ref: 063fd456b2db1518c3bd58bd378a8dedb8bc0a57
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'