metatableIndex.lua

local a = {}
local Plus = {}
local Base = {}
-- t not self
Plus.__index = function (t, k) 
    -- output Plus    true    false   false
    print("Plus", t == a, t == Plus, t == Base)
    return Plus[k]
end
-- t not self
Base.__index = function(t, k)
    -- output Base   false   true    false
    print("Base", t == a, t == Plus, t == Base)
end
setmetatable(Plus, Base)
setmetatable(a, Plus)
print(a.b)