运算符重载不起作用。

阅读 Lua 程序设计 时，我尝试了该书中给出的运算符重载示例

Set = {}

mt = {}
mt.__add = Set.union

-- 使用给定列表的值创建一个新集合
function Set.new (l)
    local set = {}
    setmetatable (set, mt)
    for _, v in ipairs (l) do
        set [v] = true
    end
    return set
end

function Set.union (a, b)
    local result = Set.new {}
    for k in pairs (a) do result [k] = true end
    for k in pairs (b) do result [k] = true end
    return result
end

function Set.intersection (a, b)
    local result = Set.new {}
    for k in pairs (a) do result [k] = b[k] end
    return result
end

function Set.tostring (set)
    local l = {}
    for e in pairs (set) do
        l[#l + 1] = e
    end
    return "{" .. table.concat (l, ", ") .. "}"
end

function Set.print (s)
    print (Set.tostring (s))
end

s1 = Set.new {10, 20, 30, 50}
s2 = Set.new {30, 1}

Set.print (s1)
Set.print (s2)

s3 = s1 + s2
Set.print (s3)

但是在最新的 lua for windows 中，我遇到了以下错误

lua: C:\meta.lua:47: attempt to perform arithmetic on global 's1' (a table value)
stack traceback:
    C:\meta.lua:47: in main chunk
    [C]: ?
{30, 10, 20, 50}
{1, 30}