ref: 77f33692fe909f74e5e1af52bebff99a596c86ab
dir: /src/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'