-- 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'