-- converts thousands-separated numbers into plain numbers
-- for example:
-- cnum(0) == 0
-- cnum(1,000) == 1000
-- cnum(3,141,593) == 3141593
-- cnum(-9,001) == -9001
function cnum(...)
    local superbase = 1000
    local sign = 1
    local result = 0

    if arg.n ~= #arg then -- maybe a nil arg? arg.n counts it
        error("group count mismatch (nil group?)", 2)
    end

    -- in the case of a single argument, be more flexible
    if #arg == 1 then
        local number = tonumber(arg[1])
        if not number then
            error("argument of wrong type: "..type(group), 2)
        end
        return number
    end

    for ii, group in ipairs(arg) do
        local number = tonumber(group)

        if not number then
            error("numeric group ("..ii..") of wrong type: "..type(group), 2)
        end

        if number < 0 and ii == 1 then
            sign = -1
            number = -number
        end

        if number < 0 or number >= superbase then
            error("numeric group ("..ii..") out of range: "..number, 2)
        elseif number % 1 ~= 0 and ii ~= #arg then
            error("numeric group ("..ii..") not integral: "..number, 2)
        end

        result = result * superbase + number
    end

    return sign * result
end