ref: 34756e8fb8c0f3899c048322a23441cb735de3b7
dir: /lua/test/readonly.lua/
-- make global variables readonly
local f=function (t,i) error("cannot redefine global variable `"..i.."'",2) end
local g={}
local G=getfenv()
setmetatable(g,{__index=G,__newindex=f})
setfenv(1,g)
-- an example
rawset(g,"x",3)
x=2
y=1 -- cannot redefine `y'